I repaired a STK module many years ago it had shorted output drivers in one of the channels. I cut the top out of the module with a dremel tool removed the old drivers and replaced them with standard transistors and that thing actually worked and sounded great. I just wanted to see if it could be done and it’s possible to repair those things as long as it’s not something further back in the circuit I imagine if someone had the right equipment it would be possible to repair further back into the pre drive stages. Great video.
I am the owner of this SX-780, and just today got it back from Kev31779. Tonight I hooked up a pair of nice Wharfedale Denton 80th Anniversary speakers and first tuned in some classical music on FM, and then listened to some Charles Mingus on vinyl (via a resto-modded Acoustic Research AR-TX turntable with a nice Grado 8MR cart). I am literally stunned by how wonderful it sounds! I am mainly a vintage tubes guy, and did not expect to be this impressed by a modest '70s solid state Pioneer receiver. I'd also like to say that Kev31779 was super to work with, and he turned it around in just about a week, including the time for the Darlingtons to arrive. Bravo!
I think the sx-780 has to be the most common and favourite reciever around! I have one...everybody has one and they sound so great. Beautiful build quality...beautiful to look at and beautiful sounding! Awesome!
I inherited, my Moms, that she bought new in 78-79. She bought new ADS bookshelves, with it. Amazing tones. She really took care of her stuff. I have it sitting in original box/packing materials. Not going to set it up, until I'm ready. Yes, got the speakers, too. Pioneer 8trac, Brought along too, but I don't think that's ever coming out. Ha! Pioneer sx780, 45wpc, but really could push my Uncle Jim's JBL floor speakers. Don't make them like this any longer.
One of the very best repair videos I've seen so far! I learned quite a bit and am anxious to watch many more of your vids. Thanks so much for sharing. Oh . . . and by the way, my first Stereo way back in 1977, was the predecessor to this one, the SX-737. Thanks again DE KM2U
Kevin your videos are fantastic. The information you provide is amazing I am learning so much from you. I cant wait to get into testing more of my own equipment. Thanks for the inspiration sir!
Thank you so much for sharing all of your knowledge here. Getting into amp repair is something I've dreamed about doing as a hobby and its awesome to see your thought process about this. I have this same amplifier too. Haven't turned it on in a while due to an issue that I think is related to the relay switch. I'm inspired to try getting it to work again.
Would have been interesting to know the actual age on those 2 older Darlington's. Clearly, on had been replaced in the past, if not both at different times. That one was still performing so well and possibly vintage speaks highly of the build quality of these older Pioneers. Well done on your trouble-shooting, sir. A quality repair with integrity.
You are a man after my own heart. Excellent. You take pride in your work, how it should be. Clean and tidy and organized. Unlike some guys who maybe fix the amp but leave it in a filthy state.
I worked for Pioneer Electronics as a Technician on Plasma TV's and Audio gear. It is good quality product old or new. I have Plasma TV ( 5th and 8th generation ) and they both going strong. Great repair video. Cheers
THANK YOU for making this video... My SX-780 powers up, I have lights, I have an FM signal, but no sound out of either speaker. I suspected that the Darlington's might have gone bad, so I ordered new ones a couple of years ago, but have not yet installed them. My receiver has been sitting on the shelf collecting dust. Your video gave me some inspiration to work on it again, soon.
Yep, my Left Channel Darlington was bad too. Replaced them both, now everything works great. Note: Solder only pins 1, 2, 3, 8, 9, 0 (first 3 and last 3). Do not solder 4, 5, 6, 7. As you can see in the schematic, pins 4, 5, 6, 7 are not used.
Never thought I'd see one of these again. I spent more time replacing the STK 0050 on these things than all other jobs combined. I swear that if you sneezed in the same room with one of these they would pop!
Did you change your channel name ? I haven't been here for quite a while and although I am subscribed, I don't recall "The Active Region". You did a nice job cleaning up someone else's mess too btw. I have a soft spot for the 780. I purchased one several years ago from the original owner who himself had purchased brand new back in the day. He included the original bill of sale with the purchase and shipped it to me here in Australia. It is absolutely mint, and I mean mint. The wood is perfect, the faceplate is perfect, not even a hair's blemish. It looks like it just came out of the box and completely original and unmolested. It's also the export model so I am able to adjust the voltage to suit our 240v we have here. Win-Win..
Hi Kev, another Ak'er here. I love watching your videos and am learning slowly. I don't have the test devices that you have, but would love to learn how to use them properly and perhaps get some of my own oneday. I have restored a Pioneer SA-7100 in the recent years and it was a total success thanks to the generosity of the guys from AK. I have since purchased a few more Pioneer gear lately, mainly Stereo receivers, both mid and high end stuff including a nice PL530 and PL 570 TTs. I also denote a certain accent in your voice. Are you from Boston, or South Boston ? You remind me of Will Hunting from the movie "Good wIll Hunting" That Boston accent. Very cool.
The Darlington Power Packs STK-0040 in my JVC JA-S22 Integrated Stereo Amplifier have lasted for 43 year so far, and still working perfectly! At 60 years old, they'll probably outlast me! The heatsink in that Pioneer Receiver looks a bit crappy! The heatsink in my JVC is much bigger and heavier and has lots of cooling fins!
That's the way repairs should be done. I further like your diagnostic method which I'd have done in a similar/same way. Especially the uncut wire on the replaced part(maybe some would say this is monkish ;-) ) and cleaning up the debris from the former thermal paste. Big thumbs up.
Hey, thanks for the video! I have the exact same model with the exact same issue. Clear sound on headphones, distorderd speakers left channel. I wonder if I should just go ahead and replace the STK or bring it to a specialist and spend top $$$ on fixing it. I do have a soldering gun and know how to use it and other tools.. but almost no experience in finding an issue (without sound generator or osciloscope available). Something also I noticed is that the Balance knob has no effect all the way to the end of the spectrum where completely turn off one channel. I assume that would need to be replaced as well... Also, where I can find the STK 0050 power packs if have to replace them (I live in Canada)?
Thanks for doing this. It is very helpful. Though a long time ago can you confirm that the Darlington for the left channel is on the right when the unit is upside down, like you mention in the video? Thank you.
Excellent video, been tearing my hair out with a customers SX780. The new darlington packs must all be junk reproductions. Lasts for a bit and sounds great with lower volume music, then when hooked up to 1Khz test tone at 150mV RMS at Aux with line powered 2 chan scope hooked across dummy load 8ohm resistors, at least one pack shows less output then shorts out when volume is raised to 3 or 4, then no more speaker relay click at startup! On new packs I initially checked +/- power supply to the darlington pack at pin 9 & 2, and the voltage spot on both of the packs. The +/- inputs at pin 10 & 1 however were not showing 1.4v & -1.4 respectively as the schematics indicate. One pack showed .552v & -1.06 the other darlington pack shows .553v & -.885v Associated transistors Q11,13,15 Q12,14,16 responsible for the packs input voltages of 1.4 & -1.4 all test correct and not shorted, just not producing correct 1.4 voltages at collectors. Don’t know where to go next. When the darlington packs short or leak internally, they appear to back feed voltage to the packs input pins where my 1.4 voltages are off, usually raising the volts, or making them each positive. Don’t know if I’m dealing with shotty STK Darlington packs from the get go or not? On my second pair now, and one or both are gone out again, and have no protection relay click. Just not sure if it is crap darlington packs, or the somewhat off input volts at pins 1 & 10 not being at 1.4 & -1.4 that is taking out the pack(s)? Is it from something tied to these input pins circuit with Q11,12,13,14,15,16, or just leaking shotty Chinese STK0050’s causing this? New packs were tested and didn’t show infinite resistances across pins 8&9 and 2&3 like I would expect to see across a collector/emitter junction when tested out of circuit “before installing”, with only ohm meter connected. Oh also did a dc offset adjustment successfully at 3mV at each channel. Does anyone have any thoughts as how to keep these STK Modules from shorting? Is there anyone with a link to these darlington packs that aren’t crap “if this be the whole issue?” Schematic provided below. Thanks Schematic: drive.google.com/file/d/1t5SM8axkI4Q-656cQOqR3VHnp4iMHg2Q/view?usp=drivesdk
Really? I'm kind of a hobbyist when it comes to electronics. That being said, a friend of mine gifted me a TON of these (Power module things)things from an eviction sale. All in original packaging. Good to know I can list them or gift them with pride.
If they are truly original Sanyo's, some may offer more than the price of reproductions as they think there is magic inside them. Although they are a to be said to be better quality, but any part can have a bad day at any time.
Hello Kev, thanks for posting this video. I just picked up an sx-780. My problem is whenever I just touch the end of an RCA cable to the Phono input, I get a loud feedback sound on either channel. If i connect an RCA cable to the left phono channel (without the other end connecting to anything, unplugged), I get a buzzing hum. The right channel also does it but not as loud. Tried 3 different sets of cables with no results. Any idea what could cause this? Thx
Those Darlington power packs seem to be fairly sturdy, so it suprises me to see one go out. Let alone two in the same unit. Makes me wonder if there is a component that is pre output that is stressing them. Like bad caps or ones that leak. The other problem is the new power packs are made poorly. I have heard of people ordering several to get a good one. Unless you get a new old stock part, but it is hard to tell. Caps can test good, but be leaky and that can stress other parts in the system.
Do you think the metal heat sinks are adequate in stock form ? What if one could place an Aluminium block, say 5mm thick, onto the original plate for extra heat dissipation ??
I had my first 780 because eBay recommends it but the sound is not normal, it is kinda muddy. I went ahead and get another and I still doesn't like it, same sound. Got another and still the same, I thought I was crazy to go this far or was it my ear? I have some Yamaha, Marantz and Sansui and I can listen to them. Is something wrong my ear?
Very good video. I have an SX-780 that has no sound and no relay click but power. I've had for about 6 yrs now. It always had one of the channels that seemed to have more distortion. I will take apart in a day or so and see if i can find the problem with my limited electronics knowledge. thanks
Hi Kev31779. A more generel question regarding these great vintage amp's from Pioneer: What is your experience with faults in their protection-circuits with the relay cutting in and out for some upstart period and then after some time, it gives some strange and quite loud spikes in the speaker(s) one or both ? PS ! I have a Pioneer SX-737 that does exactly that. Thanks in advance / Allan, Denmark
Very informational vid. I have same receiver , I noticed on my left channel, the meter is little up with volume all the way down, I checked output 1.8 v on left side , volume on zero. Any suggestions? Thanks
Where are you located? I like your meticulous troubleshooting. I have several receivers with various issues. They are not my main players but sure would like to explore getting them fixed.
Hi the original Darlingtons had numbers at the bottom of the power packs from 1 threw 10 the replacements like the ones that are in the unit like you have don't have the numbers on them, i bought a set the other day and these don't have number on the bottom of them either, my original darlingtons was soldered in differently into the board then yours also, the left channel had 3 pins at each end was soldered in, and the 4 pins in the middle of the power pack was not, the right side had 4 pins soldered in on the right side of the power pack and the left side of the power pack had only 3 pins that was soldered in and the remaining 3 pins in the middle of the power pack was not soldered in at all, so my question to you is in your professional opinion should i solder in the new darlingtons the way you have them or should i solder them in the way mine is, please let me know because i don't want to damage anything else, best regards ted
Agree in replace both channels darlings ic´s, is a matter of ethics but most people would think that is a matter of loosing money,probably the same people that leaves component´s legs uncut, with a constant risk of a short. You done a Good Job!! the only thing that I would add is recap main capacitors just for the BASS sake
Light Man what do you mean a matter of ethics? Curious because I had my pioneer sx-780 worked on recently. My tech only replaced one power pack. Funny thing is that I noticed recently the left channel is dying. I presume it's the other power pack. The issue is intermittent so I couldn't show my tech the issue when I took it back. He told me to save my money and not to replace it yet. He thought I was being too picky or something because I could only show him a slight balance issue. After watching this, I'm surprised he didn't replace both power packs originally, it makes perfect sense to me. I'm a little upset he didn't test both power packs like this video, because I have a strong feeling that's the issue. I would have gladly agreed if he would have suggested it, but it seems like he never even thought about it or cared to really dive into the issue. I'm tore between going back to him, finding a new tech, selling it or fixing it myself.
Excellent work as usual Kev. It nice to see these old Receiver be cared for in that manner. I would love to get my hands on one the output about 100 Watt or so. By the way what Generator are you using?
Hey guy, I logged in just to subscribe. Love your vids and I know who to contact when I need clean and specialized repair. I like your style/method of diagnostic and repair. Thank you for making these videos, I really appreciate them. Michael
Nice Job..I have a similar problem with mine..I have two darlingtons that I bought some time ago that I will change ..one of mine (the lower one) has about 38 volts on pins one and zero..will let you know what happens.loved your video Paul
I have found that when I removed the bad darlington there was still +- 38 volts on the 0 and 1 inputs..I ordered the transistors used in the preamp section as I think that a couple are shorted.
Really you are great Sir. I have Pioneer xv sv434, some days before., It run in 2.1 mode, means 3 channels are working out of 6 channels. 2.1 only not 5.1. I check everything with remote control, but still it plays only in 2.1. what can I do?
that can happen when the B so to speak out of the AB doesn't work, and only bias current to the Class AB is holding A at full wave until it reaches usually anywhere from 5-20 watts RMS, still watching but I'm thinking that could be a good possibility, but I've seen wired things happen too though, but that's usually what I'd think is the resistor like you said or one of the transistors lost half wave and it's only operating mainly in class A, class A is nice though, Only when it's made that way, lol
I had my SX-780 fixed after the channels were blown and just never used it after getting it repaired (1981) but now I am using it an my AM channel is really bad. Any guesses on what it might be? Please help me if you can. *** Also, can I get a copy of the service manual from you(PDF) please? Also, you should put extra heat sink and maybe a fan too. Heat is what kills a amp.
Where was his oscilloscope ground lead connected when probing signal input to darlington pack? Wouldn’t the scope measurement be taken across pin 1 & 0 of each darlington pack? It appears as if the video creator tested input to darlington between chassis ground and pins 1&0 respectively with his scope. There should be only one channel of of audio input going into to each Darlington chip and measured with scope clipped across pin 1 & 0 for measurement correct ?
Also, my left channel is cutting out. If I have both speakers on the right channel, they sound fine, but obviously this isn't how music was meant to be listened to. When I have the speakers on the left and right, the left channel cuts out past a certain volume. However, when I apply pressure or pull on the volume knob slightly, it comes back. Any tips?
Can I dare ask where you buy the solder flux from ? I have been using a flux pen, but your's seem to make things flow that bit better. Also, can you recommend a reliable STK source please ?
I use either MG chemicals or Amtech flux. Amazon may carry them. There is no reliable source for replacement STK's They are all reproductions unless you find some new old stock somwhere.
Kev. I've had a look online for either of those two Flux products. There seems to be too many AMT ones coming from China. Can you give me some specific production description so I can compare ? There are a few variations and I'm unsure which one is suitable. I may just get a 100ml tub and transfer some to a syringe dispenser. Better value. How about shelf life ?? I see a lot advertised that have expired. Is this a problem ?
I have a couple of quick questions. I’m not getting very good sound through the headphone jack. Is there an easy fix for this? Also, are there any replacement bulbs I can put in this that don’t get so hot?
Is is static or cutting out? or a hum. Static/cut out may be a bad connection or a bad solder joints. Try some Deoxit. If it is a hum, well you got bigger issued. Might be a bad filter capacitor somewhere. You can replace with the same type or led. Led might require some custom voltage taming mods.
Unfortunately the originals are not made anymore and have not been for many years. There is nothing you can get for a drop in replacement other than reproductions that many vendors sell. There is no source better than an other as there is no real way to tell who's reproduction is better or worse or what you get now will be what you get next time.
Hey Kev. I have a 1974 realistic sta 220 am/fm stereo receiver and I no longer have the owners manual . I have tried without success to find a copy of one. You have any sources? I am a member of hi fi engine. Even Radio Shack has no copies on line. Thank you......Doug
Question, when you first turn the power on, you will hear something click, which allows sound to go. What does that? Mine comes on, and then randomly will click back off and not allow anything to play. Same unit, SX780.
That click is the speaker protection relay circuit. I'm currently chasing a similar problem on my 780 as well. I've read that a common cause is bad solder joints on the three power regulator transistors (the ones with the heat sinks in a row near one of the Darlingtons). Be careful as the heatsinks are electrically live when the unit is powered on.
I have found that after I switched my speakers from the A channel to the B channel, it has resolved the issue. I can't explain why, but it did. I am running some monster Pioneer CS-C9900 speakers, so maybe it was overpowering the amp trying to run them?
One thing I noticed was how crappy the solder job was from the last person..It almost looked like they removed the power pack, blopped some solder on the pads, heated up the solder, then pressed the pins into the solder as opposed to laying the fresh solder over the pins.
Hi Great video.. The oscilloscope OWON ds6062 . Have you had any issues and are your happy with the scope. I'm undecided to buy an Rigol or Owon. My wallet tells me the Owon. Please any comments would be really appreciated. Thank you
+saarike Never, that is unsafe. To use one o your scope. If you use a isolation transformer. Use it for the device under test with much caution. Some isolation tranformers pass the ground to the output so needs to watched out for.
Ok. Thanks a lot! I just bought an oscilloskope (Hantek dso4202c). I have tested it with mesr-100 v2 esr-meter. It send 100kHz ac-signal to test capacitors. I have measured it with this oscilloscope to see, if it works and it worked well. If I use it, I will inspect computers motherboards bios-chips and clock signals. There I think it is safe to use oscilloscopes ground lead to motherboard ground points, when motherboard have no other connections than atx-power cable and 12 v supply line to cpu. Is this safe?
Nothing is 100% safe when repairing live circuits connected to the wall, one bad hookup is all it takes to make a mistake. You can use your scope that to test devices that are not hooked up to an isolation transformer but caution and prior testing needs to be made. Wherever you hook up the scope ground, "MUST" be at the same potential as the device ground if you are working on the primary part of the DC supply. If you are working with something in the dc circuit make sure to check if the negative part of the supply is at ground potential before hooking up. If that scope ground from the probe touches anything other than ground potential .. POOF. That is where the tech version of an isolation transformer comes in to play for the device under test, but not for the scope. the scope should be always grounded for safety reasons.
Would you be interested in repairing a Would you be interested in repairing a Centex by pioneer model number KH-5151? One channel out and back light not working.? One channel out and no backlight.
Since they discontinued the original part years ago and there is nothing else you can get other than used originals. They are considered alternatives to most.
I repaired a STK module many years ago it had shorted output drivers in one of the channels. I cut the top out of the module with a dremel tool removed the old drivers and replaced them with standard transistors and that thing actually worked and sounded great. I just wanted to see if it could be done and it’s possible to repair those things as long as it’s not something further back in the circuit I imagine if someone had the right equipment it would be possible to repair further back into the pre drive stages. Great video.
I am the owner of this SX-780, and just today got it back from Kev31779. Tonight I hooked up a pair of nice Wharfedale Denton 80th Anniversary speakers and first tuned in some classical music on FM, and then listened to some Charles Mingus on vinyl (via a resto-modded Acoustic Research AR-TX turntable with a nice Grado 8MR cart). I am literally stunned by how wonderful it sounds! I am mainly a vintage tubes guy, and did not expect to be this impressed by a modest '70s solid state Pioneer receiver. I'd also like to say that Kev31779 was super to work with, and he turned it around in just about a week, including the time for the Darlingtons to arrive. Bravo!
Thanks Glenn
thank you for keeping this Unit in service
That's so great he was able to help!
Glenn Sadin i did the same to a toshiba sa520 & took me 3 days parts and all
Kev31779 good job
I think the sx-780 has to be the most common and favourite reciever around! I have one...everybody has one and they sound so great. Beautiful build quality...beautiful to look at and beautiful sounding! Awesome!
I inherited, my Moms, that she bought new in 78-79. She bought new ADS bookshelves, with it. Amazing tones. She really took care of her stuff. I have it sitting in original box/packing materials. Not going to set it up, until I'm ready. Yes, got the speakers, too. Pioneer 8trac, Brought along too, but I don't think that's ever coming out. Ha! Pioneer sx780, 45wpc, but really could push my Uncle Jim's JBL floor speakers. Don't make them like this any longer.
One of the very best repair videos I've seen so far! I learned quite a bit and am anxious to watch many more of your vids. Thanks so much for sharing. Oh . . . and by the way, my first Stereo way back in 1977, was the predecessor to this one, the SX-737. Thanks again DE KM2U
The output I.C. s they used were damn good. The STK & Darlington
Nice to see these old Pioneers continue on. Great video, thanks for sharing!
Kevin your videos are fantastic. The information you provide is amazing I am learning so much from you. I cant wait to get into testing more of my own equipment. Thanks for the inspiration sir!
No problem. Thanks.
Thank you so much for sharing all of your knowledge here. Getting into amp repair is something I've dreamed about doing as a hobby and its awesome to see your thought process about this. I have this same amplifier too. Haven't turned it on in a while due to an issue that I think is related to the relay switch. I'm inspired to try getting it to work again.
Would have been interesting to know the actual age on those 2 older Darlington's. Clearly, on had been replaced in the past, if not both at different times. That one was still performing so well and possibly vintage speaks highly of the build quality of these older Pioneers. Well done on your trouble-shooting, sir. A quality repair with integrity.
You are a man after my own heart. Excellent. You take pride in your work, how it should be. Clean and tidy and organized. Unlike some guys who maybe fix the amp but leave it in a filthy state.
I like the way he troubleshoots. Also quality work.
I worked for Pioneer Electronics as a Technician on Plasma TV's and Audio gear. It is good quality product old or new. I have Plasma TV ( 5th and 8th generation ) and they both going strong. Great repair video. Cheers
THANK YOU for making this video... My SX-780 powers up, I have lights, I have an FM signal, but no sound out of either speaker. I suspected that the Darlington's might have gone bad, so I ordered new ones a couple of years ago, but have not yet installed them. My receiver has been sitting on the shelf collecting dust. Your video gave me some inspiration to work on it again, soon.
Yep, my Left Channel Darlington was bad too. Replaced them both, now everything works great. Note: Solder only pins 1, 2, 3, 8, 9, 0 (first 3 and last 3). Do not solder 4, 5, 6, 7. As you can see in the schematic, pins 4, 5, 6, 7 are not used.
Power switch relay?
excellent repair mate big thumbs up thanks for showing us your skills
No problem and thanks for watching.
Never thought I'd see one of these again. I spent more time replacing the STK 0050 on these things than all other jobs combined. I swear that if you sneezed in the same room with one of these they would pop!
Great job and video!
Did you change your channel name ?
I haven't been here for quite a while and although I am subscribed, I don't recall "The Active Region".
You did a nice job cleaning up someone else's mess too btw.
I have a soft spot for the 780. I purchased one several years ago from the original owner who himself had purchased brand new back in the day. He included the original bill of sale with the purchase and shipped it to me here in Australia.
It is absolutely mint, and I mean mint. The wood is perfect, the faceplate is perfect, not even a hair's blemish. It looks like it just came out of the box and completely original and unmolested.
It's also the export model so I am able to adjust the voltage to suit our 240v we have here.
Win-Win..
Good job Sir! Thank you for the video!
Hi Kev, another Ak'er here.
I love watching your videos and am learning slowly.
I don't have the test devices that you have, but would love to learn how to use them properly and perhaps get some of my own oneday.
I have restored a Pioneer SA-7100 in the recent years and it was a total success thanks to the generosity of the guys from AK.
I have since purchased a few more Pioneer gear lately, mainly Stereo receivers, both mid and high end stuff including a nice PL530 and PL 570 TTs.
I also denote a certain accent in your voice. Are you from Boston, or South Boston ?
You remind me of Will Hunting from the movie "Good wIll Hunting" That Boston accent. Very cool.
Really interesting. Lovely receiver
I cannot believe how much I learned from this.
That's a 1979 model...I bought the 1979 SX-580 back then for $245 Canadian in London Ontario...great vid
Thanks for video, waiting for next amp or receiver :)
Nice work, sir!
Dancing needles are a beautiful sight to see.
The Darlington Power Packs STK-0040 in my JVC JA-S22 Integrated Stereo Amplifier have lasted for 43 year so far, and still working perfectly! At 60 years old, they'll probably outlast me!
The heatsink in that Pioneer Receiver looks a bit crappy! The heatsink in my JVC is much bigger and heavier and has lots of cooling fins!
That's the way repairs should be done. I further like your diagnostic method which I'd have done in a similar/same way. Especially the uncut wire on the replaced part(maybe some would say this is monkish ;-) ) and cleaning up the debris from the former thermal paste. Big thumbs up.
excellent work.
Hey, thanks for the video! I have the exact same model with the exact same issue. Clear sound on headphones, distorderd speakers left channel. I wonder if I should just go ahead and replace the STK or bring it to a specialist and spend top $$$ on fixing it. I do have a soldering gun and know how to use it and other tools.. but almost no experience in finding an issue (without sound generator or osciloscope available). Something also I noticed is that the Balance knob has no effect all the way to the end of the spectrum where completely turn off one channel. I assume that would need to be replaced as well...
Also, where I can find the STK 0050 power packs if have to replace them (I live in Canada)?
Thanks for doing this. It is very helpful. Though a long time ago can you confirm that the Darlington for the left channel is on the right when the unit is upside down, like you mention in the video? Thank you.
Thank you for the useful video
pioneer sa9800 Is my favourite amplifier
Excellent video, been tearing my hair out with a customers SX780. The new darlington packs must all be junk reproductions. Lasts for a bit and sounds great with lower volume music, then when hooked up to 1Khz test tone at 150mV RMS at Aux with line powered 2 chan scope hooked across dummy load 8ohm resistors, at least one pack shows less output then shorts out when volume is raised to 3 or 4, then no more speaker relay click at startup! On new packs I initially checked +/- power supply to the darlington pack at pin 9 & 2, and the voltage spot on both of the packs. The +/- inputs at pin 10 & 1 however were not showing 1.4v & -1.4 respectively as the schematics indicate.
One pack showed .552v & -1.06 the other darlington pack shows .553v & -.885v
Associated transistors Q11,13,15 Q12,14,16 responsible for the packs input voltages of 1.4 & -1.4 all test correct and not shorted, just not producing correct 1.4 voltages at collectors. Don’t know where to go next. When the darlington packs short or leak internally, they appear to back feed voltage to the packs input pins where my 1.4 voltages are off, usually raising the volts, or making them each positive. Don’t know if I’m dealing with shotty STK Darlington packs from the get go or not? On my second pair now, and one or both are gone out again, and have no protection relay click. Just not sure if it is crap darlington packs, or the somewhat off input volts at pins 1 & 10 not being at 1.4 & -1.4 that is taking out the pack(s)? Is it from something tied to these input pins circuit with Q11,12,13,14,15,16, or just leaking shotty Chinese STK0050’s causing this? New packs were tested and didn’t show infinite resistances across pins 8&9 and 2&3 like I would expect to see across a collector/emitter junction when tested out of circuit “before installing”, with only ohm meter connected. Oh also did a dc offset adjustment successfully at 3mV at each channel. Does anyone have any thoughts as how to keep these STK Modules from shorting? Is there anyone with a link to these darlington packs that aren’t crap “if this be the whole issue?” Schematic provided below.
Thanks
Schematic:
drive.google.com/file/d/1t5SM8axkI4Q-656cQOqR3VHnp4iMHg2Q/view?usp=drivesdk
Really glad those replacement STK's were good. Many if not most aren't. I've had pretty bad luck with fakes. Save that seller for future reference!
Really? I'm kind of a hobbyist when it comes to electronics. That being said, a friend of mine gifted me a TON of these (Power module things)things from an eviction sale. All in original packaging. Good to know I can list them or gift them with pride.
Id love a pride gift like that haha..
Kev31779
Are they usually expensive?
If they are truly original Sanyo's, some may offer more than the price of reproductions as they think there is magic inside them. Although they are a to be said to be better quality, but any part can have a bad day at any time.
I never had an original STK bad out of the box. Used to buy the 0050's ten at a time.
Hello Kev, thanks for posting this video. I just picked up an sx-780. My problem is whenever I just touch the end of an RCA cable to the Phono input, I get a loud feedback sound on either channel. If i connect an RCA cable to the left phono channel (without the other end connecting to anything, unplugged), I get a buzzing hum. The right channel also does it but not as loud. Tried 3 different sets of cables with no results. Any idea what could cause this? Thx
Nice video. THX!!!
Muy bonito yo tengo el reciver Pioneer modelo sx580 y otro reciver San Sui modelo 3900z
Those Darlington power packs seem to be fairly sturdy, so it suprises me to see one go out. Let alone two in the same unit. Makes me wonder if there is a component that is pre output that is stressing them. Like bad caps or ones that leak. The other problem is the new power packs are made poorly. I have heard of people ordering several to get a good one. Unless you get a new old stock part, but it is hard to tell. Caps can test good, but be leaky and that can stress other parts in the system.
Do you think the metal heat sinks are adequate in stock form ?
What if one could place an Aluminium block, say 5mm thick, onto the original plate for extra heat dissipation ??
I had my first 780 because eBay recommends it but the sound is not normal, it is kinda muddy. I went ahead and get another and I still doesn't like it, same sound. Got another
and still the same, I thought I was crazy to go this far or was it my ear? I have some Yamaha, Marantz and Sansui and I can listen to them. Is something wrong my ear?
Ahh, some Friday entertainment ;)
Very good video. I have an SX-780 that has no sound and no relay click but power. I've had for about 6 yrs now. It always had one of the channels that seemed to have more distortion. I will take apart in a day or so and see if i can find the problem with my limited electronics knowledge. thanks
Power switch is bad
is it safe to turn on the unit with one of the power packs uninstalled?
Good Job👍💯
Hi Kev31779.
A more generel question regarding these great vintage amp's from Pioneer: What is your experience with faults in their protection-circuits with the relay cutting in and out for some upstart period and then after some time, it gives some strange and quite loud spikes in the speaker(s) one or both ? PS ! I have a Pioneer SX-737 that does exactly that. Thanks in advance / Allan, Denmark
Very informative video
When I set mine up, the needles don't move when I play music. What would cause that issue?
Top Job
Very informational vid. I have same receiver , I noticed on my left channel, the meter is little up with volume all the way down, I checked output 1.8 v on left side , volume on zero. Any suggestions? Thanks
Where are you located? I like your meticulous troubleshooting. I have several receivers with various issues. They are not my main players but sure would like to explore getting them fixed.
awwsome!
Hi the original Darlingtons had numbers at the bottom of the power packs from 1 threw 10 the replacements like the ones that are in the unit like you have don't have the numbers on them, i bought a set the other day and these don't have number on the bottom of them either, my original darlingtons was soldered in differently into the board then yours also, the left channel had 3 pins at each end was soldered in, and the 4 pins in the middle of the power pack was not, the right side had 4 pins soldered in on the right side of the power pack and the left side of the power pack had only 3 pins that was soldered in and the remaining 3 pins in the middle of the power pack was not soldered in at all, so my question to you is in your professional opinion should i solder in the new darlingtons the way you have them or should i solder them in the way mine is, please let me know because i don't want to damage anything else,
best regards
ted
Agree in replace both channels darlings ic´s, is a matter of ethics but most people would think that is a matter of loosing money,probably the same people that leaves component´s legs uncut, with a constant risk of a short. You done a Good Job!! the only thing that I would add is recap main capacitors just for the BASS sake
Thanks, I was told from the owner that the unit was recapped. I assume they redid them as well.
Yep I could tell right away that it was recapped when I saw those Nichicon "gold tune" caps.
Light Man what do you mean a matter of ethics?
Curious because I had my pioneer sx-780 worked on recently. My tech only replaced one power pack. Funny thing is that I noticed recently the left channel is dying. I presume it's the other power pack. The issue is intermittent so I couldn't show my tech the issue when I took it back. He told me to save my money and not to replace it yet. He thought I was being too picky or something because I could only show him a slight balance issue.
After watching this, I'm surprised he didn't replace both power packs originally, it makes perfect sense to me. I'm a little upset he didn't test both power packs like this video, because I have a strong feeling that's the issue. I would have gladly agreed if he would have suggested it, but it seems like he never even thought about it or cared to really dive into the issue. I'm tore between going back to him, finding a new tech, selling it or fixing it myself.
Excellent work as usual Kev. It nice to see these old Receiver be cared for in that manner. I would love to get my hands on one the output about 100 Watt or so. By the way what Generator are you using?
Thanks, My generator is an Owon AG2052F.
Hey guy, I logged in just to subscribe. Love your vids and I know who to contact when I need clean and specialized repair. I like your style/method of diagnostic and repair.
Thank you for making these videos, I really appreciate them.
Michael
Nice Job..I have a similar problem with mine..I have two darlingtons that I bought some time ago that I will change ..one of mine (the lower one) has about 38 volts on pins one and zero..will let you know what happens.loved your video
Paul
I have found that when I removed the bad darlington there was still +- 38 volts on the 0 and 1 inputs..I ordered the transistors used in the preamp section as I think that a couple are shorted.
well done
My Pioneer sx-780 keeps blowing fuses I hear the Darlington STK are the problem how do I figure out which one is the bad one if I can't turn it on
Really you are great Sir. I have Pioneer xv sv434, some days before., It run in 2.1 mode, means 3 channels are working out of 6 channels. 2.1 only not 5.1. I check everything with remote control, but still it plays only in 2.1. what can I do?
Ooh darlingtons!
Man this thing is so similar as my Realistic STA 2100D
that can happen when the B so to speak out of the AB doesn't work, and only bias current to the Class AB is holding A at full wave until it reaches usually anywhere from 5-20 watts RMS, still watching but I'm thinking that could be a good possibility, but I've seen wired things happen too though, but that's usually what I'd think is the resistor like you said or one of the transistors lost half wave and it's only operating mainly in class A, class A is nice though, Only when it's made that way, lol
I had my SX-780 fixed after the channels were blown and just never used it after getting it repaired (1981) but now I am using it an my AM channel is really bad. Any guesses on what it might be? Please help me if you can. *** Also, can I get a copy of the service manual from you(PDF) please? Also, you should put extra heat sink and maybe a fan too. Heat is what kills a amp.
Where do you buy the darlington packs?
Where was his oscilloscope ground lead connected when probing signal input to darlington pack? Wouldn’t the scope measurement be taken across pin 1 & 0 of each darlington pack? It appears as if the video creator tested input to darlington between chassis ground and pins 1&0 respectively with his scope. There should be only one channel of of audio input going into to each Darlington chip and measured with scope clipped across pin 1 & 0 for measurement correct ?
Also, my left channel is cutting out. If I have both speakers on the right channel, they sound fine, but obviously this isn't how music was meant to be listened to. When I have the speakers on the left and right, the left channel cuts out past a certain volume. However, when I apply pressure or pull on the volume knob slightly, it comes back. Any tips?
Try cleaning the volume pot with deoxit
780s are nice receivers. Where did you order those Darlingtons from?
Amazon
Hi i think you show is great I am new to testing stereo systems and amps where are you injecting the signal for testing
Greg
At the RCA inputs
do you know where a person can get those power packs
Can I dare ask where you buy the solder flux from ? I have been using a flux pen, but your's seem to make things flow that bit better.
Also, can you recommend a reliable STK source please ?
I use either MG chemicals or Amtech flux. Amazon may carry them. There is no reliable source for replacement STK's They are all reproductions unless you find some new old stock somwhere.
Kev. I've had a look online for either of those two Flux products. There seems to be too many AMT ones coming from China. Can you give me some specific production description so I can compare ?
There are a few variations and I'm unsure which one is suitable.
I may just get a 100ml tub and transfer some to a syringe dispenser. Better value.
How about shelf life ??
I see a lot advertised that have expired. Is this a problem ?
Im having trouble with my sx780 giving out sound to my speakers, it’ turns on but doesn’t give out sound
I have a couple of quick questions. I’m not getting very good sound through the headphone jack. Is there an easy fix for this? Also, are there any replacement bulbs I can put in this that don’t get so hot?
Is is static or cutting out? or a hum. Static/cut out may be a bad connection or a bad solder joints. Try some Deoxit. If it is a hum, well you got bigger issued. Might be a bad filter capacitor somewhere. You can replace with the same type or led. Led might require some custom voltage taming mods.
The Active Region it’s static/cutting out. If I only insert the jack 3/4 of the way it sounds fine.
Can I ask where you're buying your STK's from ?
There's so much crap coming out of China, is the a reliable source for these Darlingtons ?
Where's your shop located?
Nice repair, what was your source for the modules
Unfortunately the originals are not made anymore and have not been for many years. There is nothing you can get for a drop in replacement other than reproductions that many vendors sell. There is no source better than an other as there is no real way to tell who's reproduction is better or worse or what you get now will be what you get next time.
hola, tengo el mismo receiver sx780 y con la falla, necesito si me puede enviar el esquema o plano que ud. tienes, se lo agradezco
Hey Kev. I have a 1974 realistic sta 220 am/fm stereo receiver and I no longer have the owners manual . I have tried without success to find a copy of one. You have any sources? I am a member of hi fi engine. Even Radio Shack has no copies on line. Thank you......Doug
www.hifiengine.com/manual_library/realistic/sta-2200.shtml
My stereo is not the sta 2200. Its the sta 220. Thanks for trying Kev.
Question, when you first turn the power on, you will hear something click, which allows sound to go. What does that?
Mine comes on, and then randomly will click back off and not allow anything to play. Same unit, SX780.
That click is the speaker protection relay circuit. I'm currently chasing a similar problem on my 780 as well. I've read that a common cause is bad solder joints on the three power regulator transistors (the ones with the heat sinks in a row near one of the Darlingtons). Be careful as the heatsinks are electrically live when the unit is powered on.
I have found that after I switched my speakers from the A channel to the B channel, it has resolved the issue. I can't explain why, but it did. I am running some monster Pioneer CS-C9900 speakers, so maybe it was overpowering the amp trying to run them?
+Jay Dee Not sure about that. Sounds like the problem is in the A channel circuit only. I have no clue how to trace anything like that down though.
Where are you injecting your signal from the generator?
RCA input.
good repair. curious, how many watts is this receiver?
Judging by those darlington devices I would say 20-30watts per channel.
@@jeanious2009 rated 45 watts per channel
One thing I noticed was how crappy the solder job was from the last person..It almost looked like they removed the power pack, blopped some solder on the pads, heated up the solder, then pressed the pins into the solder as opposed to laying the fresh solder over the pins.
digital is great for sound but not warmth
Hey Kev. I have the same unit and suspect I has the same problem. Can I send u my unit for repair. Please let me know. Thanks, Rich dd
Your volume and function knobs are switched.
Hi Great video.. The oscilloscope OWON ds6062 . Have you had any issues and are your happy with the scope. I'm undecided to buy an Rigol or Owon. My wallet tells me the Owon. Please any comments would be really appreciated. Thank you
Owon are cheap but decent scope, the menu system can be hard to navigate. Cant go wrong with a Rigol. They are a step above
Thank you so much
Nice, nice, nice. Do you use isolation transformer for oscilloscope?
+saarike Never, that is unsafe. To use one o your scope. If you use a isolation transformer. Use it for the device under test with much caution. Some isolation tranformers pass the ground to the output so needs to watched out for.
Thank you very much for information!
Depending on the isolation transformer, they may need to be modified for tech use.
Ok. Thanks a lot! I just bought an oscilloskope (Hantek dso4202c). I have tested it with mesr-100 v2 esr-meter. It send 100kHz ac-signal to test capacitors. I have measured it with this oscilloscope to see, if it works and it worked well. If I use it, I will inspect computers motherboards bios-chips and clock signals. There I think it is safe to use oscilloscopes ground lead to motherboard ground points, when motherboard have no other connections than atx-power cable and 12 v supply line to cpu. Is this safe?
Nothing is 100% safe when repairing live circuits connected to the wall, one bad hookup is all it takes to make a mistake. You can use your scope that to test devices that are not hooked up to an isolation transformer but caution and prior testing needs to be made. Wherever you hook up the scope ground, "MUST" be at the same potential as the device ground if you are working on the primary part of the DC supply. If you are working with something in the dc circuit make sure to check if the negative part of the supply is at ground potential before hooking up. If that scope ground from the probe touches anything other than ground potential .. POOF. That is where the tech version of an isolation transformer comes in to play for the device under test, but not for the scope. the scope should be always grounded for safety reasons.
Unit obviously got defective power packs, where 1 took longer to go bad and hell yes replace both, there not too pricey.
Would you be interested in repairing a Would you be interested in repairing a Centex by pioneer model number KH-5151? One channel out and back light not working.? One channel out and no backlight.
Hello, I have the same receiver sx780 and with the failure, I need if you can send me the diagram or plan that you. you have, I appreciate it
www.audioservicemanuals.com/p/pioneer/pioneer-sx/pioneer-sx-780/
dear my email is : alexissonido@hotmail.com
The manuals are in the link above.
I dont know any reason why we're hearing the test tone - I have to turn it off.
They wont last long those are fake Chinese STK-0050's
Since they discontinued the original part years ago and there is nothing else you can get other than used originals. They are considered alternatives to most.
Well, considering thr price and the fact that they seem to work. One could buy a few and swap hoping for no catastrophic failures.