Wow. Seeing this receiver just took me back decades. This one one of the components in my home system along with: Teac 360s cassette, EMI speakers, Thornes Transcription turntable. That whole system did everything I wanted or needed. I can still look at this as a thing of beauty. Thanks 🙂
The guts of that thing is VERY similar to my Superscope R350. The internal layout and circuits are very close, very similar feel. Mine is a good little receiver, around 1970 with the lavender color film on the dial. Low end, ~20 watt, made for the teenager or dorm room back in the day. I love it, it sounds great in my office. My first foray into recapping and audio resto, it is solid 5 years after I bought it on ebay for $50 and went through it.
13:15 the resistors are used to keep the lamp heated to increase its resistance to protect TR304 from switching into a near-zero cold filament resistance. The reaction time impact is minimal.
That makes sense, since tungsten filament resistance is not linear. That is what makes it great for current limiting with a dim bulb. I put thermistors on the primary side of my tube mono amps to limit the inrush current of the cold tube filaments.
If you go back to the place where I showed the schematic, you will also see where the base of the transistor is being driven directly from the multiplex signal path, which is an analog signal. Depending on the hFe of the transistor, the light would be very intermittent and flickery, as there would be on set threshold for the lamp to light. The resistors serve to tame this down a bit. This is also why I had to experiment with the LED dropping resistor to find the correct value so that the light wouldn't false trigger on low signal levels.
@@xraytonyb don’t worry - I watched that part. But with all due respect, it’s misguided to say that is the reason for the resistors - it’s merely a side effect. I understand you’re an accomplished technician - and so am I - but I’m also an engineer with quite a bit of experience across many disciplines. The resistors keep the filament in a preheat state as I described. Similar circuits are used, for example, in Williams pinball machines for their “Flash Lamps”. Similar theories abound, but from a design standpoint - small signal junction devices do not withstand any overloads.
It extends the life of the bulb as well. Stage lighting did this too. That way the filament doesn't have to go so far from cold to hot. I think in stage lighting it was called preheat. You set a preset on the dimmer board just so you barely saw the filament when everything was set to minimum.
When designing high-reliability industrial monitoring equipment in the 70s, before LEDs became cheap and reliable, we always ran some idle current through incandescent indicator lamps, whether they were driven by transistors or relays, to extend lamp life. Lamps typically fail at turn-on, due to the low cold-filament resistance already noted, and pre-heat minimizes the thermal shock. This becomes important when a lamp could be switching on and off rapidly, say, in a stereo indicator circuit. The side benefit is that preheating allows you to use less beefy switching transistors, as they aren't required to handle a virtual short-circuit turn-on current.
Hi Tony. I know you did a video in the past where you discussed how FM stereo demultiplexing works. Could you do one on how the FM ratio detector works, and how it differs from how the “simple” AM detector (diode) works?
I don't get it, when repairmen complaint on the LED variant of the backlights. The light is pretty much similar, if you take the warm white. 5 years ago I made them myself, with the chassis of a fuse, two warm white LEDs per stack. And some modifications on the power supply for these. Because the ones you have, didn't exist yet. When I visit my friends where I did the mod on their receivers, they are still working. Sometimes you have to be creative. Especially when the parts are obsolete. I had to place resistors in serial. So, each LED stack had his own resistor. Luckily, I didn't have the issue that current wasn't rectified. My job is the opposite of yours, we're installing transmitters and antenna's and fine-tuning the MPX signal that goes into the receiver. Very interesting job, but not so healthy when working in transmitter sites, where a dozen transmitters are working with high power. They are centralized in Belgium, mostly on high buildings. Nice to see, how the other end tries to fine tune, the tuner of these beautiful receivers.
Hi Tony! I have the RX 454, that is quite similar to the 400. I'd like to replace its bulbs that are quite old. Can you please tell me the correct references of the bulbs (ore LEDs)? I've seen that the ones for dial and signal should be 6 x 30 fuse-type, but I don't know the others. Thank you!
Good afternoon receiver/tuners like those are far superior to the disposable junk we get today. Anyway hope you can help ,me finding a volume control for an HK 460 receiver . Cannot find here in Canada
just a thought to add about that LED panel light in the stereo indicator The bridge rectifier that you mentioned that's built into the LED is there till now the light to be put in either direction and direct the DC voltage in the right direction?. bridge rectifiers do that very well when it comes to getting DC to the point that it needs to go. in an AC situation that bulb would immediately begin working because the rectifier what convert the AC to DC so truly it's just a multi-purpose bulb.
When you buy the LED replacement lamps to replace the incandescent indicator bulbs, they will often come fitted with a built-in bridge rectifier. This is so that the LED bulbs will be compatible with AC as well as DC sources. Without the bridge, the LED would only light up on 1/2 of the cycle and would flicker. In addition, during the other half cycle, the LED will be in reverse bias, which could possibly damage the LED, depending on the circuit. Some of these replacement lamps will also include a built-in dropping resistor to limit the current to the LED, as quite often the original filament lamps will have a much higher operating voltage and current. The replacement I used had the bridge but no the resistor, so I had to add one. ;)
The LED will pose no interference to the circuit, but if it didn't have a built-in bridge rectifier, it could cause the LED to flicker when used in circuits that employ AC signals.
I really enjoy your work on 70's era gear. Love the analog nature of this stuff, the quality and serviceability.
Wow. Seeing this receiver just took me back decades. This one one of the components in my home system along with: Teac 360s cassette, EMI speakers, Thornes Transcription turntable. That whole system did everything I wanted or needed. I can still look at this as a thing of beauty. Thanks 🙂
Love it Tony, whenever you have a problem troubleshoot yourself.
When ever I encounter a problem my first internal thought is what did I do wrong.
@2.37 "before you start soiling your nappy", excellent 😀. You keep doing what you are doing Tony don't you worry about the nappy spoilers 👍
Another great session filled with great tips on using the SG-80. And that's just one of many that you reveal. Enjoyed the learning,thanks!
Great videos -- your explanations are thorough, and I have been learning consistently with your videos .. GREAT job with the Rotel ...
The guts of that thing is VERY similar to my Superscope R350. The internal layout and circuits are very close, very similar feel. Mine is a good little receiver, around 1970 with the lavender color film on the dial. Low end, ~20 watt, made for the teenager or dorm room back in the day. I love it, it sounds great in my office. My first foray into recapping and audio resto, it is solid 5 years after I bought it on ebay for $50 and went through it.
Thanks Tony, my farther had a tuner exactly the same. Used to listen to radio wales in his studio.
13:15 the resistors are used to keep the lamp heated to increase its resistance to protect TR304 from switching into a near-zero cold filament resistance. The reaction time impact is minimal.
That makes sense, since tungsten filament resistance is not linear. That is what makes it great for current limiting with a dim bulb. I put thermistors on the primary side of my tube mono amps to limit the inrush current of the cold tube filaments.
If you go back to the place where I showed the schematic, you will also see where the base of the transistor is being driven directly from the multiplex signal path, which is an analog signal. Depending on the hFe of the transistor, the light would be very intermittent and flickery, as there would be on set threshold for the lamp to light. The resistors serve to tame this down a bit. This is also why I had to experiment with the LED dropping resistor to find the correct value so that the light wouldn't false trigger on low signal levels.
@@xraytonyb don’t worry - I watched that part. But with all due respect, it’s misguided to say that is the reason for the resistors - it’s merely a side effect. I understand you’re an accomplished technician - and so am I - but I’m also an engineer with quite a bit of experience across many disciplines. The resistors keep the filament in a preheat state as I described. Similar circuits are used, for example, in Williams pinball machines for their “Flash Lamps”. Similar theories abound, but from a design standpoint - small signal junction devices do not withstand any overloads.
It extends the life of the bulb as well. Stage lighting did this too. That way the filament doesn't have to go so far from cold to hot. I think in stage lighting it was called preheat. You set a preset on the dimmer board just so you barely saw the filament when everything was set to minimum.
It does that, too, but in this case it’s to prevent damage to the small transistor because a cold filament is extremely low resistance.
@@envisionelectronics Good point, that makes sense too.
When designing high-reliability industrial monitoring equipment in the 70s, before LEDs became cheap and reliable, we always ran some idle current through incandescent indicator lamps, whether they were driven by transistors or relays, to extend lamp life. Lamps typically fail at turn-on, due to the low cold-filament resistance already noted, and pre-heat minimizes the thermal shock. This becomes important when a lamp could be switching on and off rapidly, say, in a stereo indicator circuit. The side benefit is that preheating allows you to use less beefy switching transistors, as they aren't required to handle a virtual short-circuit turn-on current.
Great work as always Tony.
A little fact on Rotel, they make there own power supply's.
My favourite amps.
Hi Tony. I know you did a video in the past where you discussed how FM stereo demultiplexing works. Could you do one on how the FM ratio detector works, and how it differs from how the “simple” AM detector (diode) works?
I don't get it, when repairmen complaint on the LED variant of the backlights. The light is pretty much similar, if you take the warm white. 5 years ago I made them myself, with the chassis of a fuse, two warm white LEDs per stack. And some modifications on the power supply for these. Because the ones you have, didn't exist yet. When I visit my friends where I did the mod on their receivers, they are still working. Sometimes you have to be creative. Especially when the parts are obsolete. I had to place resistors in serial. So, each LED stack had his own resistor. Luckily, I didn't have the issue that current wasn't rectified. My job is the opposite of yours, we're installing transmitters and antenna's and fine-tuning the MPX signal that goes into the receiver. Very interesting job, but not so healthy when working in transmitter sites, where a dozen transmitters are working with high power. They are centralized in Belgium, mostly on high buildings. Nice to see, how the other end tries to fine tune, the tuner of these beautiful receivers.
Troubleshoot yourself (and your test equipment) first! Great advice. Reduce the tail chasing.
Nice intro music😉. Iv just finisht a Marantz 2225. Its also 25 watt. But my Neigbours think its much more😂.
Thanks for you're video's.✌🏽
Hi Tony!
I have the RX 454, that is quite similar to the 400. I'd like to replace its bulbs that are quite old. Can you please tell me the correct references of the bulbs (ore LEDs)? I've seen that the ones for dial and signal should be 6 x 30 fuse-type, but I don't know the others. Thank you!
Good afternoon receiver/tuners like those are far superior to the disposable junk we get today. Anyway hope you can help ,me finding a volume control for an HK 460 receiver . Cannot find here in Canada
just a thought to add about that LED panel light in the stereo indicator
The bridge rectifier that you mentioned that's built into the LED is there till now the light to be put in either direction and direct the DC voltage in the right direction?. bridge rectifiers do that very well when it comes to getting DC to the point that it needs to go. in an AC situation that bulb would immediately begin working because the rectifier what convert the AC to DC so truly it's just a multi-purpose bulb.
When you buy the LED replacement lamps to replace the incandescent indicator bulbs, they will often come fitted with a built-in bridge rectifier. This is so that the LED bulbs will be compatible with AC as well as DC sources. Without the bridge, the LED would only light up on 1/2 of the cycle and would flicker. In addition, during the other half cycle, the LED will be in reverse bias, which could possibly damage the LED, depending on the circuit. Some of these replacement lamps will also include a built-in dropping resistor to limit the current to the LED, as quite often the original filament lamps will have a much higher operating voltage and current. The replacement I used had the bridge but no the resistor, so I had to add one. ;)
@@xraytonyb yes sir
What voltage are those lights? They look almost like the old dome light bulbs used in cars .
can you use a strong magnet to pull the tin away from the needle and then when the magnet is pulled away it can drop below eyesight?
WD40: Water Displacement 40: it basically just mineral spirits (no lubricants in it).
I Think you better put some cold white LED's in it, now its a bit yellowish green.
Why call it an S curve, its more a Z curve to me ?
My nappies are dry. No worries with the LED lights....
you don't think the LED lights interfere causing noise, should you not use a small capacity to filter it out?
The LED will pose no interference to the circuit, but if it didn't have a built-in bridge rectifier, it could cause the LED to flicker when used in circuits that employ AC signals.
@@xraytonyb but bridge rectifiers cause noise! what do they have a separate diode with the led bulbs?
😃👍
I had a Rotel RX454... 4 channel.. I see this is just 2 channel ...