Yamaha CR-420 Receiver DC Voltage at Speaker Output
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- Опубліковано 19 лис 2016
- Unit had about -10 volts DC on one of the speaker channels. This affected almost all voltages in the power output stage. Almost all transistors had wrong voltages; channel still worked though, but not correctly. Was initially difficult to troubleshoot because many transistors are tied together; what affects one then can affect many others. I then resorted to measuring the resistances to ground of the power output stage components in the bad channel, using the good channel as a comparison. A high resistance was found on the very last resistor I checked. Problem turned out to be soldering joint problem that I would have found right away if I would have taken a closer look at the underside (solder side) of the circuit board. Bias adjust was then done. Also, some electrolytic capacitors seem to have deteriorated; with lower values now then what they were rated for. It leads me to suspect that probably most if not all of the electrolytic capacitors need to be changed. Probably the same problem with all CR-420s.
- Наука та технологія
Got to love this overbuild vintage stuff, drive shafts in there are awesome to actuate the switches...
Especially fascinating are the cassette mechanisms. Thanks for commenting.
OMG! My family's receiver. 1978 (Best Christmas ever)
Hey there old-school....I was wondering if you could do a full restoration video of the following receivers: Pioneer SX-980 and the SX-636.
Yeah - Yamaha had some serious problems with the Panasonic (Matsushita) capacitors on these. Probably a good idea to re-cap for the long-term. A judgement call on a lower end unit though...
Thanks! For sure this is a judgment call as far as recapping is concerned....
i don't get isn't the .0.30 is considered high?i have about 0.39 out of my speaker tab on my mono block amp and they tell me lower is better? are they wrong?
Ideally the dc voltage at the speaker terminals should be zero, but this is never the case. I think it was about 55 millivolts DC (.055 v) after the repair. If you have a 8 ohm load (even though speakers react differently at different frequencies) you would take E squared over R giving .000378 watts. The current would be about .007 amperes. I don't think this would damage the speakers. The bias was adjusted to about 30 millivolts, as was stated in the service manual.
Do you check dc offset with the power on or off? Anyone know?
Thank you
Power on.
I have the same problem with a Pioneer sx 636, reads 16 dc volts from r channel. however after being on 30 minutes will return to normal and work normally. I have not been able to find the fault. after seeing this video I will try again. Any suggestions? Beautiful sound from this unit and I bought it new in 1973.
I have not worked on the SX-636 before; so I am not familiar with the unit. But it sounds like a thermal problem; so you might have to use heat (hairdryer) right after turning the unit on to find the problematic component (in the power amp section) and see if the problem goes away. Alternately, if the unit is working fine (after it acted up) you can try freeze spray. You could also swap the output transistors of one channel to the other channel since the channels are mirror images of each other. You just have to make sure you don't get things mixed up and make sure you put them right place. Take notes and pics, if need be. You would have to get new thermal compound. If the power board is a little extra board by itself you could also redo the solder joints. Anyway, it will take a little time. Other than that, check the audio forums on the net; maybe somebody has had the same problem. I don't use forums, but I know they are there.
Cool video, you really know your stuff! I recently tried to test a receiver someone gifted me many years ago, it's a Pioneer possibly from the late 70's. When I got my speakers hooked up the speakers had a constant very loud hum to them and fairly quickly before I realized there was a major problem it seems to have fried them. I put my meter on each one and the ohms read 1, way low for an 8 Ohm speaker so I realized they got roasted. When I put my meter in the speaker sockets, when that channel isn't activated/selected I see about 50mv DC. When I select them I see around 35VDC so I get the feeling like somehow a signal/voltage is getting blasted to the speaker channels for no apparent reason, even with nothing hooked up. I don't want to give up on this unit but I have no idea how to diagnose it from here. Is there an internal short somewhere possibly sending stray voltage to the speaker output circuit? Anyway thanks just cuirous if you had ever seen a similar problem. Cheers.
Dr. Bizz: You should really check the output transistors. You check them in-circuit, or take them out and check them if you really want to be sure; power off, of course. You might have collector to emitter shorts; meaning that the voltage from the power supply is going directly to the speakers. Which could destroy the speaker voice coils; this seems to have happened. The output transistors are separate and bolted to heat sinks or larger metal parts of the chassis. There are probably 2 for each channel. I don't know what your skill level is, so this could be a medium difficult to impossible fix. You should also check the emitter resistors of the output transistors; these are really low ohms. And check all other smaller transistors around the output transistors. For sure get the schematic/service manual. I suspect the amp section of your receiver is direct-coupled; this causes a domino effect when something goes bad; one destroyed part destroys another part and so on.
@@OldSchoolElectronics Oh wow, thank you for the response and info so much! I'm probably an intermediate skill level or below. I'm an electrican by trade but I know electronics basics so it would probably be a challenge but I know a fair amount. I got a turntable for free that works and happens to be very similar to this reciever so I'm hoping to have a matching "old school" setup for playing my records. Thanks again!
Cool! You know the electronics basics, which is a big plus. Just take your time, take pictures, and use that schematic. I think you can do it. Good luck with the repair! It's always cool to have an old school setup.
@@OldSchoolElectronics Thanks for the inspiration, peace man. 👍
Thanks for this!
Thanks for watching!
Where did you inject your sine wave signal? Thanks
The sine wave would normally be injected into the aux input.
Hi, my Yamaha CR820 is measuring approx 70mV on the all the speaker outputs and straight away I took it out of my setup.. I read that DC offset readings needs to be near the 0v. I would be grateful if you could let me know what I need to do to correct this issue. Many thanks.
I have not yet had this model. But your voltages are slightly too high--not dangerously so. If you used an 8-ohm load with 70 mV, although a speaker reacts a little differently, you would get around .009 A current. Make sure your unit has warmed up enough before you do the measurements. You have some kind of slight imbalance there; the specifications of some of the parts seem to have changed. You might want start by changing electrolytic capacitors in the output stage--might be some small leakage. This might not bring success. It could even be mismatched resistors and transistors. The zero volts is an ideal. If it was my unit, I would probably leave it alone, as long as it sounds good and check it occasionally. Especially since you could make things worse. If I ever run across a model like this; I will make a video.
@@OldSchoolElectronics Thanks very much for the advice. I read that over 100mV would damage the speakers, mines show 70mV, so i wasn't taking any chances. it sounds good, but i just didn't want to risk it. If you suggest it will be ok to use and check regularly, i may just do that, as i like this receiver very much. So if it get over 100mV, shall i stop using it and take it in for repairs? Thanks again.
Hi! Yes, if it reached 100 mV, I would take it in. Or start looking now for a competent technician (if you can find one), so you can take it to him/her when it becomes time. They should be familiar with vintage audio repair. Good luck!
@@OldSchoolElectronics Thanks very much for the advice, really appreciate it.
cool video thank you.
Thank you, sir!
I have a repair amplifier it's SAKURA 735 model 1300 watts for 4 ohms configuration.
the problem is in Left channel output have a 54.8 volts but all output transistor C5200 and A1939 is good the power supply is 64 + and 64 - is ok. I check all transistor, resistor and capacitor on driver circuit is ok. the causes that problem some leakead resistance between base, collector and emetter on transistor hole or board or amplifier board.
Hi, Is it worth owning this receiver? A recycle center near my work has one for sale for $65. not sure about the working condition but the cosmetics looks neat! Thanks for the video,really appreciate the work you do.
+Vijay Merceline Hello: I think for this price you have to make sure the outside is good. No chipped wood, big scratches, etc. Bad treatment outside can also mean bad treatment inside. Most likely some bulbs are out, but that is a small matter. For that price you should ask to plug it in and see what happens. For example, does it turn on? Or ask them if it works or what is wrong with it. It is a nice receiver with good looks and good sound. You can look up the specifications online.
I'll go and check it out today! Thanks for the reply.
I went to that store to see that amp but its seems its not that in a good shape lots of scratches and looks like it's been treated really badly over the years so I didn't purchase it! but I found a sleek silver faced Yamaha natural sound A-15 for less than $15 and it's seems its biasing is out and i have to recap it but It's phono stage is really warm! Thanks again for your input.
+Vijay Merceline You are welcomr and good luck! Good thing you didn't buy the scratched up unit. I never worked on an A-15 before. I want to get a A-960--with no big scratches.
Old Yamaha are worth restoring I think.
+iris Yes, as long as it still looks good on the outside, in my opinion.
Old School yes you are right.
Why in the world you connect a load at the speaker output? When you know there is 10V DC there!! The output transistors will get HOT!!
Better replace all those purple panasonic caps, they will fail soon, if not they already do.
Thanks for your comments! I would have replaced all the electrolytics if I would have kept the unit as part of my personal collection. I would have had to sell the unit for a much higher price (labor, replacement costs); the problem is, people don't want to pay. Buyers often want a unit in "perfect condition" for a few dollars.
Yamaha not a fan of their reliability and build reputation as noted in video. As was the case when I bought a R1000 (their flagship) and had a life of about 24 months.
I had the CR-2020, which I think was about 100 watts per channel; it did last a few years. But I have seen lots of defective Yamahas for sale...which seemed to me disproportionate compared to other brands. But maybe they also sold more than the other brands....