Yamaha CR-400 Power amp noise circuit debug help needed

rocknroller

Active Member
Picked up a CR400 on CL. Has random noise on both channels, different levels and at different times. Sounds like heavy paper rustling with loud pops. Can be perfectly clean then start minor and get really bad, then clean again. Can go for minutes in any given stage,on either channel. I do measure some peaking DC voltage on the speaker terminal when it's really bad (meter might shows 1.5 or even 2-3 volts for a fraction of a second, probably when it's making the loud pops). I believe I have ruled out every but the power amp board itself. When it occurs, it is irrespective of any front panel selections and the preamp volume can be completely off and it will still happen. With audio tester, I confirm the input from the preamp to the amp is clean and unaffected. The problem is occurring in a later stage of the amp. At least as earlier as the collector of TR609 and that path (see schematic). Definitely heavily audible along there (when it is occurring). Thinking it might be a cap and given several measured out of tolerance (uf and in some cases esr too) I have changed out all electrolytics that were 10uf and higher, except the 3300uf power caps - they do measure way out of spec about 5500uf, but esr meter shows good - also did a temp replace with a new 4700uf on one of them (closest I had) and no change to the issue on that channel at all. The amp is also passing the preamp signal all the way (when preamp turned up of course) and the signal from the preamp sounds good, but there is this underlying random noise that is clearly occurring and at times dangerous to the speaker when it gets loud and really starts popping.

Thoughts on where to measure/check in the circuit?
 

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TR609, TR610 2SC458 have a strong history of going noisy to the extent that it's a "shoot on sight" order.
KSC1815 or KSC1845 are suitable replacements.
 
Thanks - Will find some of those to replace.
Would they account for the momentary DC spikes at the speaker terminals though?
 
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Would they account for the momentary DC spikes at the speaker terminals though?
It's good that you question rather than just accept.

My answer is dunno and probably. Thinking about what an amp does, a DC spike at the outputs could be caused by a "blip" earlier on in the amp process.
TR609, TR610 is for bias however any "blip" on emitter/collector would feed through to the outputs.

From the symptoms you describe, 2SC458 is a 50/50. Nice to remove this as a possibility before looking at the others such as dry solder joint(google) and
poor contact.
 
It's good that you question rather than just accept.

My answer is dunno and probably. Thinking about what an amp does, a DC spike at the outputs could be caused by a "blip" earlier on in the amp process.
TR609, TR610 is for bias however any "blip" on emitter/collector would feed through to the outputs.

From the symptoms you describe, 2SC458 is a 50/50. Nice to remove this as a possibility before looking at the others such as dry solder joint(google) and
poor contact.
 
Intermittent popping in both channels. Is it in both channels at the same time ?
 
Sometimes. It could be one channel perfectly good, one channel light noise/light popping, or one channel light noise/popping and the other heavy noise/popping, or one channel good the other heavy noise/popping. flipped for opposite channels as well for all those cases. Really isn't a distinct pattern. It just all comes and goes though it's 80% more noise time then not and the right channel tends to be worse then the left. Measured the power supply caps and is a steady 25.X volts (+/-) regardless of the condition.
 
Using the pre-amp out on the rear panel. With a different amplifier does it still intermittently pop ?
 
Using the pre-amp out on the rear panel. With a different amplifier does it still intermittently pop ?
Sorry for the response delay. It has been a busy couple of days and I had tabled the entire unit with the plan of waiting on the parts to arrive. However I was about to say that I had checked the preamp inputs (I thought) but not from the back but where they came into the power amp board so it would be the same, except tonight I went down and hooked those external jacks up to a receiver and low and behold - noise! So, either I was touching down someplace else in the amp section then I thought (not sure where that could have been now), or my audio tester didn't reveal the noise to me and it was there all along at the preamp output stage. As before - Noise unaffected by any front panel controls including volume. Volume can be at zero and noise is still present with no change in (noise) levels if any control is moved or adjusted etc. Wider view of the schematic including the preamp section attached. I can see a few swollen caps on the preamp board but I have to pull the board out to get to anything for any test or replace and at the moment I've got a rusty stripped screw holding the faceplate on (ARRGH) and too late (2:30am) to get the drill out to remove it or the wife would have my head!! So tomorrow....
 

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Have you tried cleaning the speaker selector switch ?
Yes had done that and just did again. no change and the the source music is unaffected s o if there was intermittent contact it was affect the source music as well. Also happens in headphone with no speakers selected at all. Based on my previous post, now looking like the problem is earlier in the preamp stage anyway and maybe not actually the power amp after all....
 
Ok - Find got the preamp board out. Some caps out of tolerance but nothing showing really bad esr. Started replacing the worse ones with no real improvement so ended up doing a wholesale change out of all electrolytics on the board. Problem remains though having some time to really follow what is happening and when here are a couple more data points.
1) it generally seems to be at it's worst when powered on from sitting a while. Settles down after 30 seconds or so but never really go away. Then random moment where it spikes up again but more briefly.
2) This all occurs with no front panel controls selected (e.g. zero bass, zero treble, zero tone, Aux with nothing connected etc. ) No movement or change of any controls affects the noise at, it only adds source audio if something like the FM or AM is selected). Noise is unchanged otherwise.
3) There is noise both out of the preamp outputs , as well as the amplifier speaker/headphone outputs, but they are not the same levels. I'm running the preamp outputs into an external receiver that is twice the power (maybe 3 times) FAR louder with any source audio is present, yet the actual NOISE level is far lower then what I hear out the Yamaha amp section/speaker/headphones

Because of #3, and because I think technically shouldn't be sending ANY audio (or noise) to the amp from the preamp when the volume is set to zero, I'm actually wondering if what I am hearing at the preamp outs is really some feedback from the amp section itself?? Meaning, the noise really is originating at the amp and making it's way back to the preamp outputs, but at a far decreased volume.

My next step is to replace those TR609/610 transistors as they came in today and see if they are the culprit.
 
Replaced 609/610 no change. I disconnected one side of the preamp and found the noise was still present on the amp side, so that confirms the problem is on the amp board. I sprayed some contact cleaner in the general area of 601/605/607/VR1 on the TOP (component side) and the noise went NUTS!! Thinking maybe a bad solder contact on the bottom I touched them up but didn't resolve anything. Noise is pretty constant now, heavy on the right side. The transistor legs of 601 and others are pretty corroded but I would think intermittent contact would interfere with my source audio as well not just create the noise? Maybe I need to pull these and test each one for leakage. Tried some adjustment on VR1, think maybe dirty pots etc but no affect there either.
 
Sometimes I'll try to localize bad transistors with a Q-Tip and alcohol. Wet it, but not dripping, then rest it on top of each transistor and see if the noise is affected. The evaporating alcohol cools the Q-Tip and DUT. I'll also use a soldering iron to heat them. It wouldn't be crazy to have bad devices in both channels. You can use the same trick on resistors, just remember that a drip of alcohol can short things out, so moist only. Also, use some DeoxIT on any trimpots, or Faderlube, if you have it.
 
Sometimes I'll try to localize bad transistors with a Q-Tip and alcohol.

No luck with that so I went ahead and pulled and tested transistors ont he right channel (since that channel is the worse) Tested TR602,604,606,608, 612, & 614. Basically all on that channel except 610 (replaced earlier today with the new substitutes) and the final output pair. All tested fine, no leaks except 604, which i didn't realize until after the fact isn't a regular transistor so my meter wouldn't be valid to test that. Not sure how to test that one. When I set VR2 for 1.68 on the base, I get 2.1 and 25.2 on the other pins. beyond that, check all resistors on that channel - all in spec. Checked C604/C622 which look like possibly tants, they seem fine for value and esr. All electrolytics has already been changed out. (side note - I will say that with both the preamp board and power amp board electroytics changed out, the sound, noise/popping not withstanding, sounds darn good!)

So all this and still back where I started with no explanation of where this noise is coming from, though have ruled out where it is NOT coming from (preamp). short of a couple picafarad caps which I don't know how I'd test, and the output transistors, what else is there?
 
If 604/622 are tantalums, don't rule them out as a noise source. Check 'em for DC leakage or just replace them. Check and write down all the DC voltages on the schematic and see if they match any numbers there. Transistors can be noisy and not respond to heat and cold. Have you got a scope? Can you borrow a scope? Can you build a scope? Can you steal a scope? Have you got a sound card that, if it got destroyed, wouldn't be the end of the world? There are some scope programs that can work with that. You'd need to make a probe that would guarantee no high voltages got to the sound card- limit resistors and zeners or diodes. If you could just look at the signals at the input and output of each transistor, it might be a lot easier to find this.
 
If 604/622 are tantalums, don't rule them out as a noise source. Check 'em for DC leakage or just replace them. Check and write down all the DC voltages on the schematic and see if they match any numbers there. Transistors can be noisy and not respond to heat and cold. Have you got a scope? Can you borrow a scope? Can you build a scope? Can you steal a scope? Have you got a sound card that, if it got destroyed, wouldn't be the end of the world? There are some scope programs that can work with that. You'd need to make a probe that would guarantee no high voltages got to the sound card- limit resistors and zeners or diodes. If you could just look at the signals at the input and output of each transistor, it might be a lot easier to find this.

Didn't have any tants to replace with so used lytics temporarily just to see. No change. In terms of voltages, see attached. Portion of schematic shown is for the left channel (since they mark the expected voltages there) but I am measuring right channel values and noting major differences in Red. In the large red box - any voltage I would note specifically would be incorrect and change a second later. The voltages are all over the place there and peak up to 10-12v when the noise is blipping hard. They drop respectively when the noise quiets down. But there is obviously something wrong there, for example the collector of 609 where it should be negative (-1.25v) yet seeing random high positive voltages. This is the same location where I noted in earlier posts that I can audibly hear the noise easily with a handheld audio tester, which I have been using in lieu of an oscilloscope (with and appropriate DC high voltage protected probe)

Does this suggest that either 615/617 are leaking back into the circuit? Either through their base or emitter? They are the only two I haven't pulled yet (difficult to get to - fairly major disassembly required)

As for Scope - I did pick a T922 15 Mhz scope at an auction a couple weeks ago. Just turned it on tonight and it appears to work but I'll be frank that I'm quite ignorant how to use it, or if it even have an appropriate probe to be using (probe is simply a coax cable with two alligator clips - no apparent protection of any kind). Haven't touched on in 4 decades, though with some control fiddling, was able to pull up the preamp signal as it came into the amp board and see music playing.
 
Have to think about this. As for the scope, any coax with some clips (suggest mini-grabbers for short circuit safety) will do fine. For low voltage low impedance stuff there's no need for a 10X probe or anything special. In the earlier stages I would have suggested just following the signal an seeing where it gets noisy. The problem with a power amp is the feedback will make most problems appear everywhere you look, so it's hard to pin them down. I'm a bit suspicious of the front end diff pairs, or maybe the Vas transistors (607). Have you cleaned the trimpots?
 
The pots, though somewhat sealed (see pic) have been liberally sprayed (multiple times) and worked thru. They seem to react to voltage changes smoothly as turned and have no audible impact to the noise (positive or negative) when turning at any part of the range. The 3 transistors you mentioned have already been pulled and tested and show no signs of shorts or leaking. I don't know how to test 604 or if that would have any bearing on the issue. And again I mention the final output pair 615/617 which are also untested. But before I go to (major) effort of pulling them out would like opinions if it even makes sense that they could be the culprit, based on the high voltages seen there.
 

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