Diagnosing a fault in my Accuphase C200s

slimecity

Super Member
I have a problem all of a sudden with my much-loved C200s - audible distortion in both channels. I would describe it as "subtle" rather than "gross" distortion - ie: like the signal is perhaps 1/3rd, rather than completely distorted. Some questions -

I have a SM for the C200 - how similar is the C200s to the C200? If its not very similar then Im in trouble I suspect. I also suspect that getting hold of a SM for the C200s could be very wishful thinking.....

I am currently working to try to diagnose the issue by placing each board in one by one and testing voltages as per the schematics that I have - will this method work, or will it not give me accurate voltage readings? So far I have inserted PCB's 1 and 2 (the powersupply and headphone amp) and voltages have been to-spec on those PCBs, however on inserting the 3rd PCB (one of the filter output amp assemblies) I am getting crazy voltage readings on sections of that PCB.

Do I alternatively re-insert all of the PCBs and try to get voltages in the very small space that I have to work on there?

What is the best way to try to diagnose in this case? By signal probing? By using freeze spray, or keep going the way that I am going?

Are there FET's in this preamp? I see some 2SK97's in the phono boards. This is also a bit of a worry....

This will be a massive challenge I know, but Im willing to be patient.

Thanks
 
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I would be more inclined to have all boards loaded then check voltages. Each board/module will have it's own current draw.
This extra current will affect voltages but not in the psu.

Also this sounds like a job for a scope and signal generator, eg 1kHz input. PC based SW is available that do both, real items
are so much better.

Basics, all input sources affected? Same through headphones?
 
Cheers mbz - don't have a scope but Im inclining towards probing with a test tone.

Basics are that the line out has the distortion - the unit has a separate headphone amp board so I need to diagnose that too (however that board is the final chain in PCB's, so strongly suspect the problem will be audible thru headphones - but need to check).

Given the cascade of PCBs, suspect the issue will be audible everywhere, but I will still need to test all inputs. Here is a flow diagram. Capture.JPG
 
If you don't have a scope and are willing to try the SW (I've got 2 dead scopes so use the SW for the time being) here is the link
for the download.
https://www.zeitnitz.eu/scope_en
There should be a warning that you should not measure greater than 5V else you may damage your sound card. If you want
to try the SW then I'll track down a post that shows how to build a probe (real easy). If you have a real CRO then even better.
 
Yes, agree headphones will be affected as it just takes the L&R Line out and puts them through the headphones.
 
If you have distortion in both channels, it's most likely power supply related.

You have two gain stages in a line in situation according to the block diagram. The first is where I would be starting. Post a schematic or email me the S/M and I'll have a good look at it for you.

Bypass all the functions, tone, filters etc, feed in a line level signal and confirm the distortion is still there. If it is, and in both channels, I'd lay money on it being a regulated rail being the problem.
 
Hey John thanks for that. I also initially thought powersupply however on the 3 boards I have tested so far, all the rail voltages are fine.

I will PM you the manual for the C200 - note one of my questions - at this point Im still not certain that the manual I have is identical to my unit (the C200 S)- so far so good with the boards I have looked at however.

Sorry tried posting the schematic but I cant get the rez big enough.
 
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Start by cleaning all the edge connectors on the boards, with particular attention to the power supply board.

I would then be carefully checking all my rails- the +/-94v after the rectifier and the +/-77v at the emitters of Q1 and Q2.

The +/-64v points run the Phono Stage, whereas the +/-30v runs the gain stage. Check the +/-30v is present and not unbalanced. The voltage readings in the power supply are very well marked on the schematic, so with (insulated!) clip on probes you should be able to confirm if there's a PSU problem.

c200 psu.JPG


IMO, this is the likely place to look, there's heat, heatsinked (off board?) regulator transistors, 40 year old zeners and some caps that may have suffered heat damage and dried out. Also, as the problem is common to both channels, the Power supply is suspect.
 
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I think you are onto it John. Had a bit of a breakthru tonite. I got home from work, reinstalled all the boards, turned it on (the preamp hasnt run for a couple of days and its winter here, so everything was quite cold).

The pre sounded great - I was wondering if maybe I was imagining the distortion - checked thru the headphones - all good. Connected via the RCAs to my poweramp - all good thru all inputs. Then all of a sudden - FUZZZZZZ, a lot of distortion once the pre reached OT and I also lost about 8 or so dB and had to turn the volume control up to get the same level of fairly shit-sounding music.

The problem is happening in both channels so yea I reckon its isolated to the powersupply board 719-0001-00. However I also took some basic readings from the two TO-66 trannies mounted on the chassis (Q1 and Q2: 2SA483 & 2SC783). The readings from Collector to ground on both of these is reading at 83V, not the 94V that I should be getting.

I double-checked all the transformer secondaries and they are all OK with the exception of where I should be getting 70VAC on the red secondaries, Im getting about 3VAC less than that (probably not a major).

I'm guessing that the above means to confine a search to somewhere around Q1-Q4 on the PCB? I have removed and tested the chassis Q1 and Q2 already and they tested fine (of course however, this issue is happening once things heat up).

Is it worth investing in some freeze spray about now, that I've established chilling the problem component would probably identify it?
 
Take the PSU board out and fully inspect all the joints under magnification, especially around the heat producing parts. Redo any suspect joints by completely removing the old solder, cleaning and re-soldering.

You'll need to put your meter/s on the regulated +/-30v rails and monitor them from cold to OT (can your meter log or beep on updated minimums?). My guess is you are losing a rail or it's dropping with respect to its opposite rail once the unit gets warm.

I wouldn't worry about the minor AC voltage differences- that's not the problem, but the collector to ground voltages are a bit strange. If you are getting 67v (70v minus the 3v approx you mentioned), after full wave rectification and diode Vf loss it should be about 93v not 83v. It's remotely possible you could be losing a diode from the bridge when warm and subsequently getting a half wave rectified rail that droops under load and causes the PSU imbalance, but I think that's unlikely. You say both rails are 10v low?

I don't think it's going to be a difficult fix at all. It's a lovely preamp, but if it's original, those caps and everything else are 41 years old...
 
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Actually John this is a bit embarrassing - Im about to head away for the weekend but after pulling my accuphase apart and testing everything several times I had a closer listen to my replacement pre-amp in my system (pre-amp section in my sansui au-999) and the distortion was still there!

I narrowed it down to my Topping D30 DAC. I rebooted it and the issues went away. I could have sworn that my accuphase was still distorting when I tried other sources too.....but I really need to give this a couple of dedicated hours to confirm.

I'd honestly rather that the DAC was faulty than my accuphase.....but if its the former then I'd suspect the cheap nasty powersupply.

Will advise.......thanks for your help!
 
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