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PC2002M - one channel dropping out at low volumes

DWJames

DWJames

Active Member
Hi all,
my PC2002M power amp's been running really nice for a while, but has developed an annoying fault fairly recently I wonder if anyone can help with?

On one of the channels, if I've been listening quietly, that channel drops out almost entirely only to spring back into life if I turn the volume up sufficiently to kick it back into life. If I keep the volumes pretty loud, it seems to be fine.
When it comes back to life, sometimes it sounds over-driven with a slight edge of distortion and something like 15bd louder than it should be, but that tends to settle down fairly quickly if I give it a bit more volume.
The attenuators on the front seem to work as expected and moving them whilst an issue is present doesn't seem to fix anything, so I'm wondering if it's something with the board for that channel.
I saw a post with similar symptoms which pointed to the bias circuit, does that sound likely?
thanks,
James
 
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When this has happened to me on other amps (HK, McIntosh and Carver) it was always the protection relays. They either needed to have the contacts cleaned or be replaced entirely. I will watch this thread with interest as I have a much loved and aging PC2002M that in 10 years has not given me an ounce of trouble.

jblnut
 
DWJames said:
Hi all,
my PC2002M power amp's been running really nice for a while, but has developed an annoying fault fairly recently I wonder if anyone can help with?

On one of the channels, if I've been listening quietly, that channel drops out almost entirely only to spring back into life if I turn the volume up sufficiently to kick it back into life. If I keep the volumes pretty loud, it seems to be fine.
When it comes back to life, sometimes it sounds over-driven with a slight edge of distortion and something like 15bd louder than it should be, but that tends to settle down fairly quickly if I give it a bit more volume.
The attenuators on the front seem to work as expected and moving them whilst an issue is present doesn't seem to fix anything, so I'm wondering if it's something with the board for that channel.
I saw a post with similar symptoms which pointed to the bias circuit, does that sound likely?
thanks,
James
Click to expand...

Each channel has its own protect relay. Located on the DA/DB sheets of the amplifier assemblies.
 
Last edited:
Brilliant, thanks guys :)
I'll see if I can pop the lid off the relay and give it a good scrub with some deoxit. Failing that, I'll get it swapped out.
Cheers,
James
 
I've just dropped the DB board off and given it a good clean. The back of it was covered in a light coating of rusty gunk.... (this amp had previously been sat in a garage for a while)
I also popped the lid off the relay and gave that a blast and a good old wiggling about. It looked practically new inside, but I guess they get a bit gummy and stiff with age maybe?
It's all plugged back in and fired up again nicely with no smoke or explosions which is always good. Only had a chance for a 1 tune test though.
I'll have a proper extended listen tomorrow whilst I work. Fingers crossed...

As a side note, it's a pleasure to work on something like this where everything's nicely laid out and designed to be serviced. All my audio tweaking of late has been tiny fiddly stuff...
 
Yup, I can confirm that I got a good 7 hours of tunes in whilst I worked, in the exact same way that caused the issues before and after cleaning the protection relay for that channel, the problem seems to have gone. Good shout, thanks :)
 
DWJames said:
Yup, I can confirm that I got a good 7 hours of tunes in whilst I worked, in the exact same way that caused the issues before and after cleaning the protection relay for that channel, the problem seems to have gone. Good shout, thanks :)
Click to expand...

Congrats on your clean up and repair. Someone asked whether DeOxit was worth buying for a lowly Sansui 221? The answer is, a good tool is always worth buying, and in this case, it was exactly what you needed.

Hope the contacts in the relay will work for another good long while. :thmbsp:
 
Gahh, that channel's back to dropping out again. That didn't last very long... oh well. I'll get a replacement relay ordered and swap it in :)
Is there anything else it could be or that's worth me 'refreshing' whilst I'm placing electronics orders, have the board out and my soldering iron at the ready? I've changed all the electrolytics already.
thanks,
James
 
avionic said:
This should work. Will need to snip off the two un-used pins.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtSzCF3XBhmW8KXvo/AizVD94DI8bzZQpM=
Click to expand...
Bit of a delay in getting this sorted, but yes, I can confirm that's a perfect replacement :)

Old original yamaha part on the right, HC2-HP-DC12V-F on the left
IMG_20130812_102703.jpg


Just the end 2 pins needed snipping off the new part as the amp doesn't use the connections for the 'off' position
IMG_20130812_102751.jpg


Here's a closeup of the contacts on the old relay
IMG_20130812_112438.jpg


Would have benefited from a quick sandpaper when I last had it out, but no worries,. I have a nice fresh one now :)
IMG_20130812_112520.jpg


neatly installed
IMG_20130812_103535.jpg


I could just get my soldering iron to the rear contacts, so I soldered it in place from the side without removing the boards again. Nice and easy.
IMG_20130812_103544.jpg


Has it sorted my issue?
well, yes and no.
It seems to have solved the intermittent dropouts, which is great, but I'm still experiencing the B channel boosting in volume 15-20db intermittently. When it does so, it sounds overdriven and when you turn the volume up (using the preamp volume control), there's a crackly, distorted edge to it. Turning the amp off and on again doesn't clear the issue. Turn the volume up enough (along with the horrible crackling) and the amp seems to detect there's an issue and the level jumps back to normal, but it can re-occur again straight away at times.
Any ideas?
 
I'm now wondering if I have a problem with dirty attenuators on the front. I don't ever tend to move them, but having a play, the channel B attenuator seems to make a lot less difference to the volume than the channel A one does.
Time to get busy with the Deoxit....
 
DWJames said:
I'm now wondering if I have a problem with dirty attenuators on the front. I don't ever tend to move them, but having a play, the channel B attenuator seems to make a lot less difference to the volume than the channel A one does.
Time to get busy with the Deoxit....
Click to expand...

:yes: Sure wouldn't hurt.
 
oooh, I've just given them a good old Deoxit blast and several twistings around and I now have a proper range of control from the channel B attenuator to match it's brother and the outputs of both channels according to the meters on the front seems a lot more balanced between A and B. They were always slightly skewed towards a louder B channel.
Hopefully that's sorted it now :)
 
ahh bugger.....
I thought I had it sorted, but I'm back to my Channel B dropping out again today with low volumes :(
Something is only working when it gets enough volume/voltage across it.
Can you guys suggest any likely culprits or things to check/measure whilst this is happening?

thanks again,
James
 
It's a shame the A and B boards are different on these or it'd make troubleshooting a lot easier to swap things from side to side to diagnose. I swapped the 2SC2607 and 2SA1116 transistors from 1 side to the other just cos it's easy to do and it's been ok since, but I don't think anything's fixed and I'm kinda clutching at straws and blindly doing stuff for no obvious reason in the absence of a better plan...
I got a cracking arc off one of them when removing one of the screws, Seemed to be form the screw ot the transistor casing :yikes: I'd only turned the amp off a minute or so prior, so I guess there was still considerable voltage in a capacitor somewhere.... that woke me up a bit!
 
DWJames said:
ahh bugger.....
I thought I had it sorted, but I'm back to my Channel B dropping out again today with low volumes :(
Something is only working when it gets enough volume/voltage across it.
Can you guys suggest any likely culprits or things to check/measure whilst this is happening?

thanks again,
James
Click to expand...

Swap your preamp connections around (right input to left and left to right). See if it still drops out on the same channel.
 
avionic said:
Swap your preamp connections around (right input to left and left to right). See if it still drops out on the same channel.
Click to expand...

Thanks. I tried that fairly early on and tried a different preamp too, but the fault stays with the B channel of the power amp.

I know it won't screw together properly, but do you know if it's possible to swap one or other of the channel boards from one side to the other temporarily just for testing?
 
I'm not sure . I'm at work and away from my service manual. I'm not sure about the plug-in connectors being wired the same.
 
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