PC2002M - Need help

miklemke

AK Subscriber
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I acquired a non-working PC2002M. Replaced caps and a bad diode on the Channel A DA amp board, removed all of the acidic glue. Channel B works fine, the power supply is fine, but channel A will not come out of protection.

The voltage reading on the DA board, with the positive lead on "CT" and negative lead on "E" is -77.1v. It should be less than 10mV. Where should I start looking?

Thanks
 
I acquired a non-working PC2002M. Replaced caps and a bad diode on the Channel A DA amp board, removed all of the acidic glue. Channel B works fine, the power supply is fine, but channel A will not come out of protection.

The voltage reading on the DA board, with the positive lead on "CT" and negative lead on "E" is -77.1v. It should be less than 10mV. Where should I start looking?

Thanks

OK , let start by the beginning :

If you have - voltage on the output it is coming from defective power transistors (those connected to the negative rail voltage ) ! the C to E connection is 100% in short circuit and power supply voltage goes to the speaker relay ! it is a good thing to have such DC protection to preserve your speakers !
I don't have the service manual so I can't tell you components references but if you have it you can find the faulty transistors very easely

Come back to us with a maximum of details and we will do our best to help you (without to have the defective unit in front of us ! )
 
Here are some more details:

If I turn on the amp with both channels connected to the power supply, both channels stay in protection mode. If I disconnect channel A, channel B comes out of protection and works fine.

If I start the amp with only channel B connected to the power supply, channel B comes out of protection within 6 secs of power on and works fine. If the amp is on and working with channel B and I connect channel A when the power is already on, about half the time, channel A will come out of protection and the voltage across "CT" and "E" is an acceptable 6mV. Started in this manner, Channel A may be fine for 20 min, but then it goes into protection with the high -77 V across "CT" and "E". The other half of the time, channel A has -77 V across "CT" and "E" and does not come out of protection.

When I did the recap, I did check the power transistors with the "diode" test and they were OK. I did clean-up all of the connections where the acidic glue came into contact with components. I also inspected the board for any questionable solder joints and reflowed the ones that looked questionable.
 
Did some more troubleshooting:

If I disconnect Channel B but leave Channel A, Channel A is OK on power-on. It's only when both channels are connected that there is a problem.

As a result, I went back over the power supply and found a few suspect solder joints and one film cap that was significantly out of spec. Took care of those issues and now on power-up with both channels connected, it came out of protection within 6 secs.

After 20 mins, I dialed in the idle current. DC offset 6mV Channel A, 5mV Channel B. Looks like problem solved.:banana::banana:

Ill let it run for a few hours and re-check. Then time for speakers and music.
 
Had it running all day. The only problem remaining is that Channel A is weaker than Channel B. If I input a mono signal, I have to set the Channel A attenuator on 2 to equal the volume of Channel B set at 8. What could cause that difference? I did switch the A and B inputs with the same results.
 
Had it running all day. The only problem remaining is that Channel A is weaker than Channel B. If I input a mono signal, I have to set the Channel A attenuator on 2 to equal the volume of Channel B set at 8. What could cause that difference? I did switch the A and B inputs with the same results.

Dirty attenuator - fouled protect relay contacts- cold solder connection-dirty speaker connection - dirty plug-in connections on the weak channel.To name a few.
 
What would be a replacement for the relay.

I'll check the other items mentioned.

Thanks
 
Another issue:

While waiting for new relays, the unit was off for about ten days. On power-up, channel A stayed in protection. I took the top off and connected my volt meter to CT and E on the DA board. Powered up the unit and had a reading of about 50V and declining. The voltage continued to drop over about a 4 minute time span to an acceptable level and then came out of protection. It seemed like a lag while a capacitor charged.

I've being using it everyday since and it powers-up fine with both channels coming out of protection in about 5 seconds. I suspect if I leave it sit for a number of days, I would have the same issue.

Any ideas what might be causing this?
 
Another issue:

While waiting for new relays, the unit was off for about ten days. On power-up, channel A stayed in protection. I took the top off and connected my volt meter to CT and E on the DA board. Powered up the unit and had a reading of about 50V and declining. The voltage continued to drop over about a 4 minute time span to an acceptable level and then came out of protection. It seemed like a lag while a capacitor charged.

I've being using it everyday since and it powers-up fine with both channels coming out of protection in about 5 seconds. I suspect if I leave it sit for a number of days, I would have the same issue.

Any ideas what might be causing this?

On channel A, the next time its off for a while. Monitor the B+ and B-,on the DA board, to see if there is and DC imbalance. The DC servo opamp may be flaking out.
 
Well I waited eight days. I turned the unit on with my volt meters attached to read B+ = 80.0v ; B- = 81.4v rock steady.

Channel A came out of protection in about 15 seconds. The DC offset was erratic and ranged between +146mV to -46mV. The swings became smaller and after about 5 minutes ranged between -3mV and -2mV.

Channel B came out of protection in about 4 seconds and DC offset was steady at 4mV.

Where should I look next???

Thanks.
 
I thought that's what I did by checking the B+ and B-.

No. A good steady B+ and B- points to the DC servo as a potential issue. If you had and imbalance on the B+/- then it would point toward a different cause.
 
OK. How do I check the DC Servo?

Oops.Hold it.. Regroup.
Don't know how I over looked your statement.:scratch2:
Well I waited eight days. I turned the unit on with my volt meters attached to read B+ = 80.0v ; B- = 81.4v rock steady.

:scratch2: You have a 1400mv imbalance in your B+/- rails. Thats a potential 1400 mv of DC offset. I think your servo is working its butt off.:D


While waiting for new relays, the unit was off for about ten days. On power-up, channel A stayed in protection. I took the top off and connected my volt meter to CT and E on the DA board. Powered up the unit and had a reading of about 50V and declining. The voltage continued to drop over about a 4 minute time span to an acceptable level and then came out of protection. It seemed like a lag while a capacitor charged.

:scratch2: Stand-by..
 
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I rechecked the B+ and B-, this time with the same meter, both were at +-80v. The difference was probably due to two different volt meters.
 
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