Problem was getting worse over time.
When I opened it I discovered that someone had already been there and had replaced the Channel A fader. Maybe they suspected that this was causing the noise.
Took out the six channel board 81334 and put it on the bench. Connected +12 and -12 and to my annoyance it worked perfectly. Connected back to PM power supply (+15/-15) and it went into oscillation. At this stage I knew some component had changed in value and was voltage sensitive. Traced the source back to output of I1AA. It was in oscillation and swinging from rail to rail. Removed this IC and replaced it with a socket. Before I inserted a replacement IC, I checked the DC voltages to compare them with channel B. These two channels share the same IC1. They looked the same. On a test signal channel B looked OK but channel A looked noisier on input to the IC. Replaced IC with a new one and it went into oscillation.
At this stage it looked like a problem with the four remaining active components or associated passive components.
I removed the two 2SA1084 (diagram had said 2SB737) as I suspected these were the most vulnerable to outside interference but both tested fine. Also checked associated passive components and no sign of any issues. I then removed the two BC550C transistors. One tested OK but the other, Q2A, indicated an hfe of ‘8’ which was very different to Q4A which had an hfe of ‘425’.
I had no BC550C to hand so I used two BC546C and the channel worked perfectly. I ordered up a couple of low noise BC550C (BC549C would have been just as good). And returned the PowerMate 1000 back to its original state. Job done. Maybe someone can learn from this. Schematics are readily available.
When I opened it I discovered that someone had already been there and had replaced the Channel A fader. Maybe they suspected that this was causing the noise.
Took out the six channel board 81334 and put it on the bench. Connected +12 and -12 and to my annoyance it worked perfectly. Connected back to PM power supply (+15/-15) and it went into oscillation. At this stage I knew some component had changed in value and was voltage sensitive. Traced the source back to output of I1AA. It was in oscillation and swinging from rail to rail. Removed this IC and replaced it with a socket. Before I inserted a replacement IC, I checked the DC voltages to compare them with channel B. These two channels share the same IC1. They looked the same. On a test signal channel B looked OK but channel A looked noisier on input to the IC. Replaced IC with a new one and it went into oscillation.
At this stage it looked like a problem with the four remaining active components or associated passive components.
I removed the two 2SA1084 (diagram had said 2SB737) as I suspected these were the most vulnerable to outside interference but both tested fine. Also checked associated passive components and no sign of any issues. I then removed the two BC550C transistors. One tested OK but the other, Q2A, indicated an hfe of ‘8’ which was very different to Q4A which had an hfe of ‘425’.
I had no BC550C to hand so I used two BC546C and the channel worked perfectly. I ordered up a couple of low noise BC550C (BC549C would have been just as good). And returned the PowerMate 1000 back to its original state. Job done. Maybe someone can learn from this. Schematics are readily available.
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