Amp is FIXED!!!!! - see Post #54 for the specifics of the repair.
I bought a used PVA-7, knowing that it might have some problems. Powered up on the DBT, 60w, 75w, 150w - no issues. Checked for DC on all the outputs - none were higher than 20mV. So I moved to testing with an 8r dummy load resistors, still on DBT with no issues. Took the amp off the DBT and tested with 500Hz and 1000Hz sine waves into the 8r dummy loads up to about 50% power - again no issues.
Here is where it gets weird - connected the amplifier to some low-grade speakers for initial test. At low volumes on sine wave or music (~<1w) it sounds okay and will play. As soon as you turn up the volume on sine wave or music to 2-3w, the bias jumps up from 15mV to close to 1V and the 10r output (zobel) resistor heats up and the amplifier shuts down, protection circuit kicks in This oscillation/overheating takes about 3-5 seconds to kick-in the protection, depending on how much you turn up the volume.
I tried the same test with only the faulty channels (2 channels on a single heatsink) on DBT and will play fine up to 27Vac (max preamp volume) on the speaker outputs as long as it's playing into an 8r dummy load. As soon as I connect my speakers (~6.7r nominal load) it will cut out at around 75% volume and you can see the current ramp-up in the DBT. As the DBT lights up, so does the 10r resistor on the output.
Final test was to try each channel separately and it appears it is only on one channel, I can hook up the other channel to the speaker and play as loud as I want with no issue. I can connect to the 8r dummy load on faulty channel and still plays up to 75% before protection. But a speaker on the faulty channel or both channels hooked up to speakers and it shuts down quickly.
Amplifier board: I have tested all TO-92, pre-drivers, drivers and outputs to the working channel and everything checks out. Measured every resistor and diode going bad board to good board and they all measure within 1-2%. Output BJTs measure good and no DC on the output - so I'm at a loss to why the amplifier is oscillating.
Also - nothing except a 2N5551 has been changed in the relay circuit. Measured a little low on Hfe, so I changed it out. Everything else, including solder joints all look original - not been touched.
Can this be capacitors? I know typically you would add, change or revise capacitors in the circuit to kill the oscillation, but since this is totally stock, could it be a bad capacitor in the circuit causing the oscillation?
I bought a used PVA-7, knowing that it might have some problems. Powered up on the DBT, 60w, 75w, 150w - no issues. Checked for DC on all the outputs - none were higher than 20mV. So I moved to testing with an 8r dummy load resistors, still on DBT with no issues. Took the amp off the DBT and tested with 500Hz and 1000Hz sine waves into the 8r dummy loads up to about 50% power - again no issues.
Here is where it gets weird - connected the amplifier to some low-grade speakers for initial test. At low volumes on sine wave or music (~<1w) it sounds okay and will play. As soon as you turn up the volume on sine wave or music to 2-3w, the bias jumps up from 15mV to close to 1V and the 10r output (zobel) resistor heats up and the amplifier shuts down, protection circuit kicks in This oscillation/overheating takes about 3-5 seconds to kick-in the protection, depending on how much you turn up the volume.
I tried the same test with only the faulty channels (2 channels on a single heatsink) on DBT and will play fine up to 27Vac (max preamp volume) on the speaker outputs as long as it's playing into an 8r dummy load. As soon as I connect my speakers (~6.7r nominal load) it will cut out at around 75% volume and you can see the current ramp-up in the DBT. As the DBT lights up, so does the 10r resistor on the output.
Final test was to try each channel separately and it appears it is only on one channel, I can hook up the other channel to the speaker and play as loud as I want with no issue. I can connect to the 8r dummy load on faulty channel and still plays up to 75% before protection. But a speaker on the faulty channel or both channels hooked up to speakers and it shuts down quickly.
Amplifier board: I have tested all TO-92, pre-drivers, drivers and outputs to the working channel and everything checks out. Measured every resistor and diode going bad board to good board and they all measure within 1-2%. Output BJTs measure good and no DC on the output - so I'm at a loss to why the amplifier is oscillating.
Also - nothing except a 2N5551 has been changed in the relay circuit. Measured a little low on Hfe, so I changed it out. Everything else, including solder joints all look original - not been touched.
Can this be capacitors? I know typically you would add, change or revise capacitors in the circuit to kill the oscillation, but since this is totally stock, could it be a bad capacitor in the circuit causing the oscillation?
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