i need some help with the PPI ceramic driver cards. i've got an a1200 on the bench that was touched by another "tech" that left parts out. it needed a power supply rebuild and it was missing zeners cr3, cr4 and cr18. got those back in and now i'm having dc offset and bias adjustment problems. I checked the bias transistors, replaced the output channel drivers and i think i have proper voltages at the cards, here they are:
U3: U8:
1. +60.01v 1. +60.01v
3. -0.006v 3. -0.0132v
4. -0.1565 and climbs slightly 4. -0.007v
7. +0.0036v 7. +0.0036v
8. +0.0036v 8. +0.0036v
12. -3.41v and climbs 12. -1.38v
13. -0.43v and climbs 13. +0.99v
15. -62.15v 15. -62.15v
transformer center tap used as ground with no signal applied. bias jumpers not installed and the amp is clamped to the heatsink. I have aftermarket valve audio driver cards and they have the same effect as the original ceramic cards, no offset adjustability. The left channel has around 2 volts dc on the outputs and the right channel around 0.2v. Any ideas folks? thanks in advance!
P.S. the driver cards get holy hell hot after a minute of running too...can't even touch them hot. is that an indication of a problem on the card itself?
U3: U8:
1. +60.01v 1. +60.01v
3. -0.006v 3. -0.0132v
4. -0.1565 and climbs slightly 4. -0.007v
7. +0.0036v 7. +0.0036v
8. +0.0036v 8. +0.0036v
12. -3.41v and climbs 12. -1.38v
13. -0.43v and climbs 13. +0.99v
15. -62.15v 15. -62.15v
transformer center tap used as ground with no signal applied. bias jumpers not installed and the amp is clamped to the heatsink. I have aftermarket valve audio driver cards and they have the same effect as the original ceramic cards, no offset adjustability. The left channel has around 2 volts dc on the outputs and the right channel around 0.2v. Any ideas folks? thanks in advance!
P.S. the driver cards get holy hell hot after a minute of running too...can't even touch them hot. is that an indication of a problem on the card itself?
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i removed the board and cleaned up (removed old solder and flowed new solder) all the overheated solder joints in and around the cards. I still have adjustable offset on the left channel with no bias adjustment but now i have bias adjustment on the right channel with no offset adjustment. left offset is still between 1.5v-2v, right offset is around 0.15v. The jumpers are removed from the bias circuit.
15v regulators are outputting +-14.3vdc measured at the op-amps.
15v regulators are outputting +-14.3vdc measured at the op-amps.
I'm calling the channel with offset control and no bias "Channel A", the channel with bias control and no offset "Channel B".
I installed resistors like you showed on the Channel B driver card, put a jumper on the header and when I powered it up, the current draw spiked at ~8amps and then the resistors smoked.
I installed resistors like you showed on the Channel B driver card, put a jumper on the header and when I powered it up, the current draw spiked at ~8amps and then the resistors smoked.
i found 3 of the 4 muting j-fets bad (Q32, Q34, Q38, Q40). removed them and still no changes but I do have new readins for the tests you asked for. Channel A driver card has ~1900 ohms for test A and ~100 ohms for test B. Channel B driver card has ~560 ohms for test A and ~2500 ohms for test B.
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