So, I took over repair on this amp. The previous owner could not remember where he was at on the repair, other than all of the sink-mounted transistors were replaced (can see where the ps fets blew) After checking it over, I powered it up, and had to re-adjust the bias gains. It then idled fine with +/- 42v rail and +/-15v for the control circuits. Of course, I did find the pico fuse blown, and removed the screw that shorts to the case in the p/s section. I checked for voltage, etc on the rcas, and they would not quiet down effectively case-grounded, so I grounded the rca shields to the non-bridging channel. The left channel plays audio just fine, but the right is dead silent.
I compared the circuits between the right and left channels, with the non-bridging terminals as the reference point. Both sides appeared identical throughout all of the drive circuits near the ouput sections, at idle.
Next, I begain to trace the audio, even swapping out the module with a known good module. both channels read good in and out of the module. Finally, i came to the 4-way tl074c op-amp, which did not read even close on all 4 sections. it is attached to 2 tl431c regulators. anyways, here is the pinout:
pin1: 0.094
pin2: 0.003
pin3: 0.000
pin4: 15.02
pin5: 0.000
pin6: 0.001
pin7: 0.036
pin8: 5.xxx and slowly rising. (jumped to 14.54 after I probed pin 9)
pin9: ~2 to 2.5, and decreases with probing
pin10: 2.863
pin11: -15.13
pin12: ~2.4 and decreases
pin13: 11.38
pin14: -13.78
pins 5&6 receive audio.
Just in case, here is the pinout of the tl431c regulators.
(right channel side on board)
pin1: 11.38
pin2: 0.000
pin3: 0.000
pin4: nc
pin5: nc
pin6: 0.000
pin7: 0.000
pin8: 2.487 (connected to pin 13 of tl074c)
(left channel side on board)
pin1: 2.987
pin2: 0.000
pin3: 0.000
pin4: nc
pin5: nc
pin6: 0.000
pin7: 0.000
pin8: 2.486
I am kind of lost at this point of where to go next. thank you in advance for any help, and a schem would be great.
-Heath H-
I compared the circuits between the right and left channels, with the non-bridging terminals as the reference point. Both sides appeared identical throughout all of the drive circuits near the ouput sections, at idle.
Next, I begain to trace the audio, even swapping out the module with a known good module. both channels read good in and out of the module. Finally, i came to the 4-way tl074c op-amp, which did not read even close on all 4 sections. it is attached to 2 tl431c regulators. anyways, here is the pinout:
pin1: 0.094
pin2: 0.003
pin3: 0.000
pin4: 15.02
pin5: 0.000
pin6: 0.001
pin7: 0.036
pin8: 5.xxx and slowly rising. (jumped to 14.54 after I probed pin 9)
pin9: ~2 to 2.5, and decreases with probing
pin10: 2.863
pin11: -15.13
pin12: ~2.4 and decreases
pin13: 11.38
pin14: -13.78
pins 5&6 receive audio.
Just in case, here is the pinout of the tl431c regulators.
(right channel side on board)
pin1: 11.38
pin2: 0.000
pin3: 0.000
pin4: nc
pin5: nc
pin6: 0.000
pin7: 0.000
pin8: 2.487 (connected to pin 13 of tl074c)
(left channel side on board)
pin1: 2.987
pin2: 0.000
pin3: 0.000
pin4: nc
pin5: nc
pin6: 0.000
pin7: 0.000
pin8: 2.486
I am kind of lost at this point of where to go next. thank you in advance for any help, and a schem would be great.
-Heath H-
Yesterday, it didn't. Funny thing, i went and powered it up and it was no longer muted. Played for about 5 min, then 1 output from each side of the channel shorted. Killed power to it, and cjecked things out. Powered it back up and re-adjusted the bias again. once again, it plays on the 6 remaining outputs. While it was playing, i was able to take a few readings, and it did have audio on pins 2&3 of the tlo74c. The bias settings are a bit lower than stock. I origonally found them in stock setting. The trouble channel is near full ccw now, but i should expect it lower having 2less outputs to drive, right?
Is it possible the new outputs were previously damaged being run at the higher setting? I plan on replacing all the outputs and resistors anyways. Not sure if the amp saw a problem and muted the channel for protection.
Is it possible the new outputs were previously damaged being run at the higher setting? I plan on replacing all the outputs and resistors anyways. Not sure if the amp saw a problem and muted the channel for protection.
I don't know how you're setting the bias so I can't comment on the difference between 6 and 8 outputs.
The amp can't mute one channel. All muting is done via the relays and they're driven in parallel.
If it does it again, confirm that you have signal on both outside pins on each end of the edgecard connector. If you do, confirm that you have signal on all 4 output pins of the two op-amps near the RCA jacks.
The TL074 is for protection, not audio.
The amp can't mute one channel. All muting is done via the relays and they're driven in parallel.
If it does it again, confirm that you have signal on both outside pins on each end of the edgecard connector. If you do, confirm that you have signal on all 4 output pins of the two op-amps near the RCA jacks.
The TL074 is for protection, not audio.
to set the bias, i watch my power supply for the slightest current drain with no load or input attached.
I am trying to understand how the tl074 is working in the system. is it safe to say that each of the 4 amps in it activate with dc offset between the 2 inputs?
Section 1 and 2 appear to me to be tied in to monitor offset in right and left input channels, and the output of 4 looks to be tied to the power/protect section and outputs the -13.78. section 3 is the one that reacted to probing, but also made a hiss in the power/protect section. this is the same noise it made when the 2 outputs shorted before i killed the power supply.
Where my thoughts are headed with this is that the sections 3 and 4 are monitoring the right and left channels, and that it is receiving an offset triggering section 4, but there may be a bad component in the power/protect section, preventing it from entering protect mode, but not sure. i will have to trace the circuits from the bottom side of the amp while i am replacing the outputs.
thank you
-Heath H-
I am trying to understand how the tl074 is working in the system. is it safe to say that each of the 4 amps in it activate with dc offset between the 2 inputs?
Section 1 and 2 appear to me to be tied in to monitor offset in right and left input channels, and the output of 4 looks to be tied to the power/protect section and outputs the -13.78. section 3 is the one that reacted to probing, but also made a hiss in the power/protect section. this is the same noise it made when the 2 outputs shorted before i killed the power supply.
Where my thoughts are headed with this is that the sections 3 and 4 are monitoring the right and left channels, and that it is receiving an offset triggering section 4, but there may be a bad component in the power/protect section, preventing it from entering protect mode, but not sure. i will have to trace the circuits from the bottom side of the amp while i am replacing the outputs.
thank you
-Heath H-
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