Hello everyone,
This is my first post on this forum, and looking forward to many more. I currently have on my bench a Toshiba SA-7100 "monster" receiver. All works well, except no output. I am feeding a sine wave into the main inputs, thus bypassing everything in the unit, feeding directly into the power amps. Turning on the unit, the protection relays do engage. Touching the collector of Q507 or Q511, I get a buzz in the speakers. Touching the base of Q507, I get no output. Further there is the sine wave signal on the three small signal transistors. This problem is in both channels leading me to investigate what is in common with both channels. Of course, the power supply. I thoroughly checked that, and all voltages from the power supply are present and correct. This is not a complicated circuit, yet I am puzzled! Any of you have experience with this unit, and this problem? I sure would appreciate any help. Thanks.
This is my first post on this forum, and looking forward to many more. I currently have on my bench a Toshiba SA-7100 "monster" receiver. All works well, except no output. I am feeding a sine wave into the main inputs, thus bypassing everything in the unit, feeding directly into the power amps. Turning on the unit, the protection relays do engage. Touching the collector of Q507 or Q511, I get a buzz in the speakers. Touching the base of Q507, I get no output. Further there is the sine wave signal on the three small signal transistors. This problem is in both channels leading me to investigate what is in common with both channels. Of course, the power supply. I thoroughly checked that, and all voltages from the power supply are present and correct. This is not a complicated circuit, yet I am puzzled! Any of you have experience with this unit, and this problem? I sure would appreciate any help. Thanks.
The schematic has DC voltages. You don't say if any of them are really in there?
I am also mindful of the very complicated speaker switching S018. Aside from user-error, it could be tarnished contacts in many places. The headphone jack bypasses S018.
I am also mindful of the very complicated speaker switching S018. Aside from user-error, it could be tarnished contacts in many places. The headphone jack bypasses S018.
As mentioned above, a signal can be injected at the collector of Q507, and is heard from the speaker. This proves L601/602 is in working order. It also proves the protection circuit and relay are engaged. Voltages around Q501, 503, 505, and 507 are correct. I'm puzzled why both power amps would fail in this way, leading me to investigate what the two amps have in common. Further thoughts, please?
Two quick DC checks are all we need for now.
1/ Confirm DC offset is close to zero.
2/ Confirm output stage is drawing current. Check this by measuring the DC voltage across any of the 0.22 ohm emitter resistors in the output stage. Anything over 5 millivolts and lower than say 25 millivolts proves the stage is conducting normally.
Adjust your signal generator to give around 100mv peak/peak (50mv peak) and confirm you see this amplitude of signal on R503.
Now connect the scope to L601 and tell us what AC signal you see/don't see. You should see around 3.2 volts peak to peak of signal.
1/ Confirm DC offset is close to zero.
2/ Confirm output stage is drawing current. Check this by measuring the DC voltage across any of the 0.22 ohm emitter resistors in the output stage. Anything over 5 millivolts and lower than say 25 millivolts proves the stage is conducting normally.
Adjust your signal generator to give around 100mv peak/peak (50mv peak) and confirm you see this amplitude of signal on R503.
Now connect the scope to L601 and tell us what AC signal you see/don't see. You should see around 3.2 volts peak to peak of signal.
Thank you all for your thoughts and ideas. I did finally find the culprits. The fusable resistors R523 and R524 were both burned out causing the emitters of Q507 and Q509 to float, reverse biasing them, thus no signal propagation. Typically when both power amps fail, I suspect something in common with them, usually the power supply. Not in this case! Good lesson learned here. I spent too much time on this one, and of course, can't charge the customer for my education. Seems many repairs I do furthers my education....never ending! Anyway, problem solved, and again, thank you all for your thoughts.
Pleased to hear you found the problem.
It seems this one may have some unexplained history as failure of those resistors in both channels... well a million to one chance 🙂
I did wonder (and didn't ask 😉) how you actually coupled a generator to those devices. A low impedance generator would effectively short any injection points out.
Interesting one 🙂
It seems this one may have some unexplained history as failure of those resistors in both channels... well a million to one chance 🙂
I did wonder (and didn't ask 😉) how you actually coupled a generator to those devices. A low impedance generator would effectively short any injection points out.
Interesting one 🙂
You need to find out why the resistors fused.
Just replacing them is probably ignoring a problem somewhere.
Follow the circuit from the resistors and see if anything is shorted like electrolytics or Zener diodes.
Just replacing them is probably ignoring a problem somewhere.
Follow the circuit from the resistors and see if anything is shorted like electrolytics or Zener diodes.
Nigel, indeed, did check around thoroughly. All else is OK. Checked voltages, all OK, output sine wave is clean. I think after 40 years, these fusable resistors are simply unreliable and failed...yet, still puzzling that both opened at the same time. Perhaps the customer was playing the amp very loud, placing a load on these worn out resistors, and pop....perhaps a line spike?? Mooly, as you suggest, I may never know. This is not the first time I have seen something puzzling, repaired it with unexplained reason for failure, and never got a customer return after the repair.
Mooly, My signal injection method is the old metal screwdriver touching different points in the circuit. I know, not exactly accurate science, but it sure works for a quick circuit trace. High impedance too!
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