Thank you. I knew it would be in plain sight. 😉
There’s only 3V across it and helps dissipation in Q707 about 6mW. Surprised it‘s even there.
There’s only 3V across it and helps dissipation in Q707 about 6mW. Surprised it‘s even there.
I like JonSnell's observation--- the voltage measurements near D702 suggest that there's not enough current flowing through R710; eg. only 0.3V across D702 and Q706 appears to be slightly back biased.
I wonder if R710 might be open. Try tacking an additional 5.6k in parallel with R710 (it won't damage even if R710 is OK) and confirm with your meter that there is about 30V across the added resistor by measuring on the leads at the body of the part. Remeasure again near D702 for possible changes.
The outputs may be fried as the B -E pins were reversed.
Check all resistors ,vbe multiplier transistor , and outputs. I am assuming drivers are new.
Check for blown fuse
Check all resistors ,vbe multiplier transistor , and outputs. I am assuming drivers are new.
Check for blown fuse
I just want to give an update. I finally found what was wrong (at least one of the issue). There was a lifted eyelet on the emitter of Q757. The trace was therefore broken. @BSST's suggestion hinted me on that trace. Thank you.
I made another jumper with a wire to fix this. This board is really a mess.
Unfortunately, fixing it created another problem. Now I have +68V and 0V instead of +34V and -34V at Q707, Q757. There is never an easy one!
I disconnected the power amplifier board and the filter capacitor and the problem seem to be on the Power Supply board.
I have verified all the diodes (D802, D803, D804 and D805) of the bridge rectifier on the PS board and they are okay. Some smoothing capacitors are 19 nF instead of 22 nF so they are suspect but I don't have replacements to test that.
Q801 (Sony 2SD28 NPN) tests okay out of the circuit but strangely drops voltage in circuit from collector to emitter. If this transistor is bad, I don't have an obvious replacement. This is a TO66. I guess I would have to adapt a TO220.
I'm posting this in case someone could point the obvious to me. Otherwise, I will keep chasing parts...
I made another jumper with a wire to fix this. This board is really a mess.
Unfortunately, fixing it created another problem. Now I have +68V and 0V instead of +34V and -34V at Q707, Q757. There is never an easy one!
I disconnected the power amplifier board and the filter capacitor and the problem seem to be on the Power Supply board.
I have verified all the diodes (D802, D803, D804 and D805) of the bridge rectifier on the PS board and they are okay. Some smoothing capacitors are 19 nF instead of 22 nF so they are suspect but I don't have replacements to test that.
Q801 (Sony 2SD28 NPN) tests okay out of the circuit but strangely drops voltage in circuit from collector to emitter. If this transistor is bad, I don't have an obvious replacement. This is a TO66. I guess I would have to adapt a TO220.
I'm posting this in case someone could point the obvious to me. Otherwise, I will keep chasing parts...
I suspect a grounding issue as you still have 68V rail-to-rail, so I doubt a rectifier is the problem. Try meter probe tied to chassis and explore connections on supply board. Transformer center tap and C901, C902 should have a path to ground when all is connected properly.
@BSST, you were right, it was a grounding issue. Again a broken connection under the wires in the middle of the board.
Now back to the original problem! Still no bias on the left channel.
I have tried adding a resistor in parallel to R710. I only had 5K ohm on hand and got 26.4V across it. Voltages (without the additional resistors) are quite different from original measurements. Displayed in green now.
I'm perplex about the reading I get when probing the case of Q901. I would think I should see this across R722 but I get nothing.
Anyhow, since the last problem happened because of a broken trace, I highly suspect this is my issue. I will review them all. I damaged quite a bit the board by de-soldering/re-soldering a few times these transistors.
Now back to the original problem! Still no bias on the left channel.
I have tried adding a resistor in parallel to R710. I only had 5K ohm on hand and got 26.4V across it. Voltages (without the additional resistors) are quite different from original measurements. Displayed in green now.
I'm perplex about the reading I get when probing the case of Q901. I would think I should see this across R722 but I get nothing.
Anyhow, since the last problem happened because of a broken trace, I highly suspect this is my issue. I will review them all. I damaged quite a bit the board by de-soldering/re-soldering a few times these transistors.
I believe the case of the output transistor case is the collector, so you should see roughly speaker voltage on case of Q902, but B+ on case of Q901.
Most glaring problem I spot is path from collector of Q203 to emitter of Q705: assuming R713 is intact, there's about 4.4V base to emitter at Q705. So an open resistor or board trace or failed transistor. Try looking at Vbe of Q705 directly on the transistor's leads.
Most glaring problem I spot is path from collector of Q203 to emitter of Q705: assuming R713 is intact, there's about 4.4V base to emitter at Q705. So an open resistor or board trace or failed transistor. Try looking at Vbe of Q705 directly on the transistor's leads.
Thank you so much @BSST for all your expertise and support. It is alive again. There was a broken trace between C704 and the emitter of Q703.
Unfortunately, there is now a lot of crackles and the volume is quite low. As you can see, there are a lot of lifted pads on this board and a few jumpers.
The crackles are probably due to weak connections/traces. For the volume, could it be the output transistors (Q901, Q902, Q951, Q952)? Any other reasons for low volume?
Unfortunately, there is now a lot of crackles and the volume is quite low. As you can see, there are a lot of lifted pads on this board and a few jumpers.
The crackles are probably due to weak connections/traces. For the volume, could it be the output transistors (Q901, Q902, Q951, Q952)? Any other reasons for low volume?
Good work on your progress!
In the interest of accuracy, when I typed "collector of Q203 to emitter of Q705", I intended "Q703" but you found your way to the problem trace anyway. And judging from schematic, your "C704 and the emitter of Q703" should have been collector rather than emitter?
Low gain and pops and crackles can be almost anything including leaky transistors. But bad solder joints or traces will probably respond to tapping or prodding with a wooden chopstick, leaky transistors less likely. That you're getting audio at all is is very encouraging. Confirming that bias voltages conform to schematic notations is probably easiest path fault discovery. This is to say that Q951, Q952 are probably OK; if they are defective, they won't respond to bias current adjustment.
Where are Q901, Q902? I haven't spotted them yet.
Localizing a problem is key. Is left channel flakey, but right OK? If this is the case and left power amp is still suspect for low volume, you can lift input at C701 and connect it in parallel at right channel input; then left channel should perform identically to right. You'll determine whether the problem lies within the PA or on the preamp side.
Good luck. You're making progress!
In the interest of accuracy, when I typed "collector of Q203 to emitter of Q705", I intended "Q703" but you found your way to the problem trace anyway. And judging from schematic, your "C704 and the emitter of Q703" should have been collector rather than emitter?
Low gain and pops and crackles can be almost anything including leaky transistors. But bad solder joints or traces will probably respond to tapping or prodding with a wooden chopstick, leaky transistors less likely. That you're getting audio at all is is very encouraging. Confirming that bias voltages conform to schematic notations is probably easiest path fault discovery. This is to say that Q951, Q952 are probably OK; if they are defective, they won't respond to bias current adjustment.
Where are Q901, Q902? I haven't spotted them yet.
Localizing a problem is key. Is left channel flakey, but right OK? If this is the case and left power amp is still suspect for low volume, you can lift input at C701 and connect it in parallel at right channel input; then left channel should perform identically to right. You'll determine whether the problem lies within the PA or on the preamp side.
Good luck. You're making progress!
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Yes, I should have said collector. And I knew you were talking about Q703 given all those transistors on this board are in the 700 range.
Q901 and Q902 are the driver transistors (TO66) of the left channel equivalent to Q951 and Q952 on the right channel:
I will need to do more testing on this. Can you explain a bit more how to lift the input of C701 and to connect it in parallel at the right channel?
You would lift the input wire to C701 (extreme left of schematic above) and run a wire from left terminal of C701 to the corresponding input cap in the right channel. This will drive both left and right PA from the same signal source. Then you can do side vs. side comparison. From your earlier post, it wasn't obvious if problem was before or after C701.
Good luck!
Good luck!
@BSST, I haven't tried isolate the signal source yet but I have taken some voltages and I have a few questions before doing further investigations:
- The most obvious problematic voltage is at the collector of Q902 (the case of the transistor). Although I don't have perfect 25 mVs on the right channel, it is much closer to it. I tested the transistor for voltage drops and continuity and all is fine. Is it possible that it is leaking causing this -69 mV I'm reading? If yes, I know where I can get a replacement near by.
- The second thing that strikes me is that I am about 5 or 6 Volts below the 34 volts reference. It used to be low but I probably lost another volt or so when I redid the diode bridge with new diodes. Can this lower voltage cause the low volume I have? The transformer only outputs 25 VAC.
- When I said "crackles", I should have said distortion. I get it when I crank the volume knob since it is so low. Here how it sounds. Notice that there is a lot of hum but this is not new with this amp.
- I have replaced the Sony 2SA706 and 2SC1124 with Toshiba TTA004B and TTC004B. Could they be causing this problem too?
I doubt the B+ supply voltage (i.e. collector of Q902) is the problem. After all, isn't the other channel presented with the same voltages and yet operates cleanly? Leaky/noisey transitory can be a cause. Do you hear erratic noise and crackles without a signal applied?
Can you get access to a scope?
Can you get access to a scope?
Collector of Q902 (this is the transistor below Q901, right?) should be -25 mV and is -69 mV. And I haven't proved so far that the other channel is working correctly. From the recording, I see in Audacity that the distortion is in both channels.
I always wanted to experiment with a scope so if I find one, I will let you know.
I always wanted to experiment with a scope so if I find one, I will let you know.
I decided this morning to reinstall the old diodes on the power supply board in an attempt to get a little more B+. This did not improve the voltage but to my surprise the amp now works. In the process, I had to re-solder 2 wires going to the equalizer board. I guess, that this connection was probably weak and causing my low volume issue.
As a bonus of changing all the transistors on the power amplifier section, the receiver is now super quiet with no more hum.
The left dc offset is 37 mV (better than the previous 95 mV). Not perfect but I have made enough damage that I will leave like this.
I would like to thank you all (JohnSnell Electronic, kevinkr, stocktrader200, malibutwins) for your responses. A special thanks to @BSST for all your help and support. This forum has amazing and helpful members.
As a bonus of changing all the transistors on the power amplifier section, the receiver is now super quiet with no more hum.
The left dc offset is 37 mV (better than the previous 95 mV). Not perfect but I have made enough damage that I will leave like this.
I would like to thank you all (JohnSnell Electronic, kevinkr, stocktrader200, malibutwins) for your responses. A special thanks to @BSST for all your help and support. This forum has amazing and helpful members.
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