Tweet
I have a friend I have been mentoring in this arcane art of amp repair. I haven't had a shop in a couple years, and I miss the old problem solving. I have most of my amp assets over at his shop, and he is fixing them up for sale. I go over to talk shop, teach something worthwhile, and aid in fixing. My main fight is getting him past the "But it is solid state...".
SO I had a GK 700RB from my old shop. The thing worked, it was 100% except one of the parallel speakon jacks was bad. I had removed the jack, pending order for a new one. I had left the board dismounted, hardware in a pill vial. It suffered the move into storage and being moved around, but eventually I pulled it out and over to the friend's shop. He ordered the connector and found the amp made no sound. I knew it worked when I last had it.
My guts kept telling me the thing was OK, and something dumb had befallen it. And I don't like to ignore my guts. We spent a couple hours - we move slow as it is a social call as well as working time. So nothing out the speaker, verify both 1/4 and speakon jacks as well as the output inductor on the board, no signal. Plug into FX return, still no signal. Apply signal to regular input, nothing.
http://music-electronics-forum.com/s...ad.php?t=44027 schematic there.
I was thinking FX return went to power amp, but there is a lot of circuit after the FX return before the PA. But an input up front did yield a signal at FX send at least. Noted red LED for a few seconds at power up, but no blue LED for run.
SO what I usually say, "start at the start". So we went in at FX return and traced signal. Through C48, and present at left end of R98. Right end, no signal. R98 not open, looked around a while and found 20 ohms to ground from the right end, also pin 3 of boost pot. Followed traces between R98, D14, and boost pot, found 20 ohms to ground everywhere there and nothing obvious. All knobs off, pull the preamp board. Now let's find that 20 ohms. Well of course it is now 60k. Just what it ought to be. Friend, "but...but...but..." I said, it used to work, I am sure it works, and we probably dislodged a bit of something , solder, wire but, who knows, that was causing the short. Unit now passes signal to cable to power amp. great, amp ought to work. Reassemble and...no sound.
OK, I don;t give up easy, we fixed the preamp, the power amp still has an issue. Sigh. OK all power rails OK. I feel pretty good the thing is muted for some reason. I just know it isn't blown up. I'll skip over a bunch of general checks of the circuit. I was convinced we were muted, so where are the mutes? Took me a while to spot them on power amp. Lower left I see MUTE1 and MUTE2. I see MUTE2 kills the LM3886 for the tweeters out. But it is slaved to MUTE1. Not sure why we have Q4 AND Q5, but they work in series from the same base control so... Q4,5 have to be on to complete the diffy pair to V-. Sure enough zero volts across R21. And -86v on R23. Where does MUTE1 come from?
Found it. upper right at Q20. A 55 timer controls it. Q18 is the thermal shut down. If the 105 degree thermal sensor opens, Q18 grounds the trigger on the timer. If Q19 sees a fault signal at its base, same thing. Neither transistor was on, the trigger was free to roam. I did look around forever trying to find where FAULT came from. Eventually spotted it top center, Q21 samples the voltage dropped across the output transistor ballast resistors, and if it turns on, then Q15 turns on putting a positive on FAULT. But that wasn't happening, after all I knew the circuit worked. And we had already checked that Q18,19 were both off.
We determined the 555 was OK and putting out correctly, and getting to Q20. SO my pal, who I made do all the readings, started taking voltages on Q20. And the speakers went POP. he asked what happened, I told him he had shorted something to something with a probe, and we agreed the amp circuit worked. But as he probed we found a leg of Q20 broken off. Noted Q20 also looked mashed over. In fact two legs were broken. We needed a new 2SA1376. it will work.
Now that we know what is wrong, I know what happened. I had the lid setting loose, and in moving around the lid had come apart and the corner of the metal had smacked the board crunching the transistor. Completely my fault, but it led to a glorious couple hours of serious troubleshooting and explaining theory. I miss that.
When I go over there, I don't touch the stuff, he does all the soldering and probing and what not. I sit to the side and comment, not unlike doing it here, except it is real time and I can see the scope. We had started the night fixing up a Maestro FZ-1B. Once we got the dual pot wired in, by golly the thing worked perfectly. And the GK was a good training exercise as well as a teaching moment for how transistors work.
SO I had a GK 700RB from my old shop. The thing worked, it was 100% except one of the parallel speakon jacks was bad. I had removed the jack, pending order for a new one. I had left the board dismounted, hardware in a pill vial. It suffered the move into storage and being moved around, but eventually I pulled it out and over to the friend's shop. He ordered the connector and found the amp made no sound. I knew it worked when I last had it.
My guts kept telling me the thing was OK, and something dumb had befallen it. And I don't like to ignore my guts. We spent a couple hours - we move slow as it is a social call as well as working time. So nothing out the speaker, verify both 1/4 and speakon jacks as well as the output inductor on the board, no signal. Plug into FX return, still no signal. Apply signal to regular input, nothing.
http://music-electronics-forum.com/s...ad.php?t=44027 schematic there.
I was thinking FX return went to power amp, but there is a lot of circuit after the FX return before the PA. But an input up front did yield a signal at FX send at least. Noted red LED for a few seconds at power up, but no blue LED for run.
SO what I usually say, "start at the start". So we went in at FX return and traced signal. Through C48, and present at left end of R98. Right end, no signal. R98 not open, looked around a while and found 20 ohms to ground from the right end, also pin 3 of boost pot. Followed traces between R98, D14, and boost pot, found 20 ohms to ground everywhere there and nothing obvious. All knobs off, pull the preamp board. Now let's find that 20 ohms. Well of course it is now 60k. Just what it ought to be. Friend, "but...but...but..." I said, it used to work, I am sure it works, and we probably dislodged a bit of something , solder, wire but, who knows, that was causing the short. Unit now passes signal to cable to power amp. great, amp ought to work. Reassemble and...no sound.
OK, I don;t give up easy, we fixed the preamp, the power amp still has an issue. Sigh. OK all power rails OK. I feel pretty good the thing is muted for some reason. I just know it isn't blown up. I'll skip over a bunch of general checks of the circuit. I was convinced we were muted, so where are the mutes? Took me a while to spot them on power amp. Lower left I see MUTE1 and MUTE2. I see MUTE2 kills the LM3886 for the tweeters out. But it is slaved to MUTE1. Not sure why we have Q4 AND Q5, but they work in series from the same base control so... Q4,5 have to be on to complete the diffy pair to V-. Sure enough zero volts across R21. And -86v on R23. Where does MUTE1 come from?
Found it. upper right at Q20. A 55 timer controls it. Q18 is the thermal shut down. If the 105 degree thermal sensor opens, Q18 grounds the trigger on the timer. If Q19 sees a fault signal at its base, same thing. Neither transistor was on, the trigger was free to roam. I did look around forever trying to find where FAULT came from. Eventually spotted it top center, Q21 samples the voltage dropped across the output transistor ballast resistors, and if it turns on, then Q15 turns on putting a positive on FAULT. But that wasn't happening, after all I knew the circuit worked. And we had already checked that Q18,19 were both off.
We determined the 555 was OK and putting out correctly, and getting to Q20. SO my pal, who I made do all the readings, started taking voltages on Q20. And the speakers went POP. he asked what happened, I told him he had shorted something to something with a probe, and we agreed the amp circuit worked. But as he probed we found a leg of Q20 broken off. Noted Q20 also looked mashed over. In fact two legs were broken. We needed a new 2SA1376. it will work.
Now that we know what is wrong, I know what happened. I had the lid setting loose, and in moving around the lid had come apart and the corner of the metal had smacked the board crunching the transistor. Completely my fault, but it led to a glorious couple hours of serious troubleshooting and explaining theory. I miss that.
When I go over there, I don't touch the stuff, he does all the soldering and probing and what not. I sit to the side and comment, not unlike doing it here, except it is real time and I can see the scope. We had started the night fixing up a Maestro FZ-1B. Once we got the dual pot wired in, by golly the thing worked perfectly. And the GK was a good training exercise as well as a teaching moment for how transistors work.
Comment