So here's my story......once upon a time (actually about 3 weeks ago) I was walking through the local flea market/antique store and saw a pretty nice looking PL 700 B sitting on the floor. It has a $50 price tag on it and the booth was selling everything for 20% off that day. So, I made the purchase. The seller had taped a piece of paper to it that said "1 channel not working, may need new output transistors, transformer good".
I've done a lot of restoration work on vintage tube amps. But, my work on 1970s vintage solid state is limited to a full restore on a Pioneer SX-636 I bought new circa 1976, a full recap of a Pioneer SX-1010 and miscellaneous other work on a few other SS integrated amps and receivers.
Did not have a dim bulb tester when I started working on this amp, but built one soon after doing much reading on the mighty 700 B. The first time I powered up the amp (no dim bulb tester) it vaporized the 10 amp line fuse.....dimmed my work bench lights too.
I then discovered somebody had replaced the 5 amp supply fuses with 20 amp fuses. My guess is because they fit and can be bought at most auto parts stores, while 5 amp AGX fuses typically have to be ordered.
Following the service manual to test the outputs, I found all the outputs in the right channel were good. But the left channel had lots of blown outputs. Diode test results are shown below. Look like only two of the outputs are good and the others are shorted or open. The results were obtained by placing the red and black probes on the emitter and base pins. What I've read tells me the case is the collector.
Left bank - top to bottom
0.480 V
0.000 V
0.488 V
0.000 V
0L.
Left middle bank - top to bottom
0L.
0L.
0L.
0.000 V
0.000 V
I bought ten new TO-3s per @laatsch's recommendation.
Later pulled the right channel TO-3 outputs and diode tested them between the emitter and base pins. Here are the results.
Right bank - top to bottom
0.494 V
0.493 V
0.484 V
0.481 V
0.482 V
Right middle bank - top to bottom
0.483 V
0.492 V
0.483 V
0.484 V
0.488 V
I've also completely recapped the PCB PL-20 and am 100% certain all the new polar electrolytics are installed correctly (learned to pay attention to polarity details a long time ago).
On a whim I put a new Q2 in the left channel. AT that time @laatsch recommended I power up the amp on the dim bulb tester while the outputs were removed and test the DC offset at the speaker terminals. No problem, light burned bright, then dimmed down to nearly nothing. However, got +84.4 V on the right and -88.3 V on the left. Ouch!!!!
Then @laatsch and @d3milay on AK helped me understand the pin out on the new Q2 signal transistor is different from the original GES 97. I had read the service manual page 32 incorrectly.
Q1-Q4 have all new been replaced with new parts using @laatsch's recommended part numbers. If I have to pull these transistors to test them, my philosophy is to go ahead and install new ones for long-term reliability.
D9 and D10 and all the rest of the small diodes on the PCB tested good in circuit.
Pulled the germanium Q8 and Q9 on both sides and diode tested them.
Q8 left channel = .224 V, .222 V
Q8 right channel = .207 V, .204 V
Q9 left = .158 V, .153 V
Q9 right = .159 V, .156 V
Have I mentioned yet soldering on this PCB is a pain in the rear. Solder pads and traces lift at the drop of a hat.
Pulled and tested the 1N4004 diodes on the back wall. They all tested good, but I had new ones on hand so replaced them.
At this point (outputs still removed) decided to bring it up on the dim bulb test again. Bulb burned bright, dimmed a little bit, then burned bright again. R16 smoked itself. Hand was still on the dim bulb switch, so immediately turned it off when I saw R16 give up the ghost. All of this took about 2 seconds probably.
So, something is evidently now wrong on the PCB. Am I certain all the signal transistors are installed correctly? About 99.9% because I checked and rechecked my work. Am I sick of soldering on this original PCB? Yes, without a doubt!!
Is it White Oak PCB time?? Talk amongst yourselves and let me know what you think. I'm leaning that way.
I've done a lot of restoration work on vintage tube amps. But, my work on 1970s vintage solid state is limited to a full restore on a Pioneer SX-636 I bought new circa 1976, a full recap of a Pioneer SX-1010 and miscellaneous other work on a few other SS integrated amps and receivers.
Did not have a dim bulb tester when I started working on this amp, but built one soon after doing much reading on the mighty 700 B. The first time I powered up the amp (no dim bulb tester) it vaporized the 10 amp line fuse.....dimmed my work bench lights too.
I then discovered somebody had replaced the 5 amp supply fuses with 20 amp fuses. My guess is because they fit and can be bought at most auto parts stores, while 5 amp AGX fuses typically have to be ordered.
Following the service manual to test the outputs, I found all the outputs in the right channel were good. But the left channel had lots of blown outputs. Diode test results are shown below. Look like only two of the outputs are good and the others are shorted or open. The results were obtained by placing the red and black probes on the emitter and base pins. What I've read tells me the case is the collector.
Left bank - top to bottom
0.480 V
0.000 V
0.488 V
0.000 V
0L.
Left middle bank - top to bottom
0L.
0L.
0L.
0.000 V
0.000 V
I bought ten new TO-3s per @laatsch's recommendation.
Later pulled the right channel TO-3 outputs and diode tested them between the emitter and base pins. Here are the results.
Right bank - top to bottom
0.494 V
0.493 V
0.484 V
0.481 V
0.482 V
Right middle bank - top to bottom
0.483 V
0.492 V
0.483 V
0.484 V
0.488 V
I've also completely recapped the PCB PL-20 and am 100% certain all the new polar electrolytics are installed correctly (learned to pay attention to polarity details a long time ago).
On a whim I put a new Q2 in the left channel. AT that time @laatsch recommended I power up the amp on the dim bulb tester while the outputs were removed and test the DC offset at the speaker terminals. No problem, light burned bright, then dimmed down to nearly nothing. However, got +84.4 V on the right and -88.3 V on the left. Ouch!!!!
Then @laatsch and @d3milay on AK helped me understand the pin out on the new Q2 signal transistor is different from the original GES 97. I had read the service manual page 32 incorrectly.
Q1-Q4 have all new been replaced with new parts using @laatsch's recommended part numbers. If I have to pull these transistors to test them, my philosophy is to go ahead and install new ones for long-term reliability.
D9 and D10 and all the rest of the small diodes on the PCB tested good in circuit.
Pulled the germanium Q8 and Q9 on both sides and diode tested them.
Q8 left channel = .224 V, .222 V
Q8 right channel = .207 V, .204 V
Q9 left = .158 V, .153 V
Q9 right = .159 V, .156 V
Have I mentioned yet soldering on this PCB is a pain in the rear. Solder pads and traces lift at the drop of a hat.
Pulled and tested the 1N4004 diodes on the back wall. They all tested good, but I had new ones on hand so replaced them.
At this point (outputs still removed) decided to bring it up on the dim bulb test again. Bulb burned bright, dimmed a little bit, then burned bright again. R16 smoked itself. Hand was still on the dim bulb switch, so immediately turned it off when I saw R16 give up the ghost. All of this took about 2 seconds probably.
So, something is evidently now wrong on the PCB. Am I certain all the signal transistors are installed correctly? About 99.9% because I checked and rechecked my work. Am I sick of soldering on this original PCB? Yes, without a doubt!!
Is it White Oak PCB time?? Talk amongst yourselves and let me know what you think. I'm leaning that way.
