OK Thanks,,, TR701 and 703 are on teh bad side,,, and both transistors have been replaced when I first got it running,,, TR702-704 are the old ones including the one I soldered back together,,, they are on the good side!
I will ohm test the pot and compare Trans side to side,,,
I will ohm test the pot and compare Trans side to side,,,
Good place to look. Marantz used those type of pots a lot in they're early to late 70's gear. I used to collect and restore early to mid 70's Marantz separates. Those pots used were used for bias, DC offset and symmetrical clipping adjustment and just didn't age well. I've tried cleaning them to get them to hold steady, but in the end I had to replace some of them.
But try cleaning them first John in that one channel. See if that doesn't help.
Ok,, I cleaned and tested the pots, they seem to track well from either end... but that's just a resistance test,,, doesn't mean they are holding up under a load... VR 711-712 are 200R and read pretty close to that...
I tried to bias the amp a few times,,, set all the controls zero,, no signal,,, Tried with speakers connected and not,,, I poked around on bad side looking for a bad connection,,, I touched/poked something and it went back down to normal,, but I couldn't repeat it,, or get it to go high again... I played it awhile,, stopped the CDP,, and turned the speakers off... When I turned them back on it popped in the bad channel, (it has done this before I replaced the bias diodes yesterday, teh last time it happened it was was noisy on the bad (Left side) so I poked around and touched a cap(C726), and it stopped... I replaced C726/725 ).. Probably popped because bias was high again,, but old caps tested way high,,,
I shut it down for 30 minutes and just tried again,, and Left side is still high,,, I'll need to figure a way to read the small transistors without pulling them as there are new temporary ones on the bad side and I had to insulate the pins to cross them,,, and teh good side has the old and soldered together ones, that will be destroyed if I pull them to test... I dont have the Digi yet order so no replacements,,,
Sorry for the rambling,, just trying to remember the time line,,, I'm now thinking its a good thing teh new power transistors someone put in are so overkill, the originals may have fried with this bias problem,,,,
I tried to bias the amp a few times,,, set all the controls zero,, no signal,,, Tried with speakers connected and not,,, I poked around on bad side looking for a bad connection,,, I touched/poked something and it went back down to normal,, but I couldn't repeat it,, or get it to go high again... I played it awhile,, stopped the CDP,, and turned the speakers off... When I turned them back on it popped in the bad channel, (it has done this before I replaced the bias diodes yesterday, teh last time it happened it was was noisy on the bad (Left side) so I poked around and touched a cap(C726), and it stopped... I replaced C726/725 ).. Probably popped because bias was high again,, but old caps tested way high,,,
I shut it down for 30 minutes and just tried again,, and Left side is still high,,, I'll need to figure a way to read the small transistors without pulling them as there are new temporary ones on the bad side and I had to insulate the pins to cross them,,, and teh good side has the old and soldered together ones, that will be destroyed if I pull them to test... I dont have the Digi yet order so no replacements,,,
Sorry for the rambling,, just trying to remember the time line,,, I'm now thinking its a good thing teh new power transistors someone put in are so overkill, the originals may have fried with this bias problem,,,,
OK,,, I can bring the variac up to 90V and adjust the bias,,, any higher and the Left side runs away... So there is something wrong on the Left side,, However, the pots are too touchy to dial in on either side,,, and they dont look like they were ever very good... VR711/712 are 200R pots,,, I found these multi turn Bournes pots, that seem to be very accurate, but they are 250R,,, do ya think they will have enough adjustment to work as replacements for the originals??
TIA
TIA
Attachments
Now, that is nice and inventive. I would dial them in at about the same resistance as the old ones before you turn on the amp.
I might be inclined to epoxy-encapsulate it - at least where the leads are attached. Wrap in some electrical tape, force a bit of epoxy into the end where the wires are coming out. Wait till it cures, and remove the tape. Then it can’t get broken from vibration or fatigue 10 years from now and send the bias into runaway.
Hi Hugo,,,
I covered em with shrink wrap...
Pots are installed,,, much better control than originals,, but still same bias run away on Left channel at more than 90V (~120V line here) input on variac,,,
Here's voltage readings on first three small transistors on each channel (TR701~706) at 90V line input... Not sure if this helps,,, I can take reading at full line input, but L channel bias will reach hundreds mV... I'm trying to save new output transistors!!!!!
Thank again for the input!!!!
I covered em with shrink wrap...
Pots are installed,,, much better control than originals,, but still same bias run away on Left channel at more than 90V (~120V line here) input on variac,,,
Here's voltage readings on first three small transistors on each channel (TR701~706) at 90V line input... Not sure if this helps,,, I can take reading at full line input, but L channel bias will reach hundreds mV... I'm trying to save new output transistors!!!!!
Thank again for the input!!!!
Attachments
It looks like CE of TR705 is short circuit.
Can you compare TR705 / TR706 in diode measurement? Not resistor on DMM.
B-E
E-B
B-C
C-B
C-E
E-C
Can you compare TR705 / TR706 in diode measurement? Not resistor on DMM.
B-E
E-B
B-C
C-B
C-E
E-C
OK,,, Looks like my transistor order is scheduled to be delivered today,,, I will pull TR705 and test it,,,
Thanks for replying,,,
Thanks for replying,,,
