Wondering if anyone is familiar with this series. I was testing it out and the bias circuitry is not functioning. There is shown on the schematic a bias transistor along with one diode and pot. There is no bias transistor on the two stages, just looks like one diode. I intend on adding a diode or two to get it going.
There is a place on the boards for the transistor, but it was never there.
greg
Just to clarify. You can see the holes in the board for the transistor, but the copper is solid on the back plane. I don't figure how it could have worked right from the factory. The diode under the driver heatsink seems normal, about
6 volt drop. I might need at least 4 diodes in addition to the one. H709 is MISSING.greg
I found a service bulletin from 10/29/1975 that may or may not have an effect on your problem. If your serial number is below US 7701 or Europe
51001, replace diode H720 and diode H721 with a 1S1555. When you say the bias circuitry is not functioning, do you mean there is zero bias current? It is supposed to be 10mv from TP701-TP702 and TP703-TP704.Good luck.
-- David Farber David Farber\'s Service Center L.A., CA
If both channels are missing that transistor, and one of them works properly, then it probably was meant to be like that. You can see in the schematic that there is still a path from the base of H713 to the base of H714. If anything, H709 would lessen the voltage between the two points and decrease the bias even more. What is the reference number of the diode which you say has a 6 volt drop?
-- David Farber David Farber\'s Service Center L.A., CA
Neither channel has any current. point 6 .6 volt. I'll check on the diode specs.
greg
H720 and H721 are for the bias of a couple stages back. I'll check that out too.
greg
One mystery solved. There is a bias transistor. Its hidden under one driver heatsink. There are two positions for it on the board. One away from the heatsink. That position I first saw is vacant.
So far a bad pot on one channel is the only thing I have found wrong. I still have to figure why the bias on the other channel is not working.
greg
Doesn't each output stage have its own bias transistor?
Right. There are seaparate assemblies.
I was fixing some Sansui integrateds some years ago. A particular series had a resistor that would change value and screw up the bias. It was an esy fix, except for the conductive glue on the power supply board.
greg
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