Acoustic 320 repair. Unidentifiable resistors.

[Rytikar]

Hi,

I’m repairing an Acoustic 320 that had suffered a catastrophic failure. Four outputs short and bad burn damage to the amplifier board, several components charcoaled.
I’ve found the, quite unreadable, schematic and was able to rebuild the amplifier but for one part.

The bias transistor (Q6) has been moved from it’s position on the bottom of the board together with the pot and sits now on top of one heatsink with flying leads. The collector is wired with the red wire to the middle of a voltage divider made of two resistors that are totally burned up and open.

Has anybody the same setup and knows the values of those resistors?

Thanks in advance.

[g1]

Might it have to do with the service bulletin update of the bias circuit? See page 1 of schematic package attached.

[Rytikar]

Hi g1,

thanks for the tip. I’m using this service document, but I don’t think the bulletin refers to my problem. It adresses the bias range of the pot by reducing the ‘minimum’ resistor from 600 to 400 ohm by paralleling a 1K resistor. This has been incorporated into my pcb by using 390 ohm instead of 600.

But it gave me reason to examine the circuit and pcb one more time, and I think I get why they added the ‘flying’ bias transistor. They changed the type used and it has a different footprint (BCE) than original (EBC) and does not fit into the old holes.
Additionally they changed the collector voltage by adding the ‘split resistor’ setup I described.
If you look at the reverse engineered schematic here it shows the split as 18 and 27 ohm, where there had been 27 only originally. I hope he had a working example to take the readings from and will try with his values, although this will change the driver base voltages for one side a bit. But that may be ok, given that the NPN driver transistor has a hfe of 150 where the PNP driver only has 15.

Thanks

[PennySue]

Replying for the update.

[BettyPickens]

Hi g1,

thanks for the tip. I’m using this service document, but I don’t think the bulletin refers to my problem. It adresses the bias range of the pot by reducing the ‘minimum’ resistor from 600 to 400 ohm by paralleling a 1K resistor. This has been incorporated into my pcb by using 390 ohm instead of 600.

But it gave me reason to examine the circuit and pcb one more time, and I think I get why they added the ‘flying’ bias transistor. They changed the type used and it has a different footprint (BCE) than original (EBC) and does not fit into the old holes.
Additionally they changed the collector voltage by adding the ‘split resistor’ setup I described.
If you look at the reverse engineered schematic here it shows the split as 18 and 27 ohm, where there had been 27 only originally. I hope he had a working example to take the readings from and will try with his values, although this will change the driver base voltages for one side a bit. But that may be ok, given that the NPN driver transistor has a hfe of 150 where the PNP driver only has 15.

Thanks

Thank you.