punch 601s bias

[Hexhclass]

Anyone know what the bias voltage should be for this amp?
Can the bias be set with just one working channel?

 

[seankane]

It should be at 0 vdc across the source resistors.

The working channel should still bias properly.

 

[Hexhclass]

0.000? When I have it set there I get distortion at low volume.
With black prob on amp main ground and red prob on the gate of PS FETS I get 10.58v.
Is that ok for this amp?

 

[S Merlington]

With Blaco probe on Ground termianl of the amp what voltages do you get on all 3 legs of the power supply fets??

Leg 1:
Leg 2:
Leg 3:

 

[Hexhclass]

I think the voltage on the gate is to high but the fet is not geting hot
Leg 1:10.58
Leg 2:12.38
Leg 3: 0.00

 

[seankane]

Usually you would see 4 or 5 vdc on the gate legs.

 

[seankane]

Some of the newer amps I have worked on will have distortion in a good channel if there is a bad channel.

Some of the protection circuitry could be damaged.

To set the bias watch your amp meter on your power supply. Screw the pot clockwise just until it starts to draw current then back it off till the current draw goes away. That will be good enough for this amp.

 

[Hexhclass]

Can I used a 2A transistor in place of a 2G?
I don't remember what this amp had.Its the drivers for the power supply FETs.

 

[seankane]

You can use 2A or 2G, the 2G has a little higher current handling.

 

[Hexhclass]

Any one no in what location the PS drivers go,I took all out and don't remember
where the 1A or 2A goes .A pick will also do.

 

[seankane]

Q1 MMBTA06 "1G" or MMBT3904 "1A" NPN
Q2 MMBTA56 "2G" or MMBT3906 "2A" PNP
Q3 MMBTA56 "2G" or MMBT3906 "2A" PNP
Q4 MMBTA06 "1G" or MMBT3904 "1A" NPN

This was from an 801S schematic, I believe they are the same the 800 just has more parts on the board.

 

[Hexhclass]

more problems
When the crossover switch is set to LP the left channel gets distorted.
I change the switch but no change.Any ideas?

 

[rrae]

Something similar i had one time, the problem was preamp opamp's.

 

[Hexhclass]

If i bring the bias to 0.007mv the distortion goes away but the FETs get hot because the bias is to
high

 

[Hexhclass]

anyone have a schematic for this amp or something close.

please email it to me ranks33@hotmail.com

 

[Perry Babin]

What's the PC# on the board?

 

[Hexhclass]

pc-4333-b

 

[Perry Babin]

Sent.

For future reverence, this board diagram was sent to me as the 801S and is marked on the diagram as the G800a2. Having the board number makes it easier to find in most instances.

 

[Hexhclass]

thanks