DC Offset - ppi a600

[Sean Morrison]

I'm in the processing of replacing all 16 output transistors of this amp with the newer npn,pnp replacements. Then I plan on building a couple of makeshift clamp rails out of angle aluminum.

Can anyone correct me on what I believe is the next step of checking the offset?

Step 1: apply jumpers to some pins to defeat bias (not sure exactly what this means, where the jumpers are located, or how many there should be)

Step 2: Power up amp with no RCA inputs connected, let idle for 10 minutes, and measure each speaker output for mV of DC voltage.

Step 3: Turn adjustment pots until close to 0mV on each channel.(again, not sure exactly where these are, so any photos would help)

 

[1moreamp]

If you post a internal picture of your A600 I am sure there are many here that can directly you accordingly. The A600 is different from the A600.2 so a clear picture of your amp will help us to guide you properly.

 

[Sean Morrison]

Here are an internal pic of mine from the speaker output end of things:

 

[1moreamp]

The pots located in front of the ceramic modules are the bias adjustment pots and the pots located up front near the RCAs are your DC offset adjustments.

VR-4 and VR-5 are the DC offset adjustments.

Jumpers are located in front of the bias pots close to the edge of the PC board just center of the outputs.

 

[Sean Morrison]

Thanks for the info 1moreamp. So now, is it necessary to apply the jumpers while doing this or is that optional?

 

[1moreamp]

I don't use the bias shut down pins when I align a PPI amp. I have done it this way for many years without incident. It does require that you do a bit of back and forth meter readings to see if they have caused any drift of the bias while adjusting the DC offsets as low as possible.

I leave the bias turned on so that the forward bias of the outputs is intact so I get the true DC offset on the output. I validate the forward bias before I adjust the DC offsets.