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Thanks again Grapplesaw. I was aware of the dual rail voltage, but I wasn't sure what the intention was for it. I assumed it was similar to other amps that I have used where you set a selector switch on the back of the amp depending on the final load. My NAD switches between 4/8 ohm for low...

Quick question about how these are supposed to be run... If this will be run on 4 ohm speakers, should I leave the amp on High Z, or manually set it to low z for this application?

With many thanks to you, Perry and Ed. This thing might have killed me (or me it) if I didn't have the help.

I got the new Bias Transistors in and the bias is now stable and well within adjustable spec for both channels. I have set it to 350mV on both channels. Now I just need the new meter driver chips to come in, and I can button this up for a while.

Any pictures of what the new meters look like in an amp?

LOL... very well. Thanks again. Really appreciate all of the help from everyone.

Well Dang, Perry, thanks a lot. Would Joe's meter upgrades work on the 500, or is that a 400/700 thing only?

I grabbed a set of 2 from a USA seller. I'll give these a go and if it fixes the issue in one channel, I will get another set.

That's awesome Grapplesaw. Once the Bias is sorted, I think the last thing I need to figure out is the meters. They work and have been calibrated, but do not return to 0 without a signal. I found a snippet in the manual that states I need to check some of the diodes on the board, so I'll be...

According to the datasheet, the transistors I got are TO-92HS-3 package, and have the metal clip to mount the device to the heat sink just like the original. I assume that a lot of people sub these out for something else because of cost?... as the ones I ordered were over $5 a piece. Any...

I ordered a set of Central Semiconductor 2n3403 transistors from mouser. I will be sure to check the pin out before installing.

Jumping the emitter and collector on the bias transistor of the bad channel drops current to about 3mV, but not totally 0. Is that still indicative of a bad bias transistor?

Yes, you are correct on the .4v DC. I am running full line voltage. I only ran on the dim bulb tester to ensure no shorts. all other testing is done at line voltage. I have about 7mV on one channel and -3mV on the other as far as offset goes.

Alright... So as it turns out, I have not been measuring the Bias on the correct resistor. Now that I am measuring voltage across the correct resistor, things are looking much better. bottom of the adjustment sits at right over 2mV DC, and I am able to dial in a 4mV Bias on the left channel...

Appreciate the help Grapplesaw. I thought I had checked those resistors, but I'll open it up and check again tonight. I'll also test resistance from the solder joints instead of the resistor leads... could be that the resistor isn't making a great connection. I certainly wouldn't mind going...

OK, thanks a lot for the information and suggestions Gepetto. I'll take a stab at a higher value resistor in that spot.

I got the bias down tp .52mV with a 1k resistor for the time being.

Very well. That clarifies things a bit. Looks like it is mounted on the heat sink, not the board.

According to the service manual, the Bias is to be set between .20 and .60mV, which is why I was concerned about it being where it is.

Yes, that is correct. I am measuring across R133 and R145. In my board layouts in the service manual, I see R112 as a 820 ohm resistor, but I will see what adjusting that one does with the bias. Am I correct that q106 is the bias transistor?