Bias setting procedure on a D-500

[laatsch55]

Ed, there's a feller on AK wondering how and what to set the bias to, either in Hi Z or loZ and what the value is. He'll be scanning this thread also. thanks Ed.

 

[laatsch55]

To which Ed replied-----The 5/81 revision of the Dual 500 service manual states the following:

Step 6-6.1 Improper bias setting.
a.) Allow at least 5 minutes warm-up time; perform step 5-4 to accelerate warm-up.
b.) Remove all input and output connections, then measure the DC voltage drop across R133 and R145 located in the output stage.
c.) Verify a reading of no less than 0.30 VDC and no greater than 0.40 VDC. Adjust R119 as necessary.

NOTE: R133 and R145 are 10 ohm resistors. R119 is the 1K ohm bias trimpot.

Ed Blackwood

 

[THD+N]

There's a feller on AK wondering how and what to set the bias to, either in Hi Z or Lo Z and what the value is.

This is actually a very good question. According to the D500 SM, bias should be set with the impedance switch set to Hi Z.

After I repaired mine, I figured I would "go easy" on the circuitry and select Lo Z which keeps the rail voltage around +85VDC. In this mode, the D500 is like a "Godzilla PL 400". The 400 has +80VDC rails.

I set my bias in Lo Z and the amp appears to be happy. There is still a lot of rewiring and other basic "house keeping items" I want to do to it, but for now, it works and it does not have any particular habits.

 

[laatsch55]

What's Hi voltage on the rails??

 

[THD+N]

In this case, it is +125VDC. At that voltage the zener diodes and NPN VAS (40412VI) run pretty hot. I'll start another thread in the main section to talk about this amp in general.