Low Bias Voltage on my 2970DX

[W3FAW]

Hi, I have a 2970DX with no output. I'm getting a signal to the driver, but no signal through to the finals or to the linear amp. It is a MOSFET version with bias voltage reading about 0.98VDC on the gates of the driver and finals. The bias should be 3.6 - 3.7VDC. The IRC520's have been replaced but made no difference.

Any ideas?

Thanks, Jim

 

[Robb]

Thoughts?

Check the voltage at the junction of R274 & 276.
Might replace the the 5.6v zener diodes @ D91, 92, & 93 too.

http://www.cbtricks.com/radios/rci/rci_2950dx/graphics/rci_2950dx_70dx_sch.pdf

 

[W3FAW]

Hi Rob, thanks for the input. The voltage at that junction is 8.5VDC. I have removed the diodes and the transistors but the bias voltage is still being pulled low through R274, R275 and R276. It is about 0.95VDC on all three. It seems like another component is shorted and pulling down the voltage. All of the suspect components are SMD's and I don't have a clue about how to work with them. Where do I go from here?

 

[W3FAW]

I'm still stuck on this problem. I tried changing out the power transistor Q66 (B754) with no improvement. A discovery I made is that the ground does not go to the chassis. I wonder if that could be part of the problem. I can't tell where the connection is supposed to be made and I don't want to just ground B- to the chassis just in case it's not supposed to be at ground potential.

I sure would appreciate some additional assistance on this.

Thanks, Jim