[Greg Reid]

Originally Posted by merlotgoat

Bias adjutment pot was dirty/corroded, causing the output transistor collector voltage to be 3.4 volts instead of the normal 1.5 volts. Clean and readjust was all it needed. Other than that, radio is like new.

Sounds like there's two output xsistors that the pot is balancing? As you can tell, I'm not intimately familiar with the design.

[merlotgoat]

Just one--the radio is AM/FM mono. The AF amp is a typical Delco three-stage amp. The pot adjusts the output of the first AF amp, which feeds into the base of the second AF amp, which then determines the collector voltage on the large output amp/transistor. I told Lance that the 3.4 bias volts caused the cone of the speaker to pop outward with a LOT of force when the radio was turned on, which was the first clue something was drastically wrong. So the bias voltage was the first thing checked. I've seen this same thing before on other radios. Just moving the pot back and forth after a shot of contact cleaner takes care of the problem for the next couple decades. Use a voltmeter on the collector (speaker output wire) and set the bias at the factory recommended 1.5 volts. This can actually be done with the radio still in the dash. I'll bet many non-working radios needlessly ended up in the landfill with this problem.

[Greg Reid]

That's good information for future reference.