With everyone`s help, I`ve gotten this thing going again.
It turned out to be a two part problem. In the beginning the supply just
quit while transmitting. As mentioned in original post, I thought I was
hearing a buzz in the sidetone if drive and therefore power supply demand
was increased. I got it going again by cleaning the connectors on the power
cable. It then quit again. I checked continuity on the pass transistor.
Every lead combination checked shorted. I replaced the pass transistor and
it`s working and the sidetone buzz is gone.
The pass transistor runs hot even with a big heatsink. I`ve used a fan on it
the last few years, but I guess it just finally gave up the ghost.
Many thanks to everyone.
Michael WB4AQL
On Mon, Jul 26, 2010 at 3:23 PM, Stuart Rohre <rohre@arlut.utexas.edu>wrote:
> One of the first things to check is the diodes in the rectifier bridge.
> You may have an open bridge.
> When you plug in the supply, and turn it on, do you hear hum in the
> transformer? That would validate the transformer as receiving AC
> power. You may have to put the ear next to the supply to tell.
>
> The buzz might also indicate a capacitor was about to go; you pretty
> much have to do some point to point voltage measurements to find out
> what is wrong. You can half split a problem by looking for DC voltage
> right after the bridge rectifier ta the plus side of the first filter
> cap. Then, if you have voltage there you move toward the output, which
> might mean the regulator went. If you don't have voltage at the plus
> side of the first cap, look back toward the bridge rectifier and
> transformer.
>
> Not getting a pilot light might indicate an AC side problem, but if the
> pilot light is 12 volt type, it would be on the 12 volt bus, which is
> probably closer to 13.4 normally.
>
> These steps are general steps that work with any power supply circuit.
> But, if the 262M is a switcher, there could be other feedback paths that
> could shut down the supply. A schamtic is very helpful.
>
> -Stuart Rohre
> K5KVH
>
>
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