RadioBanter - View Single Post - Astron RS-12A Repair Help

Fred McKenzie · January 23rd 19, 06:14 PM posted to rec.radio.amateur.homebrew

In article ,
wrote:

I am having similar problems with my RS12 supply. I think I shorted a pass
transistor case to ground and output went to zero. Checked the transistors
and they seem good. Replaced the 723 chip and output returned to 13.8v, but
after cycling the supply a few times it fell to 8v. Any ideas?

Ross-

For a shorted pass transistor case-to-ground, I would expect a problem
with rectifier diodes. It is ALSO possible you now have another bad
723. If the high current rectifiers are open, you might initially
measure 13.8 volts with no load. With any load, the 723 might be
stressed.

The RS-12A uses two sets of rectifiers. One is for the low power
regulator circuit, the other has a higher current rating and feeds the
pass transistors. The diagram I have uses a single MDA2501 for the high
current rectifiers. (It is dated 6/20/88.)

Fred
K4DII