[Collins] 516F-2 power supply troubleshooting

[Gerald]

On Sun, 2005-07-03 at 17:16 -0400, Cam and Juli Hedrick wrote:
> HI all, 
> R7 (100 ohm 1/2 watt) in one of my power supplies smoked today.  What should
> I look for as the cause of this problem?  
> Thanks in advance,
> 73,
> Cam Hedrick
> WA4JKW

That is the current limiting resistor for the negative bias supply.
There are two electrolytics in the power supply for that negative bias.
Very likely they need to be replaced (they are OLD and often aren't
replaced with the main filters are replaced.

The bias rectifier is a selenium in a plastic block. Selenium rectifiers
have been obsolete for nearly as long as the 516F-2 has been about.
Selenium rectifiers in their old age develop high forward resistance and
low reverse resistance, e.g. turn from poor rectifiers into bad
rectifiers and that causes lots of current into electrolytics during the
half cycle of reversed polarity.

In my opinion ALL those selenium rectifiers should be replaced with
1N4004 or 1N4007 mounted on a two or three lug terminal strip mounted
with a 6-32 screw in the same hole used for the stud on the selenium
rectifier block. Not when they give trouble, but now.

Sometimes when a selenium fails it puts out selenium vapor that is
unhealthy. And does not have a pleasant odor.

There are disc ceramic bypass capacitors in the radios, but they are
isolated from the bias supply by the adjustment potentiometer circuit.
Even the second electrolytic in the power supply is isolated by that
adjustment circuit, so to take out R7, it has to be the rectifier and
the first electrolytic, but both electrolytics should be replaced.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer