Ha! Good one Jerry. I have also seen those resistors go bad . Just like you
say, they had cracks in them. Maybe use a ww 2 watt?to replace it?
?
Tom
?
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----- Original Message -----
Sent: Wednesday, January 02, 2013 9:47
AM
Subject: [TekScopes] bad -15v on
7704a
?
Hi,
I had a 7704A power supply that would lose -15V
after it had been running awhile. It might be after minutes or a few hours but
it would fail and the screen go blank. I could never find anything wrong with
the power supply out and on the min load.
Since the power
supply did not go into tick mode I guess that the problem must be in the pass
circuit of the -15V. I had previously replaced the pass transistor. I decided
to replace it again just in case. I happened to see R93, the 0.12
ohm resistor in series with the pass transistor. It checked okay but I decided
to jumper the resistor and test again. It worked fine. The -15V went to -15.1V
but it worked for 24 hours. I have replaced R93 with a resistor from a donor
and it is running fine.
Looking at the resistor with a
magnifying glass I can see a very small crack.
Jerry Massengale
