Re: Modifications to 577 Curve Tracer Main Power Supply

Gary Robert Bosworth

Chuck: I did not find anything wrong with my transformers, thank God.
Transformers can be a real headache in many ways. I have seen transformers
that had their core characteristics changed drastically from excessive
heating. I have also seen shorted turns, and if only 1 or 2 turns are
shorted you usually cannot measure the resistance change with an ohmmeter.
High frequency switching transformers develop their own types of problems,
what with capacitive couplings and lossy shielding.


On Tue, Mar 3, 2020 at 8:30 PM Chuck Harris <> wrote:

Have you considered that trimmer pots that are 30 years old often

Anyway, I would like to hear why you are so certain the transformer
is so very awfully terribly bad.

-Chuck Harris

Gary Robert Bosworth wrote:
Dennis: The parts in faded print are R773 and C773. They are not on the
circuit board.
The circuit design is not well described in the service manual. No
voltages and currents are mentioned in the print.
The +40V and -40V unregulated are fine.
ALL of the components measured perfectly good including the rectifier
bridges and filter capacitors.
The power supply behaves the same whether the loading connectors are
attached or not.
All of the other supply voltages are good including +200V, +12V, -12V,
I momentarily disconnected the short-circuit feedback diode CR785 when
loads were not attached and it had no effect.
The reason why I do not like this design is because of the fact that the
-30V output is used to bias the 6.2V reference diode VR772. This poses a
classic lock-up condition in that if the -30V does not exist, then the
reference diode is never turned on.
I have measured every circuit trace from beginning to end and there are
micro-cracks to open them up, and there are no solder bridges causing
This is a perplexing problem that I have not seen in 56 years of working


On Sun, Mar 1, 2020 at 11:53 AM Dennis Tillman W7PF <


Hi Gary,
Which parts on which schematics are shown in shaded text?

Dennis Tillman W7pF
TekScopes Moderator

Gary Robert Bosworth
Tel: 310-317-2247

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