Re: Modifications to 577 Curve Tracer Main Power Supply

Gary Robert Bosworth

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 normal
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, and
I momentarily disconnected the short-circuit feedback diode CR785 when the
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 no
micro-cracks to open them up, and there are no solder bridges causing short
This is a perplexing problem that I have not seen in 56 years of working in


On Sun, Mar 1, 2020 at 11:53 AM Dennis Tillman W7PF <dennis@...>

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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