Tube experts! - I need your assistance.

From: John B <>
Date: Fri Dec 3 21:42:08 1999

I don't like tubes. My experience with tubes is *very limited*. I have had a
few crash courses over the past few years......

Okay, I bought the Tek RM503 for my PDP-8/S. When I turned it on I heard
some terrible noises and found the power supply voltages were all over the
place (way off , like 10V was 500 etc..).

This scope uses a primary transformer to supply the 6.3V to most of the
tubes and has a separate winding for 6.3V for the CRT heater. Off this
transformer another winding fed to a voltage doubler and then to an
oscillator with another transformer to create a wide range of voltages. (12V
to -3000).

Picture this... the -3000 volts is fed right into the CRT heater (which
happens to be directly coupled with the primary transformer). I have been
able to locate the problem somewhat. If I remove the -3000 volt line between
the HV rectifier tube and the CRT itself then the scope works fine! All
waveforms are proper and the power supply works great (no picture of

It can be a few things... I am hoping someone here who use to work on tube
units might be able to tell me which problem below it most likely is:

#1) A bad HV rectifier tube causing the HV to come back to the second
transformer which would put a few thousand volts back into the secondary
winding taps causing high voltage everywhere.

#2) Bad insulation on the primary transformer secondary "crt heater" winding
which jumps over to the other winding that happens to be the main
powersupply winding (125V X 2)

#3) The -3000 volt wire is closely tied to the other low voltage wires. Is
insulation breakdown possible due to a crack?

My next step will be to take a reading on the primary transformer (first
transformer, secondary winding [doubled winding]) and see if thousands of
volts are there... That might help determine if there is an insulation
breakdown but from what I can tell when the -3000V is hooked up every
voltage goes crazy.

I am going to bed.. Hopefully I wake up to a great answer ;-)


PDP-8 and other rare mini computers
Received on Fri Dec 03 1999 - 21:42:08 GMT

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