Tube experts! - I need your assistance.

From: <(>
Date: Sat Dec 4 20:50:18 1999

  Quick way to find out is to disconnect 6.3 winding and use an external
transformer to supply 6.3 V to CRT.But be carefull 6.3 V transformer is rated
for 3000V. I have
a trans recovered from a 503. Willing to trade for PDP8 parts?


John B wrote:

> 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
> course).
> 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 ;-)
> john
> PDP-8 and other rare mini computers
Received on Sat Dec 04 1999 - 20:50:18 GMT

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