Tube experts! - I need your assistance.

From: Christian Fandt <>
Date: Sat Dec 4 16:21:40 1999

Upon the date 10:42 PM 12/3/99 -0500, John B said something like:
>I don't like tubes. My experience with tubes is *very limited*. I have had a
>few crash courses over the past few years......

Well John B., I *like* tubes :-) As you may have noticed I'm an electronic
historian and old radio collector. Tubes pervade my very being it seems. I
grew up learning about and fiddling with tube gear :-) Love it! Wish I
could afford getting (or even *finding* an old IBM tube machine like a 704
or such.) A few others here enjoy the same background. Anyway, I went up to
my library and dug out my Tektronix type 503 manual. The RM503 is simply a
repackaged 503 which fits into a 19" rack.

Below, I'll speak as if you've had very little exposure to tube circuits,
'scopes of this vintage, etc. as I really don't know your old technology
background as of yet. At least others here who are not tube savvy and will
someday have to fiddle with an old scope may get something out of this anyway.

>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..).

Not good of course. Limit the on-time while testing please.

>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).

I sense you have a manual too as you give a good basic layout of the power
supply and indicate expected voltages.

>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

So, by removing the -3KV line, things settle down.

First thing in mind is that either of the two pots in the voltage divider
resistor string may be arcing over to ground. They are the FOCUS and
INTENSITY controls.

Second thing and at least this is easy to check, does the CRT heater light
up? With the -3KV left disconnected from the CRT do you measure 6.3 to 6.5
volts AC across the heater connections (pins 1 and 14)?

Third thing, and most undesireable, is the CRT envelope got broken and the
tube went to air. This will *definitelly* cause arcing inside the CRT's
electron gun. You know the shiny metallic spot you often see inside vacuum
tubes? That's the gettering which basically had taken up most of the
leftover oxygen after the manufacturer had drawn a vacuum on the tube and
tipped it off. Never saw a 503 tube so can't say where to find it but it
usually would be on the inner surface of the neck somewhere maybe 5 or 10
CM in from the base. If you see a milky white spot on the inner surface of
the envelope in that region then the tube's gone to air :-(

You mention that instead of 10 volts you measured 500. Thank Heaven this is
not a solid state scope!!! Smoke City!!

>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.

A shorted 5642 HV rectifier tube would present a high frequency AC voltage
of some level on the -3KV line. Maybe 5-7 KVAC peak to peak. The freq would
be 25KHz as generated by the 6DQ6A oscillator tube. Hard to see the little
filament in the 5642 to verify whether its glowing and you really cannot do
any measurements with a standard voltmeter on this part of the circuit
(your meter would not tolerate the high voltage unless you use one designed
for, say, 5KV or more.) I'll assume you may not have such a meter and
cannot measure even the -3KVDC. Check to see if the tube envelope is
broken. This tube has gettering also and see if it's milky. The filament
could fail and flop down onto the plate thus making a short circuit. Been
there, done that. I haven't hunted for 5642's for a long time and they may
be hard to find now. But I know one of the ham radio community folks could
come thru if they have a junker Tek 'scope on hand for parts. Let me know.

>#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)

Turn off all room lights this evening, close shades if the city lights are
bright too. Turn on unit and look for faint arcing around the circuit. Keep
your hands in your pockets! ;-) You may smell ozone from the arcing. The
FOCUS and INTENSITY pots are in the resistor divider circuit and are
mounted on the front panel. Listen carefully to help zero-in on the noise.
Keep your earlobe in your pocket too! ;-) {ZZZapp!}

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

Yes. Do the lights out trick to verify . . .

>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 ;-)

Hope this helps John. Sorry to be late with this but family stuff and my
schoolwork got in the way all day.

Let me know if you need other info/help. Regards, Chris
-- --
Christian Fandt, Electronic/Electrical Historian
Jamestown, NY USA
        Member of Antique Wireless Association
Received on Sat Dec 04 1999 - 16:21:40 GMT

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