Tube experts! - I T WORKS!

From: John B <>
Date: Sat Dec 4 16:31:29 1999

ITS WORKING!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Special thanks to Tony's idea of hooking up an isolation transformer (6.3V
but rated to 3000V)..

Okay, I did some of Christians tests and I found that the Transformer indeed
was arching into the main supply. (I won two of these scopes on EBay).

Here is the funny part.. I rarely go into surplus stores but tonight I made
an exception. I told the guy at the counter what my problem was and he had a
box full of 6.3Volt isolation transformers rated to 3000KV
HAHAHAHHAHAHAHAH.. I could not believe it. I bought a few and tried one out.
The scope now has a perfect trace. On XY the dot is crisp and very stable. I
have to align the scope.. but will do so after I get all brand new tubes in

Second funny part, As I was leaving the store I went over to his lightbulb
section (get the hint yet).. Yes, brand new bulbs [equivs] for my 8/S and my
8-I still in the package!

What a day...

I can't wait to get spacewars up and running.

Thanks for the info Chris and Tony, I have one more scope to restore and
have placed orders for another 8 of them... so I have some serious tube work
over the next couple of weeks.

(comments below)

-----Original Message-----
From: Christian Fandt <>
To: Discussion re-collecting of classic computers
Date: Saturday, December 04, 1999 5:34 PM
Subject: Re: Tube experts! - I need your assistance.

>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
>>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.

The only reason I don't like tubes is because they are very flakey in old
mini computers.. From what I have heard from people who use to support them
every power cycles was a nightmare. I am trying to stick to minis that can
run off of 110/200Volt. I have a lot of design experience in transistors and
IC/analog stuff... little to no tube knowledge.

>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

Very limited.. but I have to use these old tube scopes as I am putting the
original tek scopes back into the minis.

>>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.

I did.. to about 30 seconds... Got lucky though.. the 4 transistors did not

>>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.
>>to -3000).
>I sense you have a manual too as you give a good basic layout of the power
>supply and indicate expected voltages.

Yes, I got it with the scopes.. I do have to buy a bunch of RM560s. Do
youhave any manuals for those?

>>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
>>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.

The focus *kind* of worked... nothing on intensity.
(they both work now)
>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 :-(

Thanks.. I wil check out the other scope I have to work on next.

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

I know.. What scares the hell out of me is I am hooking this thing up to my
8/S.. I am going to put some highvoltage diodes between the 8/S,8I and the
scope to make sure if the scope goes bananas I don't blow a few hundred
transistors in the minis.

>>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

The 5642 was glowing and arching inside (looked really bad).. It is happy

>(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

I do. I worked in a TV repair shop in my early teens for weekend money.. I
picked up the high voltage meter when the business went south.

>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.

I found a canadian retailer that has them in stock:

also, has all the tubes in stock (all new in the
box).. It is costing me over $300 for three sets of the tubes though :-(

6DJ8 Amperex tubes are over $30 each :-(

>>#2) Bad insulation on the primary transformer secondary "crt heater"
>>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.

Thanks for the info. It is much appreciated. I have a lot of these scopes to
quickly restore....


>Let me know if you need other info/help. Regards, Chris
>-- --
>Christian Fandt, Electronic/Electrical Historian
>Jamestown, NY USA
> Member of Antique Wireless Association
> URL:
Received on Sat Dec 04 1999 - 16:31:29 GMT

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