Tube experts! - I T WORKS!

From: John B <>
Date: Sun Dec 5 20:34:43 1999

-----Original Message-----
From: Christian Fandt <>
To: Discussion re-collecting of classic computers
Date: Sunday, December 05, 1999 10:30 PM
Subject: Re: Tube experts! - I T WORKS!

>Upon the date 02:16 PM 12/5/99 -0500, John B said something like:
>>-----Original Message-----
>>From: Allison J Parent <>
> -- snip --
>>>???? Why? The scope is an enclosed system that would be hard pressed to
>>>emit high voltages back into the system it's hooked to. It's not like
>>>HP9100 in that respect. For the RM503 to do that the HV lead would have
>>>fall on one of the input lines! As I remember the series 8 Display
>>>were driven off a pair of D/As and some single bit output to blank the
>>>beam. If you have something different I'd like to hear about that.
>I agree with Allison. Without knowing exactly the mods done by DEC that you
>mention below, I'm still assuming the output of the D/As is connected right
>into the SO-239 style front panel input connectors. In that case, the input
>circuitry is isolated from any HV. The inputs go directly to the vertical
>and horizontal amplifier tube grids (the 6DJ8's) after passing thru their
>respective attenuator switch assemblies.

They did not hook up to the front panel inputs. They used 3 connectors on
the back of the scope (x,y,z). All went into the amplifiers (except for Z of

>>Three D/As. 1 12 bit for x, 1 12 bit for y, and 1 3 bit for intensity.
>>the transformer went in this RM503 it raised the regular voltages
>>the scope to 500+ volts... in some places over 1000. That was present
>Ouch. I hope bypass capacitors and some coupling caps haven't had their
>dielectric damaged. Most of the ceramic caps and poly caps have ratings of
>500 and 600 volts DC respectively and 1KV is pushing or exceeding their
>surge voltage rating.

No, I tested it for very short times. I don't bother cycling power when I
see stuff like that.

>>through to the input. I am protecting the D/A converters as I don't need
>>PDPs taking major beatings in the future. All the supplies inside the
>>(-100,100,250,85,-3000 were going nuts). The other RM503 I have has the
>>problem. I am going to put an isolation transformer on every RM503,RM560
>>scope I get to solve the problem ahead of time.
>The other one has same problem? Interesting coincidence or possibly a
>common failure mode of this scope. I *was* going to put a benchtop style
>503 on my 'hunt for' list as it looks to be an excellent X-Y scope for my
>bench but I'll do some further thinking on that I guess.

Yes it does. But don't worry... I like this 503 and with an isolation
transformer inside that problem will never happen again.

>>The 8 Display controllers have the intensity option :-) (for spacewars of
>>course). The AX08 (XR option) and the 34D (standard) had a 3 bit intensity
>>D/A output for the scope. The RM503s were modified by DEC for intensity
>That manual you said you have, is it from DEC or Tek? I'm curious about the

Both, one from Tek and the LAB-8/34D interface manual from dec.

>mods. If you have a vanilla Tek manual like I have you can see that there
>are a pair of binding posts on the back of the unit which may be labeled
>"CRT GRID" and "GND." The GRID connection goes directly to the CRT grid
>through a 0.001 mfd capacitor.
>Now if DEC has done some fancy stuff to the 'scope by intrusively
>interfacing the PDP8 display controller directly to the deflection
>amplifier circuits in some manner then yes, I'd be leery of potential
>damage to the PDP circuits. Same for the intensity modulation (aka Z-axis
>modulation) circuit.

Yes they did... and I will do the same with some isolation added.

This scope is surprisingly stable... I just finished building a flip chip
tester and will be populating the 34D tonight... I hope it *lights* up.


>Regards, Chris
>-- --
>Christian Fandt, Electronic/Electrical Historian
>Jamestown, NY USA
> Member of Antique Wireless Association
> URL:
Received on Sun Dec 05 1999 - 20:34:43 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:31:54 BST