Tube experts! - I T WORKS! - test-drb@ccmp.vtda.org

5 Dec 1999

-----Original Message-----
From: Christian Fandt &lt;cfandt(a)netsync.net&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
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 &lt;allisonp(a)world.std.com&gt;    -- 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
the
...
 >>HP9100 in that respect.  For the RM503 to do
that the HV lead would have to
...
 >>fall on one of the input lines!  As I remember
the series 8 Display scopes
...
  >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
course).

...
 >
>Three D/As. 1 12 bit for x, 1 12 bit for y, and 1 3 bit for intensity. When
...
 >the transformer went in this RM503 it raised the
regular voltages throughout
...
 >the scope to 500+ volts... in some places over
1000. That was present right
...

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 the
...
 >PDPs taking major beatings in the future. All the
supplies inside the scope
...
 >(-100,100,250,85,-3000 were going nuts). The other
RM503 I have has the same
...
  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
control. 
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.

john
http://www.pdp8.com/

...
 Regards,  Chris
-- --
Christian Fandt,         Electronic/Electrical Historian
Jamestown, NY  USA      cfandt(a)netsync.net
        Member of Antique Wireless Association
        URL: http://www.antiquewireless.org/