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

5 Dec 1999

-----Original Message-----
From: Allison J Parent &lt;allisonp(a)world.std.com&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Sunday, December 05, 1999 3:06 PM
Subject: Re: Tube experts! - I T WORKS!

...
 <Okay.. (my lack of tube knowledge talking)... a TV
repair guy I know who us
...
 <to do a lot of tube work told me tubes only last a
couple of years. I did

In TVs where the tubes are low end and not always run at their best
operating points that may be true.  In Qualtity test equipment that is
rarely the case.

<not want to have to recalibrate this scope everytime a tube drops out. Also
...
 <I will be reselling quite a few of the tubescopes
with PDP-8s and don't wan
...

Mechanical shock and power cycling tends to ruin tubes faster.  Generally
tubes have good lifetimes and can be considered reliable (excluding the
effects of heat on surrounding parts) .

<the scopes going south a month after they get them. I've ordered the spare
<but first I'll see how long the scope will last with the ones it has now.

Best to leave be unless there is a direct indication one may be soft.
Don't forget with the exception of open filements or shorts from mechanical
shock tubes tend to fail slowly and soft.

<This one has 4 transistors in it. I am checking the caps right now.

Good idea s those (caps) do fail.

<>> The only reason I don't like tubes is because they are very flakey in
ol
...
 <>> 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

That is true, usually they get filiment failures and those are easy to
spot.  Power cycles tend to accelerate that.  then again I had a tube
organ (some 80+ tubes) and only had one failure in 8 years.

<>> 8/S.. I am going to put some highvoltage diodes between the 8/S,8I and
<>> scope to make sure if the scope goes bananas I don't blow a few hundred
<>> transistors in the minis.

???? 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. 
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
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 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.

john

PDP-8 and other rare mini computers

http://www.pdp8.com

...

Allison