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

5 Dec 1999

PDP-8 and other rare mini computers

http://www.pdp8.com

-----Original Message-----
From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Saturday, December 04, 1999 7:57 PM
Subject: Re: Tube experts! - I T WORKS!

...

 ITS WORKING!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
Excellent....!!

> 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 
I wish I had access to suprlus stores like that. The few I know over here
have bits of battered transistor radios and not much else. 
I ship stuff to the UK all the time. If you have something you are in
desparate need of (like that transformer).. please email me. If I see it I
can pick it up and ship it.

...

> 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
...
   it. 
A word of warning. There is _NO POINT_ in replacing all the valves in a
Tektronix 'scope -- most of them will be fine even after many years of
use (as an example, my 555 has had only a couple of valves replaced since
new). You _will_ cause problems if you replace valves indiscriminatately.
And you will have to do a full calibration (which is not as easy as it
looks). Some valves are probably 'Tektronix Tested Tubes', which are
essentially selected or matched ones. Replace those with ordinary valves
and you'll have a long day ahead of you trying to make it meet the specs.

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

Are there particular tubes that will go regularly and should I replace
those?

...
 The best thing to do is to run through the calibration
making as few
adjustments as possible. If some adjustment won't set up properly, then
debug the circuit. Don't necessarily replace the valve, which may well
_not_ be the problem (I've actually replaced more transistors than valves
in old Tektronix 500-series over the years!).

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

...
 Oh, and don't trust valve testers either. Certainly
not simple 'emission
testers', which certainly won't show how a valve will perform in the
circuit. But even a mutual conductance tester, like my Avo Mk 4 won't
necessarily tell you how the valve will work under the conditions used in
the 'scope. The best test for a valve in one of these units is 'Does the
stage perform correctly'.

Yes.

...
 So, buy spares if you want to, but don't replace
parts unnecessarily.

Okay.

...

> 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
...

This may well have been true when you have a _lot_ of valves. But I can
assure you it's much less of a problem in these 'scopes...

> 
Okay.

...
 > 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. 
Reminds me of what can happen when the CRT in an HP9100 develops an
internal short. It blows transistors all over one of the deflection
boards, and then applies about 220V to the outputs from the processor
chassis. This generally wipes out a number of diodes on the gating board,
and if you're unlucky kills some transistors on a flip-flop board, some
transistors on the ROM address decoder, etc.

My point exactly.. there is no way I am going to let anything over +-10V hit
the PDP-8s(plural)

...

http://www.vacuumtubes.com/ 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 :-( 
Any reason that you think they're defective? Other than they're old
(which is no good reason!).

No, they work fine. I just don't want to have to recalibrate the scope every
time one decides to die.

If that's not an issue then I will leave them alone.

john

--

http://www.pdp8.com/

...
 -tony