Tube experts! - I T WORKS!

From: John B <>
Date: Sun Dec 5 13:16:34 1999

-----Original Message-----
From: Allison J Parent <>
To: Discussion re-collecting of classic computers
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
><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.
><I will be reselling quite a few of the tubescopes with PDP-8s and don't
>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
><>> mini computers.. From what I have heard from people who use to support
><>> 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


PDP-8 and other rare mini computers

Received on Sun Dec 05 1999 - 13:16:34 GMT

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