Reviving old Televideo Terminals

[John Colt]

Hello all:

I am trying to revive some Televideo Terminals that have not been turned on in more than 40 years.

Two options:

    (1) Turn them on and hope for the best.

    (2)  Connect to Variac and bring up the voltage slowly

Any suggestions?

Thanks for your help!

John

Redmond

[Retro Cryptid]

Maybe disconnect the power cable from the power supply to the main-board, plug in the power supply and measure voltage from the power supply with a multimeter?

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[Clay Erickson]

Been awhile for me; I've worked with 950s while at Centralia Community in the late 80s and had a 925 of my own - still wish I had it.

First off, if you happen to be unfamiliar with working safely with monitor or high-voltage electronics, I would strongly discourage opening the case up in the first place; monitor voltages (especially lingering charges on the picture tube and flyback) are nothing to mess with.

If the power supply is switching, putting it on a Variac would probably be a bad idea (I've learned firsthand with an Apple IIc monitor during that era that switching power supplies don't take well to slowly increasing voltages - linear power supplies are usually okay with it).

The idea of disconnecting the power supply from the terminal electronics and then checking it with a multi-meter might tell you if it works, but it will not tell you if there are any shorted caps or other faults on the terminal or monitor electronics.  Also, if it is a switching power supply, it usually requires a load to operate without damaging itself.

Visually inspecting the caps to make sure they are not bulging or leaking before powering up might be worth considering.

OR - You might take one and simply plug it in on a surge suppressor or (if you have one) an isolation transformer (either should have an in-line breaker) and then turn it on - best done in a safe location like in a garage or some other highly-ventable place, just in case the magic smoke suddenly escapes.

That is all I can really think of for this.  If anyone else has better suggestions, I really hope they step forth and offer it.  :-)

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[Clay Erickson]

Been awhile for me; I've worked with 950s while at Centralia Community in the late 80s and had a 925 of my own - still wish I had it.  So my suggestions should be taken with a grain of salt...  ;-)

First off, if someone is unfamiliar with working safely with monitor or high-voltage electronics, I would strongly discourage opening the case up in the first place; monitor voltages (especially lingering charges on the picture tube and flyback) are nothing to mess with.

If the power supply is switching, putting it on a Variac and turning it up slowly would probably be a bad idea (I've learned firsthand with an Apple IIc monitor during that era that switching power supplies don't take well to slowly increasing voltages - linear power supplies are usually okay with it).

The idea of disconnecting the power supply from the terminal electronics and then checking it with a multi-meter might tell you if the P/S is okay, but it will not tell you if there are any shorted caps or other faults on the terminal or monitor electronics.  I've seen Adrian Black check the impedance on the power inputs to the boards to look for such shorts as a start, and then check individual components, especially tantalum or refer caps (they tend to fail more spectacularly than most - Bang!).

Also, if it is a switching power supply, it will usually require a load to operate without damaging itself.

Visually inspecting the caps to make sure they are not bulging or leaking before powering up is something to consider.

- OR - 

You might take one and simply plug it in on a surge suppressor or (if you have one) an isolation transformer (either should have an in-line breaker) and then turn it on - best done in a safe location like in a garage or some other highly-ventable place, just in case the magic smoke suddenly escapes.

That is all I can really think to suggest.  If anyone else has better suggestions, I really hope they step forth and offer it.  :-)

On 1/22/24 13:56, Retro Cryptid wrote:

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[Clay Erickson]

I should have been more specific: switching power supplies don't take well to slowly increasing voltages from zero to line voltage.  They prefer the line voltage snapped on immediately.  I had a regulator transistor go *pop* on me while trying a variac on the Apple monitor...  ;-)

Admittedly beyond that, I've never been up to speed on how switching power supplies work.  In contrast, I know linear power supplies inside and out (I suspect most people smarter than myself do).  Back in my university days, my California Computer Systems 2200 had a unregulated linear power supply that was simple to work on (regulation was done on each S-100 board).  Yet another past system I wish I still had...

Hope to see many of you this Saturday.  Cheers!

Clay.