Restoring a VT50 (VT52 actually) .

Mattis Lind mattislind at
Sat Dec 23 12:34:30 CST 2017

2017-12-23 18:51 GMT+01:00 Josh Dersch via cctalk <cctalk at>:

> On 12/23/2017 5:34 AM, Mattis Lind via cctalk wrote:
> 2017-12-23 13:32 GMT+01:00 Mattis Lind <mattislind at>:
>> Yes, the oscillator seems to work. But it is a pain to work with the VT52
>>> since the print set has very little information. Where is the component
>>> list? I cannot verify which PROM should go into what location. And the
>>> damn
>>> boards doesn't have any component designator anywhere, nor is there
>>> anything on the component placement info in the print set on the data
>>> path
>>> board.
>>> The PROMs in my machine are: 23-124A9, 23-125A9, 23-126A9 and 23-127A9.
>>> Is
>>> this the same as your VT52? Apparently there is an earlier PROM set as
>>> well.
>>> It would be very nice if you could tell me the location of the PROM chips
>>> as well.
>>> /Mattis
>>> I did some more research into the terminal after I put the christmas ham
>> steak into the oven and christmas rice pudding on the stove.
>> There were nothing going on at the microprogram outputs. All high. It
>> turned out that the CE input for the PROMS was high for ALL banks!
>> Impossible! A 7400 at E34 was not doing well. After replacing that there
>> are some life in the microprogram and I have some random klicks from the
>> key klick relay. But still not working as it should. So more interesting
>> fault finding ahead! 20+ years sitting in a cold attic probably is not
>> good
>> at all.
> I've been tinkering with a VT52 at the museum.  I found that the sockets
> for the PROMs are of poor quality and were not making good connections.
> This may not be a problem in your case, but it's something to take a look
> at if you haven't already.

God point. I did re-seat all the PROMs, but there were no difference. When
pushing a IC clip-on to the chip it appears that ICs slips out of the
socket which probably means that the force in the contacts to hold the IC
in place is not the best. Maybe I should just replace them to be sure.

I found another ITT made 7400 that was bad, E8 on the UART & timing board.
One output was always at 2V indicating that it was open circuit (pulled up
by the input of the connected TTL).


> - Josh
>>>          cheers,
>>>>            —FritzM.

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