Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default ADM-3A Wonky Until Warm

I have an ADM-3A dumb terminal and when I first turn it on the display is squashed and the top line is mangled. After five minutes or so the display looks OK but is a little jumpy. After a few more minutes, the display is rock-solid and stays that way until the next time I turn it on cold. If I turn it off back on, the display is still fine as long as I don't let the ADM cool off first. Does anyone know what the problem might be? What to look for?
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Default ADM-3A Wonky Until Warm

Tom Lake wrote:

I have an ADM-3A dumb terminal and when I first turn it on the
display is squashed and the top line is mangled. After five minutes
or so the display looks OK but is a little jumpy. After a few more
minutes, the display is rock-solid and stays that way until the next
time I turn it on cold. If I turn it off back on, the display is
still fine as long as I don't let the ADM cool off first.


Sounds like the sort of issue you could track down by a can of freezer
spray (or inverted can of air duster)

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Default ADM-3A Wonky Until Warm

On 19.06.20 9:02, Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display is squashed and the top line is mangled. After five minutes or so the display looks OK but is a little jumpy. After a few more minutes, the display is rock-solid and stays that way until the next time I turn it on cold. If I turn it off back on, the display is still fine as long as I don't let the ADM cool off first. Does anyone know what the problem might be? What to look for?

Check for bulging caps. they are failing, and work best at a bit higher
temperature.

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Default ADM-3A Wonky Until Warm

In article ,
Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display is s=
quashed and the top line is mangled. After five minutes or so the display l=
ooks OK but is a little jumpy. After a few more minutes, the display is roc=
k-solid and stays that way until the next time I turn it on cold. If I turn=
it off back on, the display is still fine as long as I don't let the ADM c=
ool off first. Does anyone know what the problem might be? What to look for=
?


A can of freeze-it spray, a plastic/otherwise completely insulated poking
device, and a very old soldered joint/failed component in the vertical
deflection section. Including inductors that may not quite be making contact
between the enamelled copper wire and its pins, capacitors/resistors with
hairline cracks in.

Once it's warmed up and "just about" working, cautiously poke, tap, or zap
with cold-spray and see if it fails.

Any sight/sound of arcing/sparking when it goes from working to not? If so,
observe it in low light conditions and keep your eyes open. Better still,
aim a camera at the board to see where it sparked, in case you miss it.
--
--------------------------------------+------------------------------------
Mike Brown: mjb[-at-]signal11.org.uk | http://www.signal11.org.uk
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Default ADM-3A Wonky Until Warm

On Friday, June 19, 2020 at 12:02:29 AM UTC-7, Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display is squashed and the top line is mangled. After five minutes or so the display looks OK but is a little jumpy. After a few more minutes, the display is rock-solid and stays that way until the next time I turn it on cold.


Either some of the video adjustments are temperature-sensitive (not impossible,
but unlikely) or you have a mechanical stress issue; maybe cracked solder joints, maybe
a dirty connector.

If you know how to safe the CRT, you can open it up and exercise the connectors,
look at solder joints (especially at heavty wires or connector pins) and maybe tap
with a chopstick at various points DISTANT FROM the anode wire while power is on.

If you care about its function, do be gentle with the CRT; those are irreplaceable.
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Default ADM-3A Wonky Until Warm

Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display
is squashed and the top line is mangled. After five minutes or so the
display looks OK but is a little jumpy. After a few more minutes, the
display is rock-solid and stays that way until the next time I turn it
on cold. If I turn it off back on, the display is still fine as long as
I don't let the ADM cool off first. Does anyone know what the problem
might be? What to look for?


The freezy spray comments are just going to waste your money. You are not
having intermittent problems when the unit heats up. Grab a heat gun and
hit the capacitors. If you have a kit of spares, just start to replace
them, or solder tack new ones across the bad ones and see which one fixes
the issue. I used to have an adm-5a, a very similar unit.

Here's a full manual, schematics start on page 57

https://www.manualslib.com/manual/10...ler-Adm-3.html

It's a great terminal, except for how that damn keyboard feels.


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Default ADM-3A Wonky Until Warm

On 06/19/2020 02:02 AM, Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display is squashed and the top line is mangled. After five minutes or so the display looks OK but is a little jumpy. After a few more minutes, the display is rock-solid and stays that way until the next time I turn it on cold. If I turn it off back on, the display is still fine as long as I don't let the ADM cool off first. Does anyone know what the problem might be? What to look for?

I agree with the others, caps are usually the culprit.
But don't waste time tracking down the individual component - that's for
modern hardware with the occasional failed cap. Old hardware like this
should be completely 're-capped'. That is, replace ALL the caps. Why?
because they degrade over time (especially electrolytics) and after a
few decades almost all of them will be due to fail soon. So just do it
now and get it over with.

-John G.
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Default ADM-3A Wonky Until Warm

Tom Lake wrote:

I have an ADM-3A dumb terminal and when I first turn it on the display is
squashed and the top line is mangled. After five minutes or so the display
looks OK but is a little jumpy. After a few more minutes, the display is
rock-solid and stays that way until the next time I turn it on cold. If I
turn it off back on, the display is still fine as long as I don't let the
ADM cool off first. Does anyone know what the problem might be? What to
look for?

Ohh, that's the exact signature of a bad elecrolytic cap in the vertical
drive circuit of the monitor. Many monitors of that vintage had bipolar
electrolytic caps in the vertical circuit. These may be hard to source, but
you should be able to find a suitable replacement.

Jon


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Default ADM-3A Wonky Until Warm

On Friday, June 19, 2020 at 8:02:29 AM UTC+1, Tom Lake wrote:
I have an ADM-3A dumb terminal and when I first turn it on the display is squashed and the top line is mangled. After five minutes or so the display looks OK but is a little jumpy. After a few more minutes, the display is rock-solid and stays that way until the next time I turn it on cold. If I turn it off back on, the display is still fine as long as I don't let the ADM cool off first. Does anyone know what the problem might be? What to look for?


After all the remedies You may also check the tube base and its connectors
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