Hello, all. About a year ago, I bought a DEC vt420 in working condition
on the well-known auction site. After sorting out all of the required connectors, it worked great as a terminal for my bog-standard Linux box.

Last week, it suffered a display failure. I had been compiling Emacs,
and the build messages were happily scrolling down the screen, when I
went away for a bit to do something else. When I looked back, the screen
was dark. That would have been normal, but the little screen-saver
spinner in the bottom right wasn't visible, and the display wouldn't
come back.

After turning it off and back on again, it displayed the startup
patterns fine (a few different plaid-like patterns of filled blocks over
the whole screen), and made the feep it makes on a successful
startup. However, the screen remained blank, instead of showing the
"vt420 OK" text dialog as normal. Pressing F3 brings up a little bit of
the setup menu, quite distorted, in the very bottom left of the screen.

I'm not a hardware guy, so I'm not sure what to do here. My default plan
is to list it on the big auction site as "for parts or repair" and let
it go to someone who can fix it and use it or resell it. But if it is
something really simple (recapping?) I might want to do it myself.

Any suggestions (or anyone want to buy it)?

--
+-----------------------------------------------------------+
| Jason F. McBrayer jmcbray@carcosa.net |
| A flower falls, even though we love it; and a weed grows, |
| even though we do not love it. -- Dogen |

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Jason McBrayer <jmcbray@carcosa.net> wrote:

I'm not a hardware guy, so I'm not sure what to do here. My default plan
is to list it on the big auction site as "for parts or repair" and let
it go to someone who can fix it and use it or resell it. But if it is >something really simple (recapping?) I might want to do it myself.

So, you know you have a serious sweep problem, or else a power supply
issue. (Power supply issues can look like all kinds of other problems
and should be the first thing to be checked.)

1. Don't turn it on until it's fixed. IF the flyback is okay, you want to
keep it that way.

2. Find a tech and have him fix it. All kinds of things can go wrong with these terminals, and if it has the original electrolytics in it, you are
likely to have a failure with collateral damage caused by a leaky electrolytic. Then again, you might just have a dirty pot.

I can't teach you how to do basic monitor diagnosis yourself online, but
these things are easy to work on, and parts (except for the flyback) are standard and readily available.

3. In the future don't power up old monitors without having them checked
out on the bench first. The VT220 is the absolute worst; there is one
bipolar electrolytic which has a very high peak voltage across it and
which consequently is prone to failure. Unfortunately when it fails,
it throws DC on the flyback and it burns up almost instantly. Stuff like
this needs to be fixed -before- collateral damage occurs. The VR270
has similar issues where the emitter resistor on the sweep transistor
goes up in value and takes the flyback out.

Anybody who can work on 1970s TV sets will have no problem working on these things.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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kludge@panix.com (Scott Dorsey) writes:

So, you know you have a serious sweep problem, or else a power supply
issue. (Power supply issues can look like all kinds of other problems
and should be the first thing to be checked.)

Thanks for your help! I'll see about finding someone who knows CRTs.

--
+-----------------------------------------------------------+
| Jason F. McBrayer jmcbray@carcosa.net |
| A flower falls, even though we love it; and a weed grows, |
| even though we do not love it. -- Dogen |

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* Origin: fsxNet Usenet Gateway (21:1/5)

On 8/29/23 09:01, Jason McBrayer wrote:

So, you know you have a serious sweep problem, or else a power supply
issue. (Power supply issues can look like all kinds of other problems
and should be the first thing to be checked.)

Thanks for your help! I'll see about finding someone who knows CRTs.

Search for a guy named Ian Primus, based in NY.

-Dave

--
Dave McGuire, President/Curator
Large Scale Systems Museum
New Kensington, PA

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