DEC VT-101 Flyback Transformer Blues

[beatkamp]

I have settled on the following snubber diode:

2CL2FM 20 kV 100ma 100ns

The 30 kV diodes I have been able to get a look at seem too large to fit. I think 20 kV should be fine given that the VT snubber is running at 15 kV when operating correctly.

The data sheet is attached here.

* this is not yet advice as I have yet to install and proof this diode

[beatkamp]

Ooof... scratch that.

I am getting:

2CL2FP 30 kV 100ma 100ns

- from the data sheet above.

I don't feel too cozy putting a diode in with so little safety margin: 15 kV operating with a 20 kV diode.
30 kV for 15 kV operating feels much better.

Now I wait for the part to arrive in Iceland.

[caseybea]

Wow. So.. This is the diode inside the flyback?

[beatkamp]

I just received the diode and visually, its a perfect match. Now to install it. Very excited!

[beatkamp]

Here it goes! I will insulate temporarily with electrical tape.

[caseybea]

OK,,this is exciting. Keep us (or me, ha) posted!

I have what I **think** is the exact same situation. But sadly, I am not the electronics expert you are, nor do I possess the equiptment you have for testing.

I have a few questions:

(A) Where did you get the diode from? Web link/url/info?

(B) If I shipped you MY flyback would you test it for me? To confirm my suspicions. I of course would pay all postage to and from and throw in a few bucks for your time.

If I have the same situation (after testing), I can do the same thing, I'm super good with my hands and have no problem soldering in a replacement component....

[beatkamp]

So, I just tested and now I have a working flyback. Great. There is also bad news. Still, I have something not right here. The video board starts to heat up terribly and the video is not stable. I must still have a bad driver transistor. The video begins to fade out as quickly as the driver board heats up.

[beatkamp]

The diode I had to get from Ebay. I paid 13 Euros for a single one. I am not in a position to recommend anyone do this as I have not been able to prove success.

[beatkamp]

Another Test:

All video, CRT, driver board disconnected. Composite video only from the motherboard to a Sony monitor.

This works well.

So I have to have a look at what I am trying to accomplish here.

The goal:
- a fully functioning dummy terminal I can use daily

I am now going to look at fully replacing the CRT elements with a LCD panel.
Perhaps this is the best long term solution?

Its times like these I really miss the slide rule generation of engineers.

[beatkamp]

I was quite disappointed yesterday having put so much hope into a bad snubber diode.

Now I am going ahead with an LCD panel install in my VT.

The main issue with this is that the composite video in the VT is very non-standard NTSC.

A Sony PVM monitor will display it but virtually nothing else will work. None of the cheap composite video converters that are readily available will handle this signal. It needs to be somehow conditioned or converted.

Lotharek.pl seems to be making something that may do the trick. I have asked them about it and am waiting to hear back,

The Medusa:
https://lotharek.pl/productdetail.php?id=135

This caught my attention due to the following description:

,, Special modes (unrecognised PAL-like and NTSC-like)
unknown PAL
unknown NTSC

[beatkamp]

Heard back from Lotharek. They said that they cannot guarantee that Medussa will work but that it likely will given that the Sony PVM does and that normal composite video devices display a shifted image. The Medussa can be configured to scale and shift vertical & horizontal.

So I am going to get it and see what happens.

[beatkamp]

I also ordered a 12.1 inch 4:3 aspect 1024x768 LED display with HDMI controller board.

It is described so: ,,HD-MI LCD Controller Board with 12.1inch M121GNX2 1024X768 4:3 LED Screen"

I won´t bother putting a link to Ebay here as it will be dead in a short time.

[beatkamp]

Some good news. i just received the Lotharek Medusa scan doubler and it fully grabs the composite video output of the VT101.

I include here some photos of the test using a VGA HP monitor. In the demo, I used a scan line filter and a red coloured overlay. These are standard functions on the Medusa.

What remains is for my target LCD panel to arrive. It may or may not work. It may be necessary to grab a few until I have something that does.

Using the composite output of the VT sacrifices the excellent brighter/dimmer function. I am going to look into harnessing the internal signal and pumping that straight into the Medussa. That is Video, HSync, VSync. If that works, then amazing. If not, I still have a very workable solution.

Shifting from 80 column to 132 column mode causes the Medusa to re-sync. The horizontal alignment is not the same as 80 column. This means that one will have to calibrate for the column mode they actively use. Perhaps using HSync/VSync will remedy this? I generally stay in 80 column mode out of stubborn habit but I would like to have the column mode switch fluidly (also out of stubborn habit).

The Medusa´s settings are persistent so from a cold start it should all simply work after configuration. It does have a start up banner on the screen. I expect that whatever LCD panel I wind up using will also have some kind of a banner as well. I have decided not to let this irritate me until everything else is working.

In any case, I have now a proven solution and a path forward to fully functional VT dummy terminal.