DEC VT-101 Flyback Transformer Blues

[beatkamp]

My lovely DEC VT-101´s display began to crap out. I instantly suspected the Monitor driver board. The famous C439 and CR408 problems or perhaps a bad 555 timer were my hope. The monitor board is the same on a VT-100 so there is a lot of archival help out there.

I recapped everything suspect and changed the parts I hoped would fix the display. No success. I noticed that when I would discharge the flyback anode to the CRT I was not getting the normal snap and blue spark. The flyback transformer appears to be finished. The screen is now utterly blank. The terminal´s composite video out and the logic board itself remain fully functional.

This part is not easy to source in my geographic area.

I do have a small collection of green monochrome monitors and was thinking of kludging in a flyback from one of these into the DEC terminal.

Has anyone had any success with installing a random flyback transformer into a monochrome display? Should I try or is this a very bad idea?

[beatkamp]

There are two flybacks that I am aware of that have been used on the VT100 family of dummy terminals:

70-17363-00 Flyback Assembly from Elston

16-16463-00 Flyback assembly from Weat

The old DEC documents refer to this part without any actual specifications. The service tips conclude with "replace".
There do not seem to be any datasheets available for these parts on the web.

Sadly, I have no idea what the ratios are for the three windings and no way to find out by testing my bad one.
I am also burdened by a complete lack of knowledge in this entire area.

[beatkamp]

I have just ordered the following flyback from Dalbani. It looks the right era and has three windings. The CRT lead comes out at the wrong angle so it will require different mounting. If it works, great. If not, I will know quickly and it cannot do any harm (I tell myself). It visually looks nearly the same.

,,Hope is the first step on the road towards disappointment." Homer Simpson

[beatkamp]

[caseybea]

I have just ordered the following flyback from Dalbani. It looks the right era and has three windings. The CRT lead comes out at the wrong angle so it will require different mounting. If it works, great. If not, I will know quickly and it cannot do any harm (I tell myself). It visually looks nearly the same.

,,Hope is the first step on the road towards disappointment." Homer Simpson

UnknownDalbaniFlyback.jpg

Hi! SO..... I have an old VT100 that was in storage, pulled it out last night. Last time I used it about 4 years ago it was fine.

SO when I fired it up: Video was at first a tiny shaky... then stable... then... it wiggled, shrank, disappered like a magic genie. Black screen.

Based on the behavior of this I have read in multiple forums, I am pretty sure it's the flyback transformer. There was a whiff of burned smell, I think it's coming from that component..

I found the same one online that you purchased. Have you tried it yet? I am dying to know, as this appears to be the only one out there. If I get one, do I just wire it up the same way as the old one? That is, the colorred wires soldered to it in the same order?

Hoping to hear about your experience!

--Kent

[beatkamp]

This project is on hold until the end of month when I will have some free time to spend on this.

What I will do is take out my frequency generator and test everything possible to get at least some
confidence that the Dalbani flyback is at least close in specification to the original and / or will not
cause serious problems.

If you suspect that your flyback is on the way out, do not power up anymore. They seem to start
failing slowly and then quickly

Depending on the failure mode, it will fry the HOT transistor and its associated capacitor. Regardless,
all the caps on the video monitor board are now beyond their service life if they are original.
When you replace them, don´t be tempted to replace the two tantalums with electrolytics. The ESR
will not match. On the other side, the tolerances for the electrolytics is conservatively over 20%.

I kept running my VT with a slowly failing flyback, as I enjoy coding on it, totally oblivious and fried
my monitor board. Thank heavens the motherboard is ok.

If you can wait two weeks, I am going to deep dive into this thing. I simply must have my dummy terminal
working. I will post everything here as I unravel the process.

Its been quite difficult, thus far, learning what a flyback transformer is, how CRT´s work, how to handle them,
not be electrocuted, measure them, etc. This is a new area for me.

[beatkamp]

I wouldn´t trust the wiring to be the same without testing it first.

If the Dalbani one is a fit, electronically speaking, that will be great news.

If the wiring is exactly the same, it will confirm my suspicion that this was a flyback made for one of the very many VT-100 clones.

[caseybea]

OK, please keep me posted. I am good with my hands (and soldering- built many electronic kits growing up), but I am NOT an electronics guru, so I have no idea what, on the video board, is the HOT component you mentioned or which is the associated capacitor......

Would apppreciate as much info you can provide once you get back to this! I will be monitoring for an update.

[caseybea]

It is worth noting mine is a VT100, not VT101. So I expect some differences in the guts from yours. I will try to post a picture of my video board for comparison.

The cool news is, for VT100 enthusiasts, this thread came up in google searches. We (mostlly you-- ha) will be providing a great service with all this info.

Also worth noting, when the video failed I did one very short followup power-on. I still get the POST beep, so there's hope not much else inside is fried if anything.

My affinity for the VT100 is because that's what I grew up on in College. Labs full of them at one time. I'm not in my early 60's. I live in the US.

[caseybea]

For what it's worth, here's my video board. After perusing some othher (often.. OLD) forum posts elsewhere, there were many refeences to C439 often blowing etc. As far as I can tell from at least a visual inspection, my video board looks well.. pretty flawless. No blown or puffed-up caps that I can see at all, no signs of overheating/scorching either.

[caseybea]

This project is on hold until the end of month when I will have some free time to spend on this.

Any update? I am DYING to know if that flyback will work... ! My VT100 is disasembled awaiting the result!

[beatkamp]

Sorry everyone for the long wait.

I have this on my desk and will get to testing the new flyback transformer and getting some solid readings from the old one as well.

For this task, I have a lovely old Heathkit frequency generator that I can run at safe voltages.

I shall fill this thread with my findings.

[beatkamp]

For what it's worth, here's my video board.
(snip)

I can see your board is the older variant without the 555 timer.

It appears that someone has already re-capped it as well. Those do not appear to be the original Sprague caps.

Back in the day, Digital did not repair those boards. They were tossed and fully replaced.

Its very interesting to see yours. Quite rare.

[caseybea]

Interesting. I have had this VT100 since 1985, when I purchased it used from GE Medical Systems. My wife's uncle worked there and there were a pile of terminals for sale. I think I spent 25 bucks. I used it for several years to dial-in to work, sat mostly idle since then. I'm surprised to see that anything on this has actually been replaced.

[beatkamp]

Ok, at last I have the prospect flyback and the original side by side.

In no way am I implying that this Dalbani flyback is a suitable replacement. I need to do a lot of testing first.

This is a faint hope operation.

Here are a few photos as I begin this process:

[beatkamp]

This is getting bojjier by the minute:

[beatkamp]

My DEC Flyback Data:

Totuku Taiwan TW MF-1100 HIPOTTED
DEC Part # 16-16463-00 Rev B

Primary
Pin 1 - BROWN
Pin 2 - YELLOW
Pin 3 - RED

Secondary
Pin 4 - GREEN
Pin 5 - VIOLET
Pin 6 - BLACK
RED LEAD Anode Connector

Impedances Primary
YEL(2) <> BRN(1) : 1.8 Ohm
YEL(2) <> RED(3) : 1.8 Ohm
BRN(1) <> RED(3) : 1.7 Ohm

Impedances Secondary
GRN(4) <> VIO(5) : 3.1 Ohm
BLK(6) <> VIO(5) : 1.7 Ohm
GRN(4) <> BLK(6) : 3.3 Ohm

DC Voltage Proof Failed
Test: 9V DC (from common battery) Positive to BLK(6) negative to RED LEAD Anode Connector
- some voltage should be present, I get nothing. This is the fault. Was expecting 2 or 3 VDC.
Note that diode CR407 is epoxied into the flyback transformer.

[beatkamp]

Not Going to Work: Dalbani´s "Unknown Flyback" Part #4-2751-48500

This is likely not a suitable replacement for the DEC Flyback.
I am posting the data I gathered on it here just for posterity.

DO NOT INSTALL THIS ONE IN YOUR VT

Primary: Pins 3, 6, 8
Secondary: Pins 1, 2, 5, RED Anode Lead

Impedance Tests
Primary:
P3 <> P6: 4.4 Ohms
P3 <> P8: 3.5 Ohms
P6 <> P8: 6.9 Ohms

Secondary:
P2 <> P1: 1.5 Ohms
P2 <> P5: 1.5 Ohms
P1 <> P5: 1.2 Ohms
RED Anode Lead has a diode

RED Anode Voltage Check:
Source Voltage 7.99 VDC
P1 <> RED Anode Lead 1.914V
P2 <> RED Anode Lead 1.914V
P5 <> RED Anode Lead 1.914V

[beatkamp]

So now I am back to the beginning. What to do?

- find a replacement flyback transformer

- install a completely different CRT into my DEC

- adapt an LCD panel as is now being done with CNC machines

And the last option.... chop away the epoxy on my bad transformer in
the faint hope that its just a bad diode.

I really have nothing to lose here. I am going to chop away. The rest of the
transformer is working as it should so this is not totally unreasonable.

At the very least I will uncover the ancient mystery of the diode, CR407,´s identity.

[beatkamp]

Behold! There is a large diode in there and a 27K, 10% resistor in series with the anode cable. It reads 22K, possibly because I clumsily shaved it with the razor.

If I am fortunate, its a bad diode and I can repair this FBT.

[beatkamp]

Eureka!

I dug out the contact points and the windings are continuous to the diode.

Its a bad anode lead diode! I believe I can repair this flyback.

This thread will continue.

[1024MAK]

Good luck

Mark

[beatkamp]

Here it is, the failed snubber diode.

I have no idea what it is. "36" is its marking. Could this mean 36 Kilovolts

Its running at 15 Kilovolts so thats not a bad safety margin.

30 Kilovolt, 100ma diodes are readily available.

If this really is a 36 Kilovolt diode then I would really need to match that or better it.

Research continues.

[1024MAK]

It's more likely to be part of a part number than the rated reverse voltage.

A 30kV diode should be fine.

Mark

[beatkamp]

It's more likely to be part of a part number than the rated reverse voltage.

A 30kV diode should be fine.

Mark

Thank you Mark. I will proceed with 30kV diode. Cheers, Kenny

[caseybea]

Well... shoot. With the "replacement flyback" being a dead end, I am at a loss what to do with mine. And, I'm not even 100% sure my flyback is bad, could be something else.

I may end up giving up and selling my VT100 on eBay "as is".

But I will continue to monitor progress here

[beatkamp]

I had to pause this work until the middle of March.

It may be the case that the snubber diode in your flyback is bad too.

Depending on the failure mode, it may have taken down the drivers on the
video board too.

There are not that many parts on the video driver board and everything is
superbly accessible on all of the DEC VT-1xx terminals. This is a finite process.

I am not giving up on this project. No matter what, I will have a working
terminal again.

In looking at the insulating material I have chipped away to expose the
snubber diode, I doubt now that it is epoxy. It seems like some kind of
silicon. I will use metal free (obviously) and acid free silicon to rebuild
the insulating layer after the new diode is installed.

I will post tonnes of photographs and anything anecdotal here.

[caseybea]

Any update? My semi-dead VT100 is still sitting hoping for a solution

[beatkamp]

I am waiting on the high voltage diode replacement.

I did try to take to pieces the flyback I purchased in the hopes it would somehow, magically, be the exact same spec. (see photos above)

What happened in this dissection was a disaster. I shaved away the white silicon (?) material until I exposed an epoxy tube into which the diode was set. I then tried to break though this epoxy and expose said diode. Nope. The entire thing shattered, the diode sheared in two. This wasn't really a smart move: in the beginning I thought it was brilliant.

I will update here with news of triumph or failure in this next phase.

[beatkamp]

ps: It was the same exact diode in the other flyback (that I ruined)