TVM monitor - no power to tube?

[ajrossnz]

I picked up a very lovely TVM MD-7E monitor which I believe to be an EGA one.

When powered up, I hear the fan spinning up, so I know it's getting juice, however I don't see/hear any sign of life out of the tube itself. Does anyone know where I should start looking. A visual inspection looks fair enough (no bursting caps etc).

Here are some images if that helps:
https://photos.app.goo.gl/FXB3Mh9jV97X3kF56
https://photos.app.goo.gl/R6LjAKAwtG65uJ146
https://photos.app.goo.gl/TTmTmHDvbrYH7NQs6

Cheers,
Alistair

 

[SpidersWeb]

Does the LED on the front light up?

If not, my first thing would be to check the outputs of the power supply module. The fan may be AC/mains powered.
If it's getting getting power to the mainboard, then since you see or hear nothing my first suspicion is the flyback circuit isn't starting.

On an Amstrad monitor I repaired, there was an IC for controlling the flyback, the tiny little capacitor used to store energy during the normal "power up" pulse and feed the IC juice for a millisecond or two had died. Replacing that 10 cent part and it came to life. Cap looked fine from the top, but had actually blown out the bottom.

But I'm not an expert, I've only repaired two and was lucky enough to have circuit diagrams.
Hopefully more experienced people will come by.

 

[ajrossnz]

Does the LED on the front light up?

If not, my first thing would be to check the outputs of the power supply module. The fan may be AC/mains powered.
If it's getting getting power to the mainboard, then since you see or hear nothing my first suspicion is the flyback circuit isn't starting.

On an Amstrad monitor I repaired, there was an IC for controlling the flyback, the tiny little capacitor used to store energy during the normal "power up" pulse and feed the IC juice for a millisecond or two had died. Replacing that 10 cent part and it came to life. Cap looked fine from the top, but had actually blown out the bottom.

But I'm not an expert, I've only repaired two and was lucky enough to have circuit diagrams.
Hopefully more experienced people will come by.

Thanks so much for your answer. I believe you are 100% spot on with the flyback. I do hear that tell-tale sound as if the CRT is coming to life when switching on, but the lack of any feedback on the tube at all , I don't however have a good idea of what to do with the flyback in terms of parts. It's also a pain to get to- if I try and remove the board, unfortunately it means cutting the earthrod which will need re-soldering when I'm finished. <Grumble>

 

[ajrossnz]

Thanks very much for your help!

I think you are probably spot on! I can hear the CRT get power, it makes that pzz sort of sound I am accustomed to when starting up a CRT, so it's getting power from somewhere, but maybe it cuts out after a moment, not sure.

The annoying thing with this monitor is, if I want to move the logic board completely out from the CRT, I have to cut the grounding wire; I don't see any way to take it off, so I'll have to re-solder it if I want to go deeper and remove the flyback assembly. Either way, I'm no electrical wiz, so I wouldn't know where to start from there on !

Cheers,
Alistair

 

[SpidersWeb]

The "pzz" will be the degauss coil, it runs for a few seconds on power up.
For me, I'd be looking up any ICs on the board, and finding out which is responsible for the horizontal feed, then check the outputs on that, and check the signals on the switching diodes for the fly back etc. But I'd use a scope for that. I have a modern scope and ESR meter if needed.

I'm also assuming you've plugged it in to a machine, just to make sure it doesn't rely on a signal to start? I know my TVM CGA's don't need that, but perhaps the EGA is different.

Also just need to reiterate I'm not an expert, just passing on my thoughts about what I would do.
In the meantime, it might be helpful if we can get good quality photos of the main PCB?