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STC Executel video board troubleshooting

Witchy

Witchy

Experienced Member
Joined
Oct 11, 2015
Messages
376
Location
Flatlands, UK
Hi folks,

*edit* I've added the CRT board layout and the video service guide from the manual.

I've obviously not been here for a week or two and we have a new look!

I've not searched for a similar topic to this because I'm pretty sure nobody has asked for help on one of these before. The STC Executel was a computerised phone system released in 1983 and featured a 5" mono TV monitor that displayed the phone applications (diary, notes, calculator, phonebook etc) but was driven by a Plessey MR9735 (or 9736) delivering Viewdata compatibility with the UK's Prestel network. The TV is driven by (as far as I can tell) an in-house developed analogue board delivering a neck voltage of 300V and an anode voltage of 8.5kV.

Schematic is here
Board layout is here
Service guide is here

I have one on the bench that gave a spark at the cluster of HV leads on the board and a little wisp of smoke at first power up then nothing else. In the schematic I've highlighted in yellow the place on the board where all 5 HV lines meet. They all have thick heatshrink on them and the yellow cable has obviously been arcing a lot since the heatshrink has solidified and broken.

I'm happy working on chassis I'm familiar with like the Philips ones that make up the Commodore 1084/Philips CM8833 type screens, but I've never worked on one of these before. Aside from the spark/smoke this one does nothing. It's powered by a 5V feed from the main board, pinout for the video cable is shown as X1 on the drawing.

Where would you start looking?

Cheers!
 
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Are all the voltages going in to the video board ok? (I see 5V, +12 V). If they are ok, next are the measurements written on the schematic.
 
*bump*

Anyone? In my original post I missed the 12V feed that actually runs things. It's a TV board, pure and simple, and I now have 3 of them that need some TLC.

Cheers

W
 
Morning!

The CRT is powered, yes. I can see a raster scan and the machine's booting using a known good display chip.
I'm up stupidly early this morning after a 'yikes! Ground plane!' type of thought but the board isn't fastened to the frame of the case - the 0v connection is back via the ribbon cable that hooks the board up to the machine. There are lots of pictures of how it all connects up in the original post.

Too late to go back to bed now, dammit :) Instead I'll figure out a way of jury rigging the display board so it can be inverted for voltage checks.

*edit* I've amended the original post to have more info in it. I also notice the schematic has waveforms for me to look for once I can get the board upside down.

Cheers!
 
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You might need to build some extension cables but from the fact you have a raster I would at least say your high voltage and sweep circuits are working fine and to see if you can spot where your video signal disappears.
 
For that board, yes. It has no sync signal and back in the early days (2016) I thought that was a fault on the video board, but it turns out that the display chip (Plessey MR9735) is supposed to generate its own sync and in this instance doesn't, so there's also a fault in the open drain side of the display circuitry on the main board. I've been chasing that problem down for months.

There's 2 others. The original one in the top message that gave off the wisp of smoke and has the hardened heatshrink on the yellow wire going to the back of the tube. The 2nd board I think is just plain dead but I'll investigate more later, see if the tube neck heats up etc.

Cheers
 
As you already alluded to, verify the waveforms on the schematic match what you are finding. I would strongly suspect the video controller blew but until you have verified it's being fed everything it needs to normally operate it's hard to say. Also I can see why you aren't just looking for another. They (625 line video controllers) don't appear to be cheap. ;)
 
Aye, £20 a pop. I did buy 2 of them last month as a sanity check even though I have one good working machine that all the other MR9735s check out in. The same display chip is also used in the Tandata TD1400 and they work in that too, so why I get no sync out of this one is a mystery unless there's a short to 0V somewhere I've not found yet.

For the analogue boards I'll build a small wood frame and extend the machine ribbon cable out to it so at least I can probe with the CRT attached.

Cheers,
 
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