Analogue monitor board repair

[Adrian Graham]

On 31/10/2016 20:51, "tony duell" <ard at p850ug1.demon.co.uk> wrote:

> 
>> My recent Executel addition has a 5" screen with associated analogue board
>> that seems to be powered from a display chip I can't find any info on, and
> 
> Well, it won't be _powered_ from that IC. My guess is that said IC provides
> the HSync and VSync signals, the video comes from other devices on the
> main PCB. But that is a guess without seeing the machine/

My earlier google-fu was hampered by getting the IC number wrong and taking
a macro photo of it has shown my error, it's actually a Plessey MR9735
Teletext/Viewdata chip that 'drives a 625 line Colour Television Receiver to
display the contents of the Page Store'

http://www.datasheetarchive.com/dlmain/4d6e79fdc0fb675d8dd857876f8d6bdd195f5
7/M/MR9735

>> Hence the question - am I wasting my time without a schematic for this
>> analogue board?
> 
> You haven't told us what the problem is.

http://www.binarydinosaurs.co.uk/STCExecutelAnalogueBoard2.jpg

Before I replaced the failed potentiometer (new one seen top left) the
display looked like its horizonal hold had gone so I reasoned that's what
the pot controlled. I can *nearly* get a steady picture but the brightness
is out as well, despite there being working pots for that and contrast. The
brightness seemed more constant before I replaced the radial cap with the
PET one which is why I wondered if the choice was wrong.
 
> Do you get anything on the screen? Is the CRT heater glowing? Have you
> measured the CRT pin voltages?

There's a recognisable picture, yes.
 
> I can see one IC on that 'analogue board'. Looks to have metal heatsink
> tabs so I am going to make a wild guess that it's a TDA1170 vertical
> deflection IC Are there any other ICs on the board? My guess is that
> the 'analogue board' could be reverse-engineered without too much
> work.

It is indeed a TDA1170, there's also a TDA1180P.

Here's the machine in all its glory:
http://www.binarydinosaurs.co.uk/STCExecutel01.jpg

Board:
http://www.binarydinosaurs.co.uk/STCExecutelBoard.jpg

Cheers!

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?