Problem Issues with a68aga20x101 Monitor

[JohnT]

Bought a machine on eBay a while back that was listed with a monitor issue. The tube is a Thompson a68aga20x101 and the chassis is a CH-288-C29.

I couldn’t get a picture with the multi game board it shipped with (only horizontal lines with a green colour), but managed play around with the pots to get a picture with a Pandora box, but it’s not great.

Main issues are:

- Cant size the picture correctly to the screen
- colours seem washed out
- with the best picture I can get you can see horizontal lines behind/at the sides
- loud Buzz coming from the Anode cap area
- Purple colour in the top right corner

Looking For some general guidance on this monitor/chassis and also if anyone’s come across these kind of issues and could recommend away to resolve?















Cheers

John

 

[gunblade]

possible weak tube but more possible is poor setup
green cut off is too high, screen volts are too high
horizontal size is far too high
discolouration is either degauss not functioning or magnetic source near the top right

 

[JohnT]

I’ll have another go at adjusting tomorrow and see if I can get anything better setup wise.

 

[Retroman839]

There is another brightness pot on the
Kneck card
Turn flyback down & brightness /gain up on
Small controller pcb & crt neck card pcb.

Also b+ is on the small vertical pcb attached to large heatsink.

does this screen jump when a white flash appears in a game ?

250v 220uf cap check if bulging also

That R318 mod was made to change the source for VCC to the vertical circuit. Prior to that the supply came from the flyback, if the monitor was used with a gun game, each time the gun was shot the high white flash would cause the highvoltage to jump.


This in turn would cause the vertical VCC to jump and result in a jumpy picture vertically. The resistor from D306 to the empty hole ar R318 should be 680 ohms.


I see vertical fold over on right of screen

As far as the vertical foldover goes, change C318, it causes weird stuff when it is off value, everything from foldover, jitter and collapse.

C318 is a 2.2 uf 16v dipped tantalum
I got some new just pay postage if you
Get that far.

 

[gunblade]

of course do not adjust b+ unless you know the level and are reading with a muiltimeter
screen volts and b+ are very different beasts

 

[Retroman839]

of course do not adjust b+ unless you know the level and are reading with a muiltimeter
screen volts and b+ are very different beasts

Be Safe !! Yes pull up schematic find test point and Correct volts for b+
be safe
With electricity high voltage on monitor chassis can kill. Only work on /in & around chassi it if you know what your doing! take care High Voltage area it’s very Dangerous be safe and take all necessary precautions.
You can get seriously hurt.

Schismatic

https://www.arcade-museum.com/manuals-monitors/Pentranic%20CH-288%20Switchable.pdf

 

[JohnT]

Definitely going to be a learning curve and sounds like an excuse to get a proper test bench setup.

 

[Retroman839]

Im working on this chassis
At the moment .
I also have that same tube
But I’m using a Samsung tube currently with it.
But like you I have it dialed in but it’s parameters are very slim.

And from factory it seems it was badly designed ..