Service Manual for Ball TV-90 monitor?

Tony Duell ard at p850ug1.demon.co.uk
Sat Jun 20 14:17:46 CDT 2009


> > Decided to dig out my IBM 5120 and try to get its monitor working,  
> > at long last.  It's a Ball TV-90.  Anyone out there have the  
> > service manual for this?  (Worst case, there's a copy on eBay right  
> > now, but before I go through that I thought I'd bug you guys...)
> >
> > I mentioned this issue a few years ago, here's a photo of the  
> > interesting distortion I'm getting: http://yahozna.dyndns.org/ 
> > scratch/5120disp.jpg.  Thus far I've done a complete cap kit with  
> > no effect.  I hate working on monitors, but I really want to get  
> > this running again...
> 
>    Wow that's a bizarre distortion.  I'd love to see the horizontal  
> drive waveform on an oscilloscope.  It almost looks like it might be  
> some sort of zero-crossing distortion in an amplifier.

I tired to look at that URL earier (when I was on a machine with a 
graphical display), but for some reaosn I couldn't access it. So I've not 
seen the actual distorted image.

However, very few monitors use any form of linear amplifier stage to 
apply a sawtooth current to the horizontal defliection coil (they do for 
the vertical, of course). Rather, they use the inductance of the yoke as 
part of the sawtooth generator. The horizontal output stage is 
essentially a switch.

Before you go any further, check the supply to the monitor. If there's 
something wrong in the PSU you might get significant ripple at the 
horizontal frequency which will produce 'interesting' displays.

Things I would check : 

The horizotnal output transistor and its drive (although if it's not been 
turned hard on and off, it'll overheat and fail very quickly0

The flyback 'damper' diode

Any damping resistors in paralell with inductors in the yoke circuit

A horizontal linearity coil (if one is fitted -- and check it's the right 
way round. It may be a 2-terminal inductor, but it is polarised). 

The yoke coupling capacitor (OK, you've replaced this)

-tony



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