Defeated by a Commodore 1950 Monitor

[tony duell]

> 
> I have a Commodore 1950 monitor that came along with an Amiga 3000, and I think it has defeated me. Its 
> failure mode is that there's no vertical scan; just a single, horizontal line. Vertical drive is still working, because I 
> can shift the line up and down a bit using the vertical centering control, but whatever circuitry generates the 
> vertical scan doesn't appear to be doing it.

It is not uncommon for the vertical shift control to apply a DC bias to the yoke, and thus the output amplifier
stage could still be the problem.

> Trying to debug it has been challenging, even though I found a service manual online. The monitor appears to 

Do you have a URL for this manual? I might give it a look and see if I can deduce anything.

> have been designed to be slapped together without much consideration of service accessibility, and the service 

That is quite normal, alas.

> manual does't describe the circuitry in much detail. It includes a block diagram, with no indication of which 
> actual circuitry corresponds to which blocks. The schematic diagrams show connectivity between the parts, but 
> don't identify the functionality of most of the ICs.

> I think I've narrowed down to one IC that's a likely candidate for the failed component, but it's still just an 
> educated guess that it's even the IC responsible for generating the vertical scan. And it's a no-name IC with the 
> monitor manufacturer's internal part number (IC401, with part number 56A326-1), so I don't think I'm likely to 
> find a replacement for it unless I find another model 1950 with a different failure.

Older Commodore monitors were Philips inside. I have an idea this 1950 is an AOC chassis, but I might
be wrong. That IC number doesn't look like a Philips number, certainly, It is quite possibly a standard part,
house coded. The vertical section of a monitor is much simpler than the horizontal section, and standard
ICs are common. Often they are also used in TVs (the deflection rate, etc, is the same after all).

Often the back emf from the vertical deflection coils during retrace is used to power the output 
stage of the vertical amplifier. Look for a diode and capacitor connected to the output IC. And 
check them, they often cause this sort of trouble.

-tony