IBM 5155 analogue display fault

[Tony Duell]

> 
> 
> Just picked up an IBM 5155 luggable which was working up until the point
> the chap switched it on to show me it working, when it promptly
> broke :-)
> 
> On switch on it gives a narrow horizontal band of amber about 1.5" high;
> after a couple of seconds the whole screen changes to flickering amber
> with brighter zig-zag flyback lines visible.

You have tried fiddling with the 2 user controls? 

What I would do is : 

Pull the case (it's the 6 screws on the front), take off the sheilds, etc 
if they're there. You'll see the CGA card in the slot on the left, next 
to the PSU/monitor. The monitor video is the yellow/black pair plugged 
into this card. Unplug it, also unplug the monitor power. I can't 
rememmber where this 2-pin connector is located, it may well be in the 
cable tunnel under the chassis.

Connect a CGA monitor to the DE9 on the CGA card and power up. Even 
without a boot disk, it should do the memory test and fall into ROM 
BASIC. If not, you've got a logic problem to debug.

Try again, this time with a composite monitor (US TV rates) connected to 
the RCA socket on the CGA card. Ifyou get no display this time, debug the 
composite video circuit on the CGA card. If that works, then it's a good 
bet there's a signal on the intneral monitor connector too, the only 
thing that could remove it would be a cracked track.

OK, now we need to look at the monitor itself. The first problem is 
getting inside it. From what I remmeber, it's assembled with Bristol 
Spline screws, with the odd tamperproof Torx in the PSU. The former are 
not easy to get drivers for, I have a set, but they weren't cheap. 

The monitor is fairly conventional (and I have the IBM TechRef schematic, 
which I think has a few typos on it -- 'H2VDC' is really '+12VDC' (!). 
It's clearly a Zenith chassis, and the only IC has a Zenith part number 
( 221-86 ). That's clearly the line (horizontal) oscillator and may be 
something like an LM1391, but I am not certain.

Still, you're not looking for a horizontal fault yet.

The next useful thing is that the first digit of the component reference 
number gives the area it's used in :

1 : Horizontal
2 : CRT
3 : Vertical
4 : Video

Some useful component references for you : 

Q401 Video Output
Q402 Video Driver 
Q403 Video Amp 2
Q404 Video Amp 1
Q405 Video Amp 3
Q406 Video Drive 1
Q407 Video Clamp
Q408 Not used
Q409 Sunc Seperator

Q301 Vert Osc
Q302 Osc 1
Q303 Diff Amp
Q304 Vert Driver
Q305 not used
Q306 Vert Output
Q307 Vert Output
Q308 Vert Retrace

That should be enough to get you started. 

-tony