IBM 5120 monitor issues

Josh Dersch derschjo at
Wed Oct 17 08:18:45 CDT 2007

Christian Corti wrote:
> On Tue, 16 Oct 2007, Alexandre Souza wrote:
>>> But I'd like to see what I can do about the monitor.  The picture 
>>> exhibits a strange behavior that I can't say I've seen before.  The 
>>> horizontal retrace seems to "fold" over itself in the middle (like 
>>> the electron gun is doing a "loop" in the center of the screen for 
>>> each line).  I've taken a (terrible) picture of the fault and put it 
>>> at:  The picture's 
>>> also a bit jumpy.  I've tinkered a bit with the various adjustments 
>>> on the CRT's PCB but it doesn't make any appreciable difference.
>>   Have phun on the horizontal circuit caps. There is one shorted or 
>> with a huge loss probably on the yoke line.
> Ehmm, you've replied to a posting that has apparently never appeared 
> on the list, at least it didn't arrive here... Who was the original 
> poster?
> Anyway the machine's not working as it halted at $01F6, or did you 
> (->original poster) halt the machine with internal HALT switch? The 
> routine around 01F6 (specifically 01EE-01FA) is POST 'G' "Keyboard 
> Test". It enables interrupt 3 and busy waits for a short time:
>                         ; --- Test G ---
>                         ; Keyboard Test
> 01EE    8BC7            LBI R11, #'G'
> 01F0    7B50            MOVB (R5)+, R11
> 01F2    1440            CTRL $4, #$40           ; enable int.3 and 
> typamatic
> 01F4    AF00            ADD R15, #$01
> 01F6    CF04            SS R15
> 01F8    F005            BRA $01F4               ; small delay
> 01FA    1442            CTRL $4, #$42           ; disable typamatic
> Christian
It was a response to my post (which took almost 2 days to show up for 
whatever reason :)).  I know the machine's not working, it has several 
hardware issues -- the most apparent being the monitor :).  There's also 
a keyboard issue (which is why it's halted in the pic) -- it seems to 
think there's a stuck key when there isn't (I've disassembled and 
cleaned the keyboard, etc...) -- if I swap in the keyboard (apologies to 
Tony) from my 5110 this goes away.  The last issue is a ROS error, which 
I'm betting will be very tough to solve, given the odd (proprietary?) 
packaging of the ROS chips.  (Again, if I swap ROS boards from my 5110, 
this goes away and the machine works OK, minus the monitor issue.)

- Josh

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