Re: IIe troubleshooting

[Alex Freed <alex_news@mirrow.com>]

Steven Hirsch wrote:
> All,
>
> I could use some suggestions.  I'm trying to troubleshoot one of my IIe 
> motherboards, but with little success.  The symptoms are a bit odd.  If 
> I do not have an AUX slot video board inserted, it works just fine 
> (albeit in 40-column mode only).  With any board (I've tried several) in 
> the AUX slot, the display becomes total garbage - and it's not limited 
> to an "every other character" issue.  My first thought was a problem 
> with one of the custom gate-array chips and started substituting from a 
> second, known good board. This got me nowhere.
>
> Next step was to sub every socketed chip on the motherboard!  Didn't get 
> me anywhere either.  Now, I'm down to suspecting either memory, or one 
> of the soldered-in 74-series logic chips.  However, it's hard to see how 
> it could be memory, since the machine works fine with nothing in the AUX 
> slot.
>
> Any ideas?

The AUX slot board supplies latched video data during ph1 low period. 
PH1 is on pin 36 of the AUX slot. If for whatever reason it is stuck 
low, the AUX card will always drive the video data bus resulting in 
garbage.
Bad buffer on the AUX card (LS374) will do the same.

If you don't have a scope, buy at least a logic probe. Cheap and quite 
useful.


-Alex.



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