Help resurrect my Xerox Alto

Tony Duell ard at p850ug1.demon.co.uk
Wed Sep 22 13:05:13 CDT 2010


> 
>   Thanks Al,
> 
>      We are doing component testing of all the electronics in the 
> monitor first, before we do any further testing with the actual alto.  
> We do see the tube glowing, also the transistors all seem to be 
> working.  We are however NOT getting any static on the screen, I don't 
> think the flyback is outputting high voltage, we are working our way 
> backwards to find the culprit.
> 

I seem to rememebr seeing schematics for the Alto monitor on bitsavers 
(but I can't rememebr them in detail).

I assume you don't have an EHT voltmeter, otherwise you'd have measured 
the fianl anode voltage on the CRT by now :-)

OK, the first thing to do is see how the CRT heater is powered (since you 
know that's working). Most of the time it just comes from a PSU 
regulator, but occasionally, it's powered by a little winding on the 
flyback transfotmer. If the latter, then if the heater is glowing, you 
know the horizotnal output staage is doing something.

Next masure the CRT pin voltages. On a monchrome CRT you'd expect, 
perhaps, 700V on some of the electrodes, but not much more. Which means 
the votlages are witin the range of most multimeters. If you can't find 
suh voltages, it's a petty good bet you have problems in the horizontal 
section.

One trick I hesitate to mention, since there's the posibilty of doing 
damage, is to _momentarily_ short the cathode to grid (g1) on the CRT. By 
removing hte -ve grid bias, you should get a very bright screen. But 
don't leave thsoe electrodes shorted together, you can damage the CRT due 
to excessive beam current. Just a quick flick with a bit of wire.

-tony




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