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Old 4th Jan 2021, 9:10 pm   #38
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: Non-working Commodore PET 3016

I think the relevant diagram for the timing circuit section is this one:-

http://www.zimmers.net/anonftp/pub/c...N/320349-6.gif

Among other things, this part of the circuit generates the 'Horizontal Drive' and 'Vertical Drive' signals. You can find these signals on pins 3 and 5 of connector J7 which looks to be the mainboard's output connection to the monitor.

If those connections are difficult to get a physical connection to you can, alternatively, find the 'Vertical Drive' signal on pin 11 of IC 'UG10' and the 'Horizontal Drive' signal on pin 2 of IC 'UH7'. Try looking at each of those (with the power on of course, and with the meter in 'Hz' mode) and see if you have steady frequencies on both of them. 'Vertical Drive' I would expect to be in the region of 50-70Hz, and 'Horizontal Drive' somewhere in the tens of Khz. If you do find steady frequencies on those points it would also be interesting to know what the 'Duty Cycle' (%) figure is for each of those signals as well - that's another unusual, but potentially useful feature your new meter has.

If you don't find anything which looks like a steady frequency on those two points try looking at pin 3 of IC UG5, where you should see a frequency of 8.00MHz, and pin 7 of IC UG5 where you should see a frequency of 1.00MHz. (The main clock frequency, at 16MHz, is a bit too high for your meter to be able to measure).

All measurements to be taken with the black meter lead on a known 0V / GND point.

Edit: Crossed with Tim, who said much the same in more concise terms.

Last edited by SiriusHardware; 4th Jan 2021 at 9:23 pm.
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