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Old 7th Nov 2020, 1:29 am   #73
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
Default Re: I found it! A very sorry looking MK14.

Reverse 'C' is, as we remember, 0x0F, or 00001111 on the databus. That's what is on the data bus when nothing else is driving it.

We have two proven examples of the issue VI working here already so we know the PCBs are OK, assuming yours was made from the exact same design files. I think I'm right in saying that the 2111 / 6xxx RAM type select link on the underside of the board should already be made in favour of 2111, but please do check that.

One possibility is that one of your bipolar PROMs may have gone faulty - they are allegedly prone to a variant of 'tin whisker disease' which happens whether the device is powered or not so it is quite possible for them to work one minute and not the next. There are various ways to check the PROMs, but in any case I think you have two sets there don't you - 'old' OS and 'new' OS - so try the other set.

Assuming you have some sort of device programmer, is it one which can also functionally test TTL ICs and maybe RAMs as well? If so, consider just running all the standard ICs through that to see if they are all OK.
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