Thread: Zx81
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Old 28th Apr 2020, 6:25 pm   #283
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 8,544
Default Re: Zx81

Originally Posted by John Earland View Post
I have checked the pins to the components on the board and they are all connecting.
That is quite a general statement so I'm not sure precisely what you have and have not done.

For example, you've checked out the connections from the socket contacts to the pins on the underside of the board which is certainly a valid test / task, but really you need to check that there is a connection from every one of the 40 pins of the ULA to every other place that each of those pins should be connected to. That will be difficult to do in the purely physical trace-by-eye sense because tracks disappear under ICs, zip through holes from one side of the PCB to the other and back again somewhere else, making them very hard to follow.

What we need now is for you to start to get more familiar with reading circuit diagrams, so that you yourself can see, from looking at the diagram, which connections need to be checked out.

Lucky this is a ZX81 and not your PET, where each data line alone would probably swing past around a dozen ICs.

I don't know how you want to proceed from here - you are tremendously single minded and diligent when in pursuit of a problem so I have no problem with trying to walk you through testing all the connections but it will take a lot of time and numerous posts.

As a final resort, if you wanted to send it (just the PCB) up to me I could take a look, I have an 'open plan' ZX81 where the ULA is in a socket, so the first thing I would do would be to put your super-ULA in my ZX81 to test that out and make sure the ULA has not somehow suffered damage before looking around elsewhere. I could also read out the ROM and compare it with one of my known good ROMs or with a known good ROM image from the net. I have all the other required accessories including a keypad so I would not need the power supply, video lead, RAM pack, etc.

I don't have a listing of the ZX81 ROM but I am wondering if your problem (freezing when trying to invoke load from cassette) points to the unit having problems when trying access a particular part of the ROM or a particular address range within the ROM. It could even be the ROM itself faulty (one or more corrupt bytes) but if so it is odd that you didn't seem to have this problem earlier... or did you?

I have a feeling this one is not going to be easy, as it seems to have behaved badly and intermittently at every stage even before you changed the ULA - it seemed pretty clear that the original ULA had a thermal fault so it is disappointing that you are still having problems after doing such good work to replace it.
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