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Ken · 4th Aug 2017, 8:33 pm

Hello Jay,
Just watched your video. Very interesting. I tried to scope pin 16 of the ula. I have the instructions to build the ZX81 and this has a circuit diagram. I noticed that pin 16 was connected to the lead that passes through a sort of plastic grommet on the side of the modulator so I connected the scope to this and earth. This gave an output which did not appear to change with key presses. However, as I said I rarely use my scope and I was not sure I had it set up properly to do this. I could however detect a voltage of about 2.8V with a digital multimeter. This fell by a bit over 0.1V each time a key was pressed. The ula also ran fairly hot. Now I'm not sure if this suggests that the ula is working but I thought it was hopeful. I am wondering if I should just build your little circuit to give composite video and depending on whether I see the cursor or not it would tell me if it was the ula or the modulator at fault.

Regards,

Ken.

4th Aug 2017, 8:33 pm	#52
Ken Hexode Join Date: Oct 2003 Location: Near Glasgow, UK. Posts: 309	Re: Sinclair ZX81 Hello Jay, Just watched your video. Very interesting. I tried to scope pin 16 of the ula. I have the instructions to build the ZX81 and this has a circuit diagram. I noticed that pin 16 was connected to the lead that passes through a sort of plastic grommet on the side of the modulator so I connected the scope to this and earth. This gave an output which did not appear to change with key presses. However, as I said I rarely use my scope and I was not sure I had it set up properly to do this. I could however detect a voltage of about 2.8V with a digital multimeter. This fell by a bit over 0.1V each time a key was pressed. The ula also ran fairly hot. Now I'm not sure if this suggests that the ula is working but I thought it was hopeful. I am wondering if I should just build your little circuit to give composite video and depending on whether I see the cursor or not it would tell me if it was the ula or the modulator at fault. Regards, Ken.