ZX81 resurrected
Posted: Sat Dec 01, 2018 8:32 pm
I've had this ZX81 sitting in a box for about 15 years gathering dust and after regaining ym interset in old 8 bit computers after repairing a few other machines I thought I'd give this a go.
There was no PSU with it so I hacked one togethor from a 9V 1A adaptor and a 3.55mm power plug from an even older PSU which I had lost years ago (but still had all the plugs for).
Unfortunately there was no video output at all, no matter how much I tuned the TV, so figuring it was because it just didn't like the LCD TV I cracked it open for some surgery.
First problem was, as you probably guessed, the ribbon cable for the keyboard snapped as I removed the motherboard. I thought I could get away with trimming it back but the 8 pin socket had suffered as well and it wouldn't retain the cable. I put that aside for later as my first goal was to get a display.
Inside was an Issue 3 motherboard and it seemed almost pristine condition. I first attempted the transistor modification on the modulator but again, no signal on the TV. I then replaced the 3 capacitors on the motherboard but again no joy on the video output.
Checking the power supply I could see 4.93V going into the CPU, ULA and memory chips which seemed a little low so I popped a new 7805 regulator in. Now seeing 5.03V but still no video.
Thinking I had c****d up the video mod I splashed out on a ZXVID board and swapped out the contents of the modulator box. Still no video signal
I thn broke out the cheapie oscilloscope I had bought and never used. the clock pin on the ULA showed a wave form but it was nothing like a clock signal. IN fact the same waveform was seen on just about every pin on the board so I concluded the ULA was faulty.
Fortunately I was able to get a replacement with a heatskink attatched so I removed the suspect ULA, put a 40 pin DIL socket on the board and plugged in the new ULA.
WIth fingers crossed I applied power again and..... a grey screen with a black 'K'! Success!
The final step was to replace the broken keyboard connector with a new one and since I was feeling generous I peeled the old keyboard off and stuck on a brand new one.
SO now I can add this to the pile of reconditioned 8 bit computers in the mancave.
There was no PSU with it so I hacked one togethor from a 9V 1A adaptor and a 3.55mm power plug from an even older PSU which I had lost years ago (but still had all the plugs for).
Unfortunately there was no video output at all, no matter how much I tuned the TV, so figuring it was because it just didn't like the LCD TV I cracked it open for some surgery.
First problem was, as you probably guessed, the ribbon cable for the keyboard snapped as I removed the motherboard. I thought I could get away with trimming it back but the 8 pin socket had suffered as well and it wouldn't retain the cable. I put that aside for later as my first goal was to get a display.
Inside was an Issue 3 motherboard and it seemed almost pristine condition. I first attempted the transistor modification on the modulator but again, no signal on the TV. I then replaced the 3 capacitors on the motherboard but again no joy on the video output.
Checking the power supply I could see 4.93V going into the CPU, ULA and memory chips which seemed a little low so I popped a new 7805 regulator in. Now seeing 5.03V but still no video.
Thinking I had c****d up the video mod I splashed out on a ZXVID board and swapped out the contents of the modulator box. Still no video signal
I thn broke out the cheapie oscilloscope I had bought and never used. the clock pin on the ULA showed a wave form but it was nothing like a clock signal. IN fact the same waveform was seen on just about every pin on the board so I concluded the ULA was faulty.
Fortunately I was able to get a replacement with a heatskink attatched so I removed the suspect ULA, put a 40 pin DIL socket on the board and plugged in the new ULA.
WIth fingers crossed I applied power again and..... a grey screen with a black 'K'! Success!
The final step was to replace the broken keyboard connector with a new one and since I was feeling generous I peeled the old keyboard off and stuck on a brand new one.
SO now I can add this to the pile of reconditioned 8 bit computers in the mancave.
