Sinclair ZX80 / ZX81 / Z88 Forums

Hi Mark

With TR4 and 4116's out of circuit (i've never put them back in, wanted to ensure all my voltages were right, first) the current draw is 397-404mA

Hi Mark The 4A currently has it's ram all removed. I think something must have given out when i was scoping it out for the clock pulses for comparison with the 4S when I was sorting it out My limited knowledges suggests something must be shorting to ground for it to be tripping the current limiter l...

So here's where i'm at: Issue 4A: I got some ZTX213s to replace TR5 again, did so - still trips current limiter, and tested the ZTX753 that I put in (it's OK too) Even replaced the 7805 and tested the ZTX650 on TR4 - both OK So i've gone from a 4A with working 5v, -5v and low 12v with a number of br...

Quick update

4A: I've ordered an original ZTX213 to replace TR5 - hasn't arrived yet.

4S: - replacing TR7 and D13 resurrected the audio, next to get it loading tapes!

I found where I thought TR5 could be replaced with a BC213.... a diagram on a website belonging to one 1024MAK

Since received some ZTX753, but the bench supply trips it's 800mA limiter with one of those for TR5
Back to the drawing board!

I did a composite mod to the 4A to ensure the RF modulator wasn't in the way. Initially I had a screen (white border, funky corrupt content) which I expected given i had no lower ram installed, then it went away :-) further inspect finds R59 and R60 OK, but the clue is... i'd already replaced TR5, b...

TR4, and all electrolytic caps on the 4A have already been replaced.
I do have spare ZTX650s, so I guess I could double check the caps out of circuit and replace TR4 again just in case

Good news on the 4S Armed with clues from other posts from other people, I checked clock pulses and found I had none. I removed the RF modulator and cleaned some corrosion from the underside, and replaced the X1 crystal. Hey presto, clock pulse returned, and this 4S now boots As you can see from the...

As with all things, i'm doing things wrong and backwards and posting the requested "hello" post after already seeking help on the boards. My first experience of Sinclair computers was when my dad borrowed a family friend's ZX81 - I was about 4yrs old at the time and didn't get to use it, but remembe...

Not getting too far here - not convinced the cheap and nasty Hantek USB oscilloscope I purchased is either working or any good, since it doesn't ever appear to show square waveforms, only a constant DC power level - will see if I can test it on something known good But some voltages: ULA IC1 pin 14 ...