Sinclair ZX80 / ZX81 / Z88 Forums

No lower memory chips hot.

Cut track and voltages now ..

TR5 emitter (should be the same voltage as the +5V rail) 3.04v
TR5 base 3.45v
TR5 collector 4.23v
R59 left hand lead (should be the same as TR5 base) 3.45v
R59 right hand lead 0.0v
R58 left hand lead (should be the same as TR5 base) 3.45v
R60 right hand lead (should be the same as TR5 collector) 4.23v
R60 left hand lead 9.48v
R61 left hand lead 9.49v
R61 right hand lead 9.87v
Also check the polarity of C74. Correct

Then with the power still on, test the voltage across R62. That is with the meter positive/red lead on R62 right hand lead and the meter negative/black lead on R62 left hand lead. 0.028v


This is a picture of the fuzzy screen I'm now getting

That's very odd.

What voltages do you get across these capacitors:
Across C50 (next to C34)
Across C34 (near the coil, right hand side of board)
Across C44 (near front of board near the Sinclair logo)
Across C45 (next to C44)
And on pin 1 on any of the 'lower' DRAM chips (IC6 to 13)?

Mark

1024MAK wrote: ↑Fri Feb 02, 2024 10:20 am That's very odd.

What voltages do you get across these capacitors:
Across C50 11.82v
Across C34 4.95v
Across C44 1.92v with track cut, 0.079v with track fix
Across C45 1.89v with track cut, 0.082v with track fix
And on pin 1 on any of the 'lower' DRAM chips? 0.059v with track cut, 0.078v with track fix

I added the voltages above. All these problems started whin I did the DCDC mod. I'm thinking of reverting back to the original design and undoing the mod.

Unless you have broken a track, damaged a through-hole plating or similar, your DC /DC modification should be working. Trying to reverse it may cause future problems.

The voltages in your last two posts are inconsistent with each other, hence I'm confused.

Can you please show me where you cut the PCB track.

Mark

Mark,
I think my board has more serious issues which have got worse the more I test it and it's powered on. When I first did the DCDC mod I had just a low 12v line with monochrome screen, but the Sinclair copyright message was displayed. Now the voltages are completely wrong with a fuzzy screen, following all the testing while it's powered up. Perhaps I shorted something while I was doing this. My hands are not as steady as they used to be
I checked the diodes in the DCDC circuit and I think they are all blown as meter shows a voltage reading checking both ways on the diode check on my multimeter.
So no point in myself doing anything else on this board - I've realised it's time to quit on it. I'll just try and find another working one.
(I did cut the track in the same place as your photo and the continuity check confirmed open circuit. I have soldered over the track again)

Thanks for your help Mark it has been appreciated

I now know what killed my board. I have a faulty dandanator. Second working board now has same symptoms as first one - same screen as last posted. No luck at all