Sinclair ZX80 / ZX81 / Z88 Forums

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

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, fol...

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...

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 ...

Thanks I will try this tomorrow evening

Just to be sure cutting this track is enough... I don't need to lift or cut any legs on memory chips when I do this?

1024MAK wrote: ↑Wed Jan 31, 2024 4:54 pm
The only way to tell beyond doubt is to cut the PCB track for the +12V supply going to the 'lower' DRAM chips.

Mark

I can do this as it is reversible, where would be the best place to do this on the 3B board?

I suspect you have a "lower" DRAM chip fault. Are any of these 4116 (or equivalent)(IC6 to IC13) DRAM chips getting hotter than the others? Are you okay with cutting a PCB track and cutting or lifting a pin on each of the 4116 DRAM chips? Mark I ran the diagnostics from dandantor a few days ago whe...

So having another look at this. I'm getting 0.71v on this test. Does that mean the TR5 may need replaced?Screenshot_2024-01-29-20-35-23-450-edit_com.android.chrome.jpg This appears to be in circuit testing with the power off. I need to check this before answering. However, I would like you to carry...

Can you please check the polarity of the two electrolytic capacitors that are fitted as part of the DC/DC converter upgrade. One (C78, 22µF) I'm not sure of the polarity from the photo. The second (C79, 1µF, in position D16) looks like it may be fitted with the incorrect polarity. Mark Hi Mark, the...

So having another look at this. I'm getting 0.71v on this test. Does that mean the TR5 may need replaced?