Sinclair ZX80 / ZX81 / Z88 Forums

Was booting to K, but now getting this and an occasional black screen. I'm assuming the ferranti is on its way out?

After leaving for a while, and trying again, I'm now getting a fully white screen, but no K.

Done the logic tests, seems fine except A14 and A15 on the processor. On the known working zeddy, these pulse slow enough for me to see the LED change. Not on this one. It's pulsing, but the pulsing indicator LED is solid.

Possibly RAM?

I'm sure Mark will be along to solve this one when he reads it so all I can offer is that the only time I've had the "snowy" screen effect as in your picture is when I've not supplied enough voltage to a Zeddy fitted with a switching regulator. They are either on or off I find unlike the linear regs that will give at a least a picture albeit with a black wavy line running top to bottom due to being under voltage.

Just a thought nothing more and probably the wrong one.

I waited a while and just fired it up again and it came back to a K prompt. A print peek of 3901 returned 253, so likely a 649 bios.

The Mostek versions usually are.

If it is the socketed motherboard issue one we are talking about then perhaps a clean of the chip pins and a reseat may improve things.

EDIT.

Forgot to add that Mostek chip pins tend to tarnish far more readily than other chips in my experience so the above may help the ROM at least which, judging by your picture of that board, the ROM pins look almost black or is that just bad lighting?

A black screen or "static" is normally a problem with the video circuitry, which may include the ULA.

A white screen without an inverse K, can be a problem with any of the chips. But normally is either a RAM problem or ROM problem. Also possible is a processor problem, an address, control or data line fault (including open circuit or short circuit faults) or a partial ULA failure.

If the fault is intermittent, as Moggy suggests, check the chips in sockets. Also check the soldering in case there is a dry, cracked or broken solder joint or PCB pad/track (trace).

A poor 9V connection can cause erratic operation and strange faults as well. So check that. And the solder joints for the 7805.

Mark

Thanks for the advice.

What's the best way to clean chip pins and the sockets please? ie. safest, least damage risk.

Poor 9v connection? We're talking the jack socket, or possible 7805 failure? How would I check this please?

Personally I use a fibreglass stick of the kind used by coin collectors and/or polishing electrical power tool commutators, for the pins.
The sockets I find in general to be ok as is and generally clean enough , except for those types with the really tiny holes where the pins just fit, as found on some ZX80s, the socket internals tarnish very easily in those types in my experience.

There will be other methods I'm sure that those who use them will post about.

I use one of these, only cost a few pounds -


Random example :

https://www.ebay.co.uk/itm/111741724919 ... media=COPY

Sounds the same as Moggy’s suggestion.

You may be able to get it clean enough just by removing the chip and replacing it a few times but that can be risky.

Maybe a pen eraser?