Sinclair ZX80 / ZX81 / Z88 Forums

Moggy wrote: ↑Sat Sep 07, 2019 5:18 pm Tried 4 known good and tested CPU's

So it’s not a faulty Z80 CPU then.

Moggy wrote: ↑Sat Sep 07, 2019 5:18 pmReplaced ROM with ZX81 ROM as I have a glut of them and can afford to lose probably one or two trying.

I presume these are known to work in a ZX81, so it’s not a ROM chip fault then.

Moggy wrote: ↑Sat Sep 07, 2019 5:18 pmReplaced the composite video board with original modulator just in case that had blown.

With a white screen, I think this unlikely anyway.

Moggy wrote: ↑Sat Sep 07, 2019 5:18 pmOnly other thing to mention is that on my composite zx81's there is a small DC voltage at the video outlet that is missing on the ZX80.

I would not worry about this. On the ZX80 the video signal comes from two 74 series chips via resistors, so you will get a different average DC voltage.

Going on what you say so far, I’m still suspicious of poor socket contacts.
The other possibility is that one of the 2114 SRAM chips has developed a problem. First off, try reseating them in their sockets.

Mark

Unfortunately the RAM chips are not socketed but tried with RAM pack and no difference.

Checked for continuity from underside of board to each pin on CPU and ROM and all ok with nil resistance.

Hmm, I’m going to have to think about this....

Mark

Too much whisky and too tired to go further but if needs be then I will happily solder the chips directly to the board.

Many thanks Mark by the way for the time and trouble you are spending on this. I am very very grateful.

Just as a side issue of no importance.

I have converted a ZX81 back to ZX80 courtesy of our German brothers and just for giggles and things I connected a classic ZXpand cw the ZX80 spand EEPROM chip and ended up with a monstrous hybrid that obeyed in a crude fashion the ZX80 ROM but also some ZX81 commands and would work in both fast and slow modes and at the same time could count above 32767.

Ahhhh to much booze methinks!!

I recommend fitting turned pin sockets. It makes life a lot easier for the future.

Mark

Well I got up this morning after drowning my sorrows over my busted 80.

I went over Marks advice and recommendations again and the bit about loose sockets seemed the only answer even though I was getting continuity from the bottom of the board to the CPU pins.

So yesterday I replaced the ROM socket and still no joy. Today I thought sod it do the CPU one as well even though I had continuity.
Now luckily with 80 the pads are nice and big and the solder soft so no problems removing socket without causing damage.

Then the flaming obvious hit me...........
The CPU is also connected to tracks on TOP OF THE BOARD!!!!! Hmmm why does the pad on the top track going to pin 20 look a funny colour compared to the rest?

Any how I bit the bullet and not having the CPU socket I thought I had in my spares box went and soldered the CPU to the board feeling I had nothing to lose.
Connect up, power on and YEEESSSSSSSSSSSSSS!!!! A lovely K cursor appeared so I ran a little test program and all is now well in Moggy's world.

I would like to express my fulsome praise and gratitude to Mark not just for his expertise and in this case spot on diagnoses, but also the time and trouble spent to arrive at same.

Please accept my thanks Marks for all you've done.


Best Regards.

Moggy.

Good to hear that your ZX80 is up and running again

Mark

Moggy wrote: ↑Fri Sep 06, 2019 2:44 pm
As for the RWAP EPROM I'm not sure if the one he sells now is just a standard ZX80 coded one or the one initially sold with the ZX80 ZXpand kit consisting of spand, extension cable and EPROM containing the extra routines for the spand.

The EL-2364 EEPROM I supply as a replacement is blank - we (well SirMorris) programs them for me if required with whatever ROM image you need

Thanks for that information Rich, handy to know.

I have found a Sinclair 16K RAM pack that works! Although strictly speaking it is a Timex 1016.

I have plugged it into my ZX80 and switched it on and it switches on as normal - instantly producing the K. How do I get the 80’ to recognise it? No idea if the 80 has a built in routine to scan for extra memory.

Thanks!