ZX80 Oddity with Replacement ROM

[RWAP]

I have a customer who has a ZX80 and has purchased the ZXpand expansion kit, which includes a flash EL-2364 EPROM programmed with the ZXpand version of the ZX80 ROM (done by SirMorris).

Oddly his ZX80 works fine with the original ROM, but when he swaps it for the EL-2364 EPROM, his ZX80 does not start up.

I sent him a replacement EL-2364 EPROM to try, but it was the same result. SirMorris has also tried one in his own ZX80 and it works fine.

Any ideas?

[Moggy]

At the risk of stating the mundane, has he inserted the EPROM the correct way round?

I made that mistake with my 80 as the new EPROM didn't seem to be as well marked as to orientation as the original ROM.

Logic dictates that what works with the original device should also work with the new one or what's the point?
There were two issues of the ZX80 as far as I can recall maybe there is some electronic difference between the two?
EG the new EPROM may work on an issue two but not an issue one due to some obscure timing differences, for example.

Just a speculative guess on my part though.

[britain4]

As a complete stab in the dark replacing the Z80 solved the problem you described on mine (although I never swapped the ROM) so if it was mine I'd be tempted to try a different one to eliminate any weird timing anomalies as a cause

[RWAP]

At the risk of stating the mundane, has he inserted the EPROM the correct way round?

I made that mistake with my 80 as the new EPROM didn't seem to be as well marked as to orientation as the original ROM.

Logic dictates that what works with the original device should also work with the new one or what's the point?
There were two issues of the ZX80 as far as I can recall maybe there is some electronic difference between the two?
EG the new EPROM may work on an issue two but not an issue one due to some obscure timing differences, for example.

Just a speculative guess on my part though.

That was my thought too - SirMorris was going to take a photo of the correct orientation, but hasn't yet!

[VectrexRoli]

Hi, I was that customer and Rich proposed I should post some photos here. Maybe someone has a good idea what else I could try before I give up the idea using my wonderful ZXpand also on my ZX80 machine?

This is my ZX80 "ISSUE 2A" PCB and I did the following modifications, before I wanted to try the ZXpand conversion kit:
I replaced the 2 electrolytic capacitors with new ones, I built in a "ZX8-CCB" video out mod and I replaced the old 7805 voltage regulator with a modern 5V Tracopower TSR 1-2450 DC-DC converter. The heatsink is not really needed anymore, but I kept it in. I use a modern strong voltage regulated power supply set to 9 Volts for DC input.

The machine runs fine with the old original Sinclair 4k ROM, I get a nice looking video signal, I can run, load and save Basic programs. I neither connected the ZXpand nor any other memory expansion so far. I can measure 4,98V on the power pins on my ROM socket.

Next I take out the original ROM and put in the replacement ROM, it sits in the socket perfectly no bent legs (see photos), I can see using my multimeter that I have contact for each pin of the replacement ROM down to the soler pads of the PCB, so the ROM socket is also fine.
When I switch on my machine (again neither the ZXpand nor a memory expansion is connected), the screen stays black and the machine is not booting up. I changed the relacement ROM with another one Rich sent me and I also replaced the Z80 CPU with another working one, no change in behaviour.

Rich had the idea now, that I put the replacement ROM in the wrong way, so I just tested that with 1 of the 2 replacement ROMs I got and I put it in the other way round (which would be the wrong way in my opinion) and as expected it got very hot after 2 seconds, so I switched it off immediately and this makes me pretty confident the first way was the correct one.

Anyway you can see all of that on the attached photos, both Z80 CPUs I tried, the working screen and both ROM types. If someone has any further good idea, it would be highly appreciated. Thanks, roli.

[1024MAK]

This is my ZX80 "ISSUE 2A" PCB and I did the following modifications, before I wanted to try the ZXpand conversion kit:
I replaced the 2 electrolytic capacitors with new ones, I built in a "ZX8-CCB" video out mod and I replaced the old 7805 voltage regulator with a modern 5V Tracopower TSR 1-2450 DC-DC converter. The heatsink is not really needed anymore, but I kept it in. I use a modern strong voltage regulated power supply set to 9 Volts for DC input.

I would not expect any of these modifications to make any difference in respect of changing the ROM for a modern equivalent.

The Z80 CPU in the picture, is this what your ZX80 came with, or is that the "replacement" CPU that you tried? The Sinclair ROM in the pictures, same question, is that what came with it, or is it a "replacement"?

Only back in 1980, the Z80 CPU would have been a NMOS type. And so would the ROM. Where as modern chips (CPU and the ZXpand ROM) are modern CMOS. This should not make any difference, but it is possible that something is causing a communication problem between them.

As the CPU does everything including the screen display, if the CPU does not get the correct instructions from the ROM, there will be no meaningful picture.

Apart from the multimeter that you have, do you have any other test gear (or have access to any)?

Mark

[britain4]

It's annoying having weird problems like this, I will be interested to see what it is when it gets sorted

Have you checked the resistance of the pins down to the solder pads on the PCB not just continuity - I don't think it's beyond the realms of possibility that the contacts could have corroded in such a pattern that a replacement chip would not make proper contact but the original ROM still does

Also to echo what 1024MAK said I would still want to try a modern CMOS Z80 chip just to rule it out, and even if it doesn't solve the problem, it'll run cooler and more reliably when you do get it sorted

Lastly just to cover all bases could you have accidentally touched the ZX8CCB - I had a ZX81 I thought was broken for about 18 months until I realised I'd accidentally moved it out of adjustment

[1024MAK]

The Z84C0020PEC Z80 CPU in the pictures is a CMOS type.

Mark

[Moggy]

I can confirm what Mark says in his post about the replacement parts making no difference as my ZX80 is Identical(issue2) to yours, Same Traco Reg, same video device as yours and same cap replacement.

I do differ however in having original NMOS Zilog z80.

I notice that the "original" ROM has a sticker placed over it denoting what it is rather than the "printed" on decal that was standard practice for these chips. I have owned five 80's over the years and never seen a "stickered" ROM so as Mark suggests is this a replacement chip with perhaps a timing difference?

I only suggest this as forum member Andy Rea discovered a timing incompatibility with ROMS made by certain manufacturers for the ZX81 when creating,I think, his ULA device and I myself found a certain brand of ZX81 ROM that had timing issues with a Hunter board I own.

[britain4]

The Z84C0020PEC Z80 CPU in the pictures is a CMOS type.

Mark

Oh sorry, I misunderstood what you said, ignore that then!