ZX80 trouble shooting

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: ZX80 trouble shooting

Post by 1024MAK »

Lardo Boffin wrote: Fri Sep 06, 2019 1:04 pm Where do I get the 5 and 9V rails on an 80? I’m not familiar with the layout of these.
7805 regulator pins or edge-connector pins are the easiest places.
Apart from lacking the ROMCS line, the edge-connector pin outs are the same,

Mark
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Re: ZX80 trouble shooting

Post by Moggy »

Lardo Boffin wrote: Fri Sep 06, 2019 1:07 pm
Moggy wrote: Fri Sep 06, 2019 11:34 am To use the spand on the 80 requires the modern EPROM equivalent provided by rwap which contains the extra code required for the spands commands etc. Also a couple of pins have to be soldered to the classic spand board just under the sd card slot.
When a jumper is placed on the pins then it's in 80 mode, remove the jumper for 81 mode.

Just a thought Lardo.

When I want to experiment with different EPROMS on the 81 I burn whatever image to a 2764 EPROM, rewire the pins and off you go kind of thing. A while back the lads on the German forum showed how to turn a ZX81 into a ZX80 by burning the 80's ROM code into a 2764 add a few resistors to the CPU and lo instant ZX80.

My point being is I still have that EPROM and if you can bide a while I'm willing to try it in my own 80 and if it works you can have it with my blessings.
Thanks for the info and very kind offer Moggy!

Would I have to modify my 80 to get the EPROM to work?

I assume I would still need to get an RWAP EEPROM to get the ‘spand up and running?
Alas the 2764 EPROM didn't work. :( It is an 8k device so maybe that makes a difference?

I do have some 4k 2732 chips (CORRECTION JUST ONE!!)and if anyone knows, with suitable pin swapping, they are a viable alternative then I will happily burn one for you.

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.

There are no mods required other than the new EPROM,extention cable and jumper pins soldered onto the spand.

As for my own 80 I fitted the switching reg for cool running plus one of pokemons video boards.
Last edited by Moggy on Fri Sep 06, 2019 4:34 pm, edited 1 time in total.
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Paul
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Re: ZX80 trouble shooting

Post by Paul »

Hi Moggy, did you burn the 4k ROM twice into the 8k Eprom? I guess it needs to be there from 0-4k and from 4-8k
Kind regards Paul
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Re: ZX80 trouble shooting

Post by Moggy »

Paul wrote: Fri Sep 06, 2019 3:39 pm Hi Moggy, did you burn the 4k ROM twice into the 8k Eprom? I guess it needs to be there from 0-4k and from 4-8k
Kind regards Paul
Hi Paul.

I did burn the image twice but without checking I'm not sure if the pin wiring I did for the ZX81 is suitable for the ZX80.

I also found this picture on Grant Searle's page which shows an easy fix for a 2732 which I might try just for the exercise if nothing else.

Kind Regards.

Moggy.
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Moggy
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Re: ZX80 trouble shooting

Post by Moggy »

Not a happy tabby at the moment. :(

The good news.

Yes a 2732 EPROM does work in the manner suggested by Mr Searle YAY! :D

The bad news.

I seemed to have done my usual moggy trick of F!@%king up everything I touch!
In short I now have no display just a white screen.
Refitted original ROM chip, tried various good CPU's etc no joy.
I would add that this happened after I removed the 2732 and only when messing around with other CPU's and ROMS so would have thought the 2732 played no part in it.

The only test gear I have is a multi-meter and each chip shows 5v where 5v should be, the power supply isn't being dragged down to something ridiculous as in a short somewhere so can only assume I've buggered up one of the many chips. :( :( :( So any help would be most welcome from the forum gurus.

In the meantime I have converted a ZX81 into a ZX80 with grateful thanks to our German brothers and the only bother is going back to tape loading.
Lardo Boffin
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Re: ZX80 trouble shooting

Post by Lardo Boffin »

Argh! Sorry to send you down the road to disaster Moggy!
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Moggy
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Re: ZX80 trouble shooting

Post by Moggy »

No need for any apologies lardo. :D

I don't have the collectors reverence for these old puters and am willing to accept any damage caused by my electrical musings. :lol:

I'm sure it can be recovered and if not I still have the cases and no doubt I will get another board someday also the 2732 EPROM works so some good came of it!
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1024MAK
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Re: ZX80 trouble shooting

Post by 1024MAK »

Hi Moggy

So just to be clear, the only chips you changed were the Z80 and the ROM/EPROM chips, yes?

The symptoms are a white screen only (no inverse K) and the +5V supply is present and correct on all chips.

So my questions so far are (and forgive me if I start with the obvious things that you may have already checked):
  • You have checked for bent pins?
  • You have put back the previously known good ZX80 ROM chip?
  • You don’t give any details of the Z80 chips that you were trying, were all of them known good Z80s? Or did you put back the original known good Z80?
  • Have you tried the Z80 chips in other machines to see if they still work?
  • Have you had a close look at the Z80 socket and the ROM socket to see if any of the socket contacts have become damaged? A magnifying glass and good daylight / a good light are useful for this.
  • Have you checked the underside of the board to see if there are any dry or cracked solder joints? Start with the Z80 socket and the ROM socket. Again, use a magnifying glass and good light.
I’ll be back later with some tests. It would help if you can give me some details of your multimeter. I presume you don’t have a logic probe?

Mark
ZX81 Variations
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ZX81 Video Transistor Buffer Amp

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Re: ZX80 trouble shooting

Post by Moggy »

1024MAK wrote: Sat Sep 07, 2019 1:56 pm Hi Moggy

So just to be clear, the only chips you changed were the Z80 and the ROM/EPROM chips, yes?

The symptoms are a white screen only (no inverse K) and the +5V supply is present and correct on all chips.

So my questions so far are (and forgive me if I start with the obvious things that you may have already checked):
  • You have checked for bent pins?
  • You have put back the previously known good ZX80 ROM chip?
  • You don’t give any details of the Z80 chips that you were trying, we’re all of them know good Z80s? Or did you put back the original known good Z80?
  • Have you tried the Z80 chips in other machines to see if they still work?
  • Have you had a close look at the Z80 socket and the ROM socket to see if any of the socket contacts have become damaged? A magnifying glass and good daylight / a good light are useful for this.
  • Have you checked the underside of the board to see if there are any dry or cracked solder joints? Start with the Z80 socket and the ROM socket. Again, use a magnifying glass and good light.
I’ll be back later with some tests. It would help if you can give me some details of your multimeter. I presume you don’t have a logic probe?

Mark
Some excellent advice there Mark as always. :D

Only the ROM and CPU's changed but I will try the CPU in other zeddies. I have put back original ROM chip.

One point you make is about the sockets.

The sockets fitted to this board are of the type with the tiny holes so I was very careful about bent pins etc BUT whereas normally with this kind of socket a firm press is usually needed to seat the chip, the ones fitted to this 80 are so slack that chips could fall in under their own weight!! Exaggeration I know but you get the picture.

I will remove and replace with better sockets and report back.
I will try all CPU's in other zeddies (mixture of NEC Zilog and the Italian one)
The board underside is very good on this 80 solder all nice and shiny and must have been kept in a dry environment.

My multimeter is just the usual digital cheap and cheerful type volts amps and resistance etc.

5volt where 5volt should be and no dramatic pull down of the input voltage in the manner of a short etc, 12 volt drops to about a shade over 9.5v with RAM pack fitted 10volt without. which is normal for this set up.
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Re: ZX80 trouble shooting

Post by Moggy »

Story so far.

Tried 4 known good and tested CPU's

Replaced ROM with ZX81 ROM as I have a glut of them and can afford to lose probably one or two trying.

Replaced the composite video board with original modulator just in case that had blown.

No joy just a white screen on power up.

I'm now officially stumped!

Only 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.
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