Dead ZX81.

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Dead ZX81.

Post by 1024MAK » Mon Sep 17, 2018 1:42 am

The ULA chips always run hot. That’s normal. The ULA is also the most likely cause of the failure.

The photo you posted, that’s an issue 3 board. It is functionality the same as the earlier issue 1 board (there are no known issue 2 boards in existence). The ULA and Z80 CPU can be swapped between boards if needed.

The wires to the ROM chip are due to the chip being an EPROM instead of a custom mask ROM. There are some differences between the pin allocations (pin-out) for these chips. We suspect the factory making ZX81 boards ran out of the custom mask ROM chips. Custom programmed EPROMs are quicker to be produced compared to custom mask ROM chips.

As this ZX81 was produced in 1983, maybe Sinclair did not want to spend lots of money on a new batch of custom mask ROM chips (they have to be ordered in large quantities). As the ZX Spectrum was continuing to sell well, but sales of the ZX81 were falling.

Both the custom mask ROM chip and the custom EPROM do the same thing as far as the CPU and ULA are concerned.

Mark

flaviosr
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Re: Dead ZX81.

Post by flaviosr » Sun Sep 23, 2018 8:50 pm

1024MAK wrote:
Mon Sep 17, 2018 1:42 am
The ULA chips always run hot. That’s normal. The ULA is also the most likely cause of the failure.

The photo you posted, that’s an issue 3 board. It is functionality the same as the earlier issue 1 board (there are no known issue 2 boards in existence). The ULA and Z80 CPU can be swapped between boards if needed.

The wires to the ROM chip are due to the chip being an EPROM instead of a custom mask ROM. There are some differences between the pin allocations (pin-out) for these chips. We suspect the factory making ZX81 boards ran out of the custom mask ROM chips. Custom programmed EPROMs are quicker to be produced compared to custom mask ROM chips.

As this ZX81 was produced in 1983, maybe Sinclair did not want to spend lots of money on a new batch of custom mask ROM chips (they have to be ordered in large quantities). As the ZX Spectrum was continuing to sell well, but sales of the ZX81 were falling.

Both the custom mask ROM chip and the custom EPROM do the same thing as far as the CPU and ULA are concerned.

Mark
Ok. "I have decided" the ULA is the issue... but I do want (neither I have the abilities) to unsolder the "other" ULA to make the test! :(
So now I have to find another ULA... I have also read that some people "created new ULAs" but I have also read they are not sold in this period! :(
I correct myself!!! They are!!! I think I will be a new tester for these new ULAs...

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Andy Rea
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Re: Dead ZX81.

Post by Andy Rea » Tue Sep 25, 2018 12:07 pm

For unsoldering the old ULA ( if your certain you don't want to keep it ) cut each of the pins one by one with a small pair of side cutters close to the chip body, you'll then have 40 pins sticking out of the board, you can then easily remove the pins one at a time with tweezers and a soldering iron ( sometimes a little fresh solder can help )

Once all pins are removed using a solder sucker or flux and good quality solder braid clear each hole before refitting a good quality socket ( after you have clean and inspected the board for any damages )

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

flaviosr
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Re: Dead ZX81.

Post by flaviosr » Tue Sep 25, 2018 4:39 pm

Andy Rea wrote:
Tue Sep 25, 2018 12:07 pm
For unsoldering the old ULA ( if your certain you don't want to keep it ) cut each of the pins one by one with a small pair of side cutters close to the chip body, you'll then have 40 pins sticking out of the board, you can then easily remove the pins one at a time with tweezers and a soldering iron ( sometimes a little fresh solder can help )

Once all pins are removed using a solder sucker or flux and good quality solder braid clear each hole before refitting a good quality socket ( after you have clean and inspected the board for any damages )

Regards Andy
There is a bit of confusion.
My ZX81 that IS NOT working has all 3 chips on sockets.
My ZX81 that IS working has all 3 chips soldered on the mobo.
So I can easily make tests on the (up to now) dead ZX81 but I do not want the take the risk to ruin the working Zeddy...

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1024MAK
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Re: Dead ZX81.

Post by 1024MAK » Tue Sep 25, 2018 4:50 pm

Yeah, what you want is one of these shiny new ones.

Mark

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Andy Rea
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Re: Dead ZX81.

Post by Andy Rea » Tue Sep 25, 2018 7:09 pm

flaviosr wrote:
Tue Sep 25, 2018 4:39 pm

There is a bit of confusion.
My ZX81 that IS NOT working has all 3 chips on sockets.
My ZX81 that IS working has all 3 chips soldered on the mobo.
So I can easily make tests on the (up to now) dead ZX81 but I do not want the take the risk to ruin the working Zeddy...
ah my bad :cry:

so this is good then that the broken zeddy has sockets and should be fixable.

Regards andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

flaviosr
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Re: Dead ZX81.

Post by flaviosr » Wed Oct 24, 2018 9:39 pm

Some good news! I bought a not working ZX81 with socketed ROM and Z80A and welded ULA.
I put on socket the ULA so that I could make changes and test between different ZX81! :)
I also bought new Z80As.
AND!
Both Z80 of the two not working ZX81s are dead!!! And both ULA are working (with the issue of the B&W TV).
The "new" ZX81 is now fully working (B&W issue but I have also found a new LG full HD TV that works well!!! :) ...)
While the "old" ZX81 is resisting. I have also bought a new ZX81 ULA and made the Z-Video mof.
The last thing I can do is to replace both transistors and capacitors. They seems ok but who knows... expecially transistors can be bad beasts!

I keep you updated! I have to find new components that are compatible...

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1024MAK
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Re: Dead ZX81.

Post by 1024MAK » Thu Oct 25, 2018 2:15 am

The transistors used in the ZX81 are fairly robust and failure of them is very, very rare. So unless you have a good reason to suspect them, my recommendation would be to leave them alone.

Mark

flaviosr
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Re: Dead ZX81.

Post by flaviosr » Thu Oct 25, 2018 8:22 pm

1024MAK wrote:
Thu Oct 25, 2018 2:15 am
The transistors used in the ZX81 are fairly robust and failure of them is very, very rare. So unless you have a good reason to suspect them, my recommendation would be to leave them alone.

Mark
The ZX80A and the ROM are good since I have tested in the working ZX81.
The resistors are ok (they are visually ok).
When I turn on the ZX81 I feels that the problems is the same as for the "other" ZX81 with only a dead Z80A...
I have finished my ideas... :( apart the transistors and the capacitors.
I do not think that the two memory chip, even if dead, could stop the startup...

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1024MAK
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Re: Dead ZX81.

Post by 1024MAK » Thu Oct 25, 2018 11:32 pm

A faulty Z80 CPU, a faulty ULA, a faulty ROM or a faulty RAM chip could and would prevent start-up.

A poor connection (socket, dry joint, broken/cracked track) in any of the many critical circuits could also prevent start-up.

Mark

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