ZX80 Repair.

[avrovulcan]

Hi all,

Looking for some advice. I bought a ZX80 about 5 years ago of Ebay. Only things i could see that we're obviously wrong was IC6 (74LS157) was cracked and I replaced this quite early on. Since then the computer has been sitting in my 'to-do' pile. Today I pulled it out and replaced the resonator, which was taped to the back of the PCB when I received it way back when.

Only thing else I have noticed is that there is piece of wire connected to pin 13 of the ROM. Should this be connected somewhere, to another pin on the ROM? The ROM is a Hitachi HN462532.




Cheers in advance

Andy

[Moggy]

A quick search of the forum will bring up a topic or two on the ZX80 ROM.

For example...

https://www.sinclairzxworld.com/viewtopic.php?t=3281

A couple of pictures showing the required mods for 2732 and 2532 EPROMS.

I can vouch for the 2732 version as I have tried it and it works, as for the 2532 I have no actual experience and can only presume the mod as shown is correct.

[1024MAK]

A photo would help...

Pin 13 is a data bus line. It's unusual for any wiring to be connected to any of the data bus lines.

ZX80 ROM

Mark

[avrovulcan]

Hi thanks for the replys.

I have taken a couple of pictures already. But I'm assuming I have to use a photo hosting site to upload. Can anyone suggest a free one as I'm loathed to pay a fee for something I will only use on the odd occasion.

Cheers

Andy

[msknight]

There is a function for attachments. You'll see it at the bottom of the post. I'd recommend reducing the resolution to a reasonable level, as some cameras these take take gajillions of pixels. I normally reduce mine to about 1,200 pixels on the longest axis, or less if it still looks reasonable.

[Moggy]

I believe the forum software shrinks photos to fit Perhaps Mark could clarify?

Just use the ATTACHMENTS tag, the ADD FILES tag, it's self explanatory from there.

[avrovulcan]

Hi guys.

Cheers for the help. Please see the picture.
I have no background on this machine, other than the solder looks professional rather than kit built. TBH the wire could have been jammed in there for 'reasons'??
There appear to be no other roach or bodge wires anywhere on the PCB or on the ROM. Certainly there appear to be no lifted pins etc.

Only other thing for me to do is change the 7805 as a matter of course. Again, the liklihood is this machine is unlikely to have been powered up in 35+ years. I'm not a big believer in changing components for the sake of it, but.

Andy

[Moggy]

Looks wrong at a guess.

I would think the mod shown in the top picture I posted is probably correct as I seem to remember that this EPROM needs to have its programm voltage pin held at 5 volts when in use and the mod as shown would certainly achieve that.

[1024MAK]

I believe the forum software shrinks photos to fit Perhaps Mark could clarify?

Yes, photo/picture/image files will be automatically compressed to save space.
There is also an absolute restriction on file size for any attachment. Also, the number of attachments and the allowed file types.

TBH the wire could have been jammed in there for 'reasons'??

That looks, from your photo, like an off-cut of a piece of wire that has got lodged there. I don't know of any reason for this wire being there. If that was mine, or someone had asked me to service or repair it, I would carefully remove the EPROM to check the socket.

If you don't have experience of removing chips from sockets, please ask me to point you to some advice.

Only other thing for me to do is change the 7805 as a matter of course. Again, the liklihood is this machine is unlikely to have been powered up in 35+ years. I'm not a big believer in changing components for the sake of it, but.

7805 voltage regulators are generally speaking, reliable devices. The primary reason that they get replaced now, is that they are not very efficient and they produce a lot of heat. Hence DC/DC converters/switching regulators (which run cool) are preferred if people are not bothered about keeping their machine 'original'.

If you suspect the 7805, supply the board with a 7V regulated supply and measure the output (+5V) with a multimeter.

If the output is within spec (4.75V to 5.25V) then it's likely to be fine.

If you do decide to replace it with another 7805, do buy a legitimate part, not a cheap substandard part from eBay or other trading site.

Looks wrong at a guess.

I would think the mod shown in the top picture I posted is probably correct as I seem to remember that this EPROM needs to have its programm voltage pin held at 5 volts when in use and the mod as shown would certainly achieve that.

Agreed. The VPP pin needs to be connected to the +5V supply. Being connected to address line A12 is not recommended.

Mark

[msknight]

If you do decide to replace it with another 7805, do buy a legitimate part, not a cheap substandard part from eBay or other trading site.

Is there any way of telling these substandard parts from the legit ones, perchance? ... asks the person who certainly did not, in any way, shape or form, just buy some of these https://www.ebay.co.uk/itm/155949855428 from ebay.

Michelle.