My ZX80 Restoration

Discussions about Sinclair ZX80 and ZX81 Hardware
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avrovulcan
Posts: 45
Joined: Sun Nov 03, 2019 9:56 am

My ZX80 Restoration

Post by avrovulcan »

Part 1

Here's a ZX80 I pick up off ebay about a year ago. It was sold as non-working/condition unknown and without a case.
Still even for a non-runner £45 is a bargain, even if I include any replacement parts.

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First, the obvious issues. The ceramic resonator X1 is missing.

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I have replacement resonators and have also bought a crystal oscillator. I've yet to decide which way to go as regards which one I'll fit.

Secondly one of the 74LS157 multiplexer's is actually cracked right across the package

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These first two issues are easily solved. However of more serious concern is the EPROM.

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As you can see the protective cover that should be over the quartz window, is protecting nothing. Now although these types of EPROM's can retain their programming for decades, we all know that exposure to UV light of any source wille eventually erase the contents. I have no knowing how the machine was stored before I came by it, I.e. was it kept in darkness out of the sun or other UV light source.
It's not a total disaster though as have several diffrent EPROM programmers that will reprogram this version of EPROM used in the ZX80, and if the device is faulty it's not too bigger problem as I collect old IC's, processors and EPROM's and I'm sure in my largish collection of EPROM's I will have a suitable replacement. First off though I'll insert the device into one of my programmers to see if it can read back any of its contents. This will give me an idea if the ROM is still viable.
The repair of this machine was started about six months ago, unfortunately some of the other retro computers I have bought have taken a bit more priority.
In the next part I'll detial how I replaced the damaged 74LS157.

Regards
Andy
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Daeve
Posts: 35
Joined: Sat Feb 23, 2019 8:33 am
Location: Leics, UK

Re: My ZX80 Restoration

Post by Daeve »

Looking forward to reading about this, I’d love to pick up a reasonably priced ZX80 and learn to repair it.
avrovulcan
Posts: 45
Joined: Sun Nov 03, 2019 9:56 am

Re: My ZX80 Restoration

Post by avrovulcan »

Daeve wrote: Tue Nov 05, 2019 10:00 pm I’d love to pick up a reasonably priced ZX80 and learn to repair it.
And that's the big problem. There's so few of these in circulation now that working examples are horrendously expensive.
And chances are that most of the broken or faulty ones were disposed of years ago.

Regards
Andy
avrovulcan
Posts: 45
Joined: Sun Nov 03, 2019 9:56 am

Re: My ZX80 Restoration

Post by avrovulcan »

Part 2

Replacing IC 6


For some reason the 74LS157 (IC6) has a crack right across the package. The entire package is cracked and broken free from pins 6 to 11.

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This isn't ment to be a tutorial on soldering or removing broken devices. Although I've been in the electronic's industry for 30 odd years my technique is no better or worse than anyone else's. And the way I look at it, is there is always someone better than you.

First off I need to remove the IC from the board. Normally I'd use my Duratool desoldering iron. Unfortunately I keep this iron at work and I was doing this at home so I had to resort to plan B.

Plan B involves using a very sharp knife to carefully cut through where the pins attache to the package. Gentle but firm pressure is all you need. Taking extreme care not to damage the tracks underneath.

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Here the package has been cut away from the pins

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To desolder the pins I flip the PCB on its back and using reasonable pressure on the Iron heat each pin untill the solder melts and then gently 'tap' the board so the pin falls out. I use a Thermaltronics iron, this heats up instantly when soldering or touching a solder join and automatically applies the right amount of heat. If you are using a more traditional variable iron I would set a temprature of about 320 degrees as a starting point and go from there. Remember the tracks, through hole plating and via's can be very delicate on old PCB's

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Both sides are then cleaned up with solder wick.

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To ensure the tracks are sound I performed a continuity check on all 16 pads to the next solder joint or via.

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Like most other people I concider it good practice to solder in an IC socket rather than solder the replacement IC directly to the PCB. I always use turned pin sockets as opposed to the single/dual leaf varieties. The diffrence in cost of the turned pin type is very minimal.

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The socket is solder to the board

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The replacement multiplexer is then fitted to the socket.

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In the next part I'll replace the missing oscillator and try to read back the contents of the EPROM before trying to power the computer up.

Regards
Andy
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