ZX Spectrum 48K Mainboard Issue 4S - Help!

terrymr
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by terrymr »

Current Z80 cpus work just fine in the ZX81 so I imagine they’d work just fine in the spectrum.
mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Thanks for all your additional replies/advice. The new Z80 chip arrived this morning, managed to get all the pins through the board with a little coaxing with the long edge of a steel rule. Hopefully I've oriented it correctly, it sits flush and the pins are level by eyeball on the rear. Here's the front:

zx-spectrum-front-z80.jpg

I'm going to attempt to solder the pins in tomorrow, I've applied some Kapton tape either side and down the centre of the pins to hopefully prevent any unwanted bridging with spots or exposed trace. Here's the rear:

zx-spectrum-rear-z80.jpg

Apologies for the light/quality of the images. If anyone has any further advice prior to me butchering those pins in, now would be the time! I'll report back once I've made an attempt.

Cheers,
Mike.
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1024MAK
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by 1024MAK »

Well, normally when carrying out repair or servicing that involves large DIL chips, most people use a IC socket. As that then makes any future chip changes a lot easier. It also has the benefit of almost eleminating any possible PCB track damage should the chip need to changed again.

Whichever way you do it, I do strongly recommend you carry out a very close visual inspection of each pad and it’s PCB tracks, both on top of the board, and on the underside. Then ‘bell out’ (test using a multimeter on the continuity range) each of the pads to the other points on that ‘net’.

By ‘net’, I mean to the other chips, resistors etc. that are shown on the circuit schematic diagram.

By doing all this, you find out before you start soldering if there is a problem that requires a wire to replace a broken PCB through-hole plated connection (that joins the top pad to the underside pad) or a broken PCB track. Fixing these problems is a lot more difficult once a socket or chip has been refitted.

And trying to trace a fault caused by such a problem is a nightmare.

Mark
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mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

1024MAK wrote: Fri Aug 27, 2021 11:22 pm Well, normally when carrying out repair or servicing that involves large DIL chips, most people use a IC socket. As that then makes any future chip changes a lot easier. It also has the benefit of almost eleminating any possible PCB track damage should the chip need to changed again.
Thanks Mark for your reply - this is solid advice I think. My (fuzzy) logic was to solder the Z80 directly back into the board then buy a ULA or equivalent to see if it would start. If it doesn't, then I could remove the ULA (as long as I don't fry it) and buy another mainboard that didn't have one or was borked. Of course, by doing this it doesn't leave me the option of removing the Z80 so easily should I need both the Z80 and ULA in a future project.

I'm going to suspend the soldering and buy a couple of 40-pin IC sockets on your advice. If worst comes to worst and I butcher the IC socket or the machine won't start, I can keep the Z80 and use it as a back-up. I've had a good look at the traces around the through-holes on the IC2, there does appear to be some small flecks of resin that have come away from a couple of them leaving some bare spots, which is why I've applied some Kapton tape to (hopefully) prevent any solder bridging. Under my magnifying glasses, none of them appear to have any breaks.

As I've mentioned, this board really is a 'free hit' - If I don't manage to repair it, it was knackered anyhow. I'll report back once I've got the IC sockets.

Cheers,
Mike.
popeymon
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by popeymon »

I second what Mark is saying. You should check for shorts. You should check for continuity. And do this using the beeper or bell or whatever your continuity tester has on your multimeter.

This may be a pain in the short run but it is better to know ASAP, and especially before you solder anything else in.

And yes, you should use a socket for this.
mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Hi again,

Apologies for the delay in replying, ordered the IC sockets early last week and they only arrived today with a number of bent/creased pins to throw into the mix. Managed to get them straightened out with some needle nose pliers and went to work. Here's the results:

The soldering went okay, couple of stubborn pins but eventually managed to get a decent flow on all of them and avoided bridging thanks mainly to the Kapton tape. Pin R17 was a particular pain.

zx-spectrum-rear-soldered.jpg

The IC socket sits flush to the front of the board after soldering, some issues initially with the 5L pin socket as you look at it but managed to pull it firmly home from the underside beforehand so all even.

zx-spectrum-front-ic-socket.jpg

The CPU is seated firmly and flush to the IC socket, again a slight snag getting this in but the steel rule guided all pins home nicely.

zx-spectrum-front-ic-cpu.jpg

The CPU seated in the IC socket from another angle. Apologies for the quality of the image here.

zx-spectrum-front-ic-cpu-angle.jpg

I didn't test for shorts as my multimeter has gone missing, so I'll leave that to luck. I'm going to look around for a ULA now, worst case scenario is that after buying and fitting the parts the machine doesn't work, but I'll have both a removable CPU and ULA to fit into something else. As soon as I've got a ULA, power supply and cables, I'll fire it up, see if it goes pop and report back.

Thanks for all your help so far.

Cheers,
Mike.
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