ZX Spectrum Issue 3 repair

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Thu May 04, 2017 1:41 pm

The pad for the + connection of C46 has lifted. I can't quite tell from your picture, but if there is or was a track going to another component, a thin wire will need to be fitted to replace the gap where the pad/track was. On the end that connects to the leg of the capacitor, form the striped end of the wire into a small loop or hook.

Please also check the pad and track on the underside of the board.

And yes, you need to remove the bit of damaged pad so that it is no longer shorting to the nearby PCB track. The positive end of C46 has a "12V" AC signal on it that may kill any semiconductor chips if it can get to them.

If the upper RAM chips get hot, it may well indicate that they are damaged as well.

Mark

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Thu May 04, 2017 2:48 pm

I suppose I could very well have fried all the ICs myself with that pad touching the trace and distributing 12v. I was getting low readings before recapping and that pad lifting so perhaps the ICs were already dead. I'll never know, it was a board that had quite a lot of abuse that I'd taken on. I'll continue on with the repair. I've bought a ULA and ROM now so hopefully once the board repairs are complete and everything checks out I can eventually see this board live again. I'll sort that lifted pad first and then look at the upper RAM

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Thu May 04, 2017 6:09 pm

If it makes you feel any better, given that the board was missing the PSU connector when you got it, I suspect that either an incorrect PSU had been connected to it at some time, or it had been connected with the wrong polarity, and that had caused most of the damage.

The type of silicon chips used in ZX Spectrums are more hardy than people think. But applying a AC voltage, a reverse DC voltage or a voltage significantly greater than the normal supply voltage will kill them.

Mark

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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Thu May 04, 2017 10:58 pm

I'm pretty certain that much of this board was toast with the state it was in. The ULA was certainly dead having been tested in the working 3b board before working on this board which is the most important thing. I'll get this board back to a working state eventually :)

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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Sat May 06, 2017 11:36 am

Continued this morning, moving the lifted capacitor pad off the trace with a micro screwdriver and a fine tip on the iron fortunately worked well so that is out of the way now. I tested continuity before fixing that and it was certainly in contact with the track. Hopefully that hasn't caused me more issues. The trace looks to be going to the group of diodes which perhaps are for the keyboard. Several of the upper RAM chips still get hot after a minute and the DC converter still has a buzz so it looks like the next step is to remove all the upper RAM. With no ICs then left on the board I'm hoping that the DC converter is silent and all the voltages show correct readings.

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Sat May 06, 2017 11:46 am

Keep going, you got to fix it now :mrgreen: ;)

Mark

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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Sat May 06, 2017 3:09 pm

Thanks Mark, I've just bought one of those cheap Duratool desolder stations so will hopefully it make these jobs a little easier. It should arrive next week.

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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Tue May 09, 2017 11:33 am

My desolder station arrived this morning, what temperature do you recommend I set to remove the ICs? I've cut out the dead Z80 and ROM so I'll just be desoldering the separate legs there individually but the upper RAM chips are still in place. I don't know whether that makes any difference when I desolder each leg?

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Tue May 09, 2017 12:27 pm

The temperature depends... (can't be bothered to list all the variables...)
Have a read of this. I often use a setting of between 330C and 350C.

Mark

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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Tue May 09, 2017 12:37 pm

Thanks Mark, I've read through the comments on that link. I'll give it a go today and hope to have an undamaged board at the end of it.

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