ZX Spectrum Issue 3 repair

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Tue May 09, 2017 2:21 pm

Well, I've now used the desolder station for the first time, I sucked the solder from all the legs giving the tip a small wiggle once the solder had melted and the legs looked pretty clear of solder. There was enough solder remaining to keep them in place however so I still had to resort to heating each leg with the iron and pulling them with pliers. The upper RAM is still firmily in place so I will need to cut the legs again. I removed the lower RAM originally with a manual pump, and hot air but cutting them off seems to be the safest method to avoid damaging the board and/or components
Last edited by Tezz on Tue May 09, 2017 3:00 pm, edited 2 times in total.

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Paul
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Re: ZX Spectrum Issue 3 repair

Post by Paul » Tue May 09, 2017 2:52 pm

Did you add fresh solder before desoldering? This improves the results pretty much ...
In theory, there is no difference between theory and practice. But, in practice, there is.

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Tue May 09, 2017 3:10 pm

Paul wrote:
Tue May 09, 2017 2:52 pm
Did you add fresh solder before desoldering? This improves the results pretty much ...
I've only been adding new solder when the old solder doesn't seem to flow well, should I have added new solder to all the legs before desoldering?

Which ever method I've tried up until now there always seems to be a small amount of solder remaining on the legs on the top side of the board to keep them in place. Hot air works well at that point to finally remove them (that is if I want to save the IC rather than cutting it off). I prefer to desolder each leg after cutting them instead of so much heat going through the board. I should buy some good quality side cutters really, the cutters I'm using are too large.

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Tue May 09, 2017 3:50 pm

How easy it is to remove DIL chips, partly depends on the size of the PCB hole. If there is very little clearance, it can be tricky.
Clean the solder joints with IPA before starting. If very dull, you will need to either apply some liquid flux, and / or new 60/40 solder.

Remember, the components and pad have to be hot enough for all the solder to melt. It also takes a little bit longer for the heat to make it to the middle of the through hole. I once desoldered a 14 pin PCB mounting DIN socket off an Atari ST (I was moding it to use an external HD FDD and needed to cut a track under it). The normal signal pins were bad enough, but the ground pin was a nightmare :shock:

But if the chips are known to be defective, or are strongly suspected, then by far the easiest is to cut the chip off the legs.

Mark

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Tue May 09, 2017 7:47 pm

Wow, removing the upper RAM took me an absolute age today. There was still quite a lot of solder on all the legs after using the desolder station this morning, I found some new cutters that I'd stored away and forgotten about but they were still a little large so it was fiddly getting to all of the legs without catching anything. I had one mishap when I started to carefully lift out IC16 one leg wasn't cut all the way and the trace pulled up with it :o I thought I would be facing a jumper repair but when everything was out of the way it looked like it remained connected and I was able to carefully wick off the solder and partial leg and push the trace back into place (see photo) hopefully that is ok, I haven't checked continuity yet. Removing each leg with the iron and pliers seemed to be much tougher than the previous ICs, the top legs on the ground trace took a while. I found the hot air removal of the lower RAM so much quicker and easier. I have an upper RAM replacement module to install so not so many connections to resolder there, I just need to make sure that all the traces are ok and nothing is bridged or broken. Should I stick the trace that lifted down with epoxy do you think if it checks out? Maybe I should also check the voltages of the upper RAM pads before I install the modules? It'd be bad to still have a fault and damage the replacement modules and ICs.

I use my flux pen on the legs on both sides of the board before desoldering and then clean everything off with IPA at the end, normally I clean the boards quite a lot with IPA when I get them although I also washed this one first in de-ionised water (leaving it to dry out naturally for a few days). That brown flux they used back then is pretty messy to clean away.

See attached photo of the progress so far ..
issue_3_ics_removed.png
issue_3_ics_removed.png (758.6 KiB) Viewed 212 times
lifted_trace.png
The trace in the center that lifted between IC16 and IC17
lifted_trace.png (148.06 KiB) Viewed 212 times

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Wed May 10, 2017 1:13 pm

Happily everything looks good with the continuity test. I forgot that you can't install the upper RAM module directly on the board with other components inbetween them so I will need to install eight sockets. I'll need to ensure that the trace I lifted is fixed in place. Can I carefully apply regular epoxy with a pin over the trace?

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Wed May 10, 2017 5:52 pm

I've never tried to glue down a lifted PCB track before. So try by all means.

I do however suggest that you use a fine strand of wire (one strand from a multstrand cable/wire) or thin fuse wire and thread it through the hole of the pad/track that lifted. On the top, line it up on top of the good track leading away from the lifted area, then carefully solder it to the track, but not the through-hole. Leave the end through the hole until you are happy with the top. Then insert the socket in place. Then when you solder the pin of the socket on the bottom, the wire gets soldered too. Cut the excess wire off. Now you know that even if the pad on the top has a hairline crack, the wire will provide continuity ;)

Mark

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Wed May 10, 2017 6:05 pm

thanks Mark, that's a good idea. I'll make sure to do that. I did a quick Google search today and epoxy seems to be a common method to reattach lifted traces. I thought maybe it might need to be the high temperature epoxy but it should be ok.

Tezz
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Re: ZX Spectrum Issue 3 repair

Post by Tezz » Thu May 11, 2017 11:12 am

I thought I'd do a final voltage test at the pads of the lower RAM now that the upper RAM has also been removed. 12v now shows 12.12, 5v shows 4.96, -5v shows -4.78, 0v shows 0.01. The DC converter still has the same buzz so I'll look into whether this is expected. Perhaps if a modern regulated supply would make a difference.

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1024MAK
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Re: ZX Spectrum Issue 3 repair

Post by 1024MAK » Thu May 11, 2017 11:53 am

Are you sure that it is the Traco Power DC/DC converter module that is making the noise?

Only, the coil (transformer) that together with TR4 and TR5 on the Spectrum PCB (that produces the +12V and -5V supplies) often buzz. Some rather loudly.

Mark

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