Fixing ZX Speccies... FIXED AT LAST (See last post)

[tezza]

[quote="kevman3d"EDIT : Unless someone can tell me "No stupid, that's NOT how you check for broken tracks!", I used the continuity test on the multimeter to make sure that there was a connection - and nothing appeared to be broken from what all the beeping seemed to indicate... I noted a handful of bared tracks (the green had been scratched off them) - some very tiny, but nothing looked like it had a crack or break in it...[/quote]

One thing when checking tracks is to make sure one end of the track is disconnected from anything. Otherwise is can look like it's connected but it may because there is a connection to the other end through the circuitry via various components rather than the suspect track.

[kevman3d]

I did buy a conductive pen today - it is a lot trickier then it looks when the fluid comes out a little more then I'd like, so I had to wipe it off a little...
I figure just a quick covering over the small piece of track that was exposed might help...

On another update...

I decided to take a stab at the other Speccy yesterday. Just replaced out the old regulator and the paired resistors again - just because they tend to be a common source of problems as most people have said online. Interestingly, my older Speccy seems to have just solder underneath the PCB, and not on top of the pads unlike the $10 one I picked up. After clipping off the 7805, I also noted that one leg appeared to have a dry joint... However its now replaced anyway.

That makes me wonder how many other dry joints might be there as well... I might pull out the magnifying glass and see if I can spot any just in case.

I did also notice that moving the ULA from the old one into the newer one changed its problem from a corrupted screen to what looks like a reboot - then a screen with a few flashing squares - then a reboot... Constantly. That's possibly a clue that there may be some RAM issues?

As for the second one, I decided to remove the upper 32k of ram to convert it back to a 16k model and got nothing come up on screen - in fact, it wasn't even working! Then I suddenly discovered I'd forgotten to reinsert the ULA and it was sitting on top of the socket! Doh! But alas, no such luck and still same issues (much more corrupted looking then the other when it comes to display).

What I am considering is getting the lower 16k replacement from Zigg... But of course, its spending money in the hope that it'll make a difference. If it doesn't, that's more money spent for no result. And I'll need to desolder the 4116's from the PCB which could be fun (I'm actually now enjoying soldering/desoldering more then I thought I would - once the confidence kicked in, and the soldering in that second speccy suddenly looked clean and tidy...)

Soldiering on... Or soldering on which sounds more appropriate.

[kevman3d]

One quick question for those experienced here - I was wondering how safe it would be to use THIS solvent cleaning spray on the older speccy, just to remove the build up of grime/dust/etc on the components? It could do with a clean off... I've run over it a little with Meths and a cotton bud or two, but its got its grime still... If not, any suggestions on best approaches to cleaning old machine internals?

Apparently that spray is non-corrosive, quick drying and doesn't contain lubricants... But I'd rather not spray things if I'll just end up destroying them...

There is a different one at Jaycar called Electronic Circuit board cleaner - but it almost looks like the same thing... Would it be much different from the one above?

[tezza]

I've never had any issues with these sprays myself. They should be ok.

Yes, I think the two are similar.

[kevman3d]

I've never had any issues with these sprays myself. They should be ok.

Cool! I might give one of those sprays a bash then just to clean up that other Speccy a little more. Tested the Speccy once I let the conductive pen dry for a day and a half - and no luck unfortunately - but I'm going to suspect that its 'possibly' not a scratch on a track (I say possibly in that it could be - depends how good a job I did with the pen lol!)

But we'll see - I'll take another peek at the other Speccy for suspected dry joints when the weekend rolls around again (thanks to a shorter week and all) - given its got solder on just the lower part of the PCB and clean pads/holes on top (which makes it look very clean and new) I guess its possible that the solder joints below may have been affected by the years of storage abuse...

In the meanwhile, I went ahead and ordered one of the Zigg.net lower 16k replacement units. I'll look at removing the old RAM and fitting the sockets I bought a while back from one of the machines and cross fingers. That's not likely to be here for a couple of weeks obviously. I'm sure I can be patient and wait...

[kevman3d]

Here's an update... I've linked some of this post to a few photo's I've snapped as well...

I got the Zigg 16kb replacement unit (it arrived within 5-6 days of it shipping from the UK - nice and fast) so I could swap out the lower 16k. The other day I removed all the 8 chips. Today I installed 8 sockets, and then clipped in the 16k module... Its a cute little thing, and fits pretty snuggly into those sockets (though a little tightly and needing to be pressed fairly hard - but that's likely the new sockets)

Plugged in - powered up - ready to cheer... Nope. Same dark screen, no change...

HOWEVER...

As I was doing this, I spotted a few of the small brown capacitors were shriveled and blistered (click here to see my piccy). I suspect that it may be not actually "seeing" these in the beginning may have been the reason for the machine not working - There's a few spread around the PCB so I think it may be a case of replacing these and testing again. I was inspecting my lovely soldering job with the sockets and I spotted a dry joint on the speaker as well... So, a few more things to fiddle with... I just have to find out what capacitors I need (I'm pretty sure they look the same, but they're not going to BE the same. )

When I get around to that next step, I'll post another update.

FYI - I've started to write up a guide to all these things I've been doing, things I've learnt and links to great resources. The plan is to post up a PDF for all other noob enthusiasts online who want something that doesn't inflict the same frustration scouring the net looking for information (since I've already done that part for them). I figure its a resource I wished I could have had before I started this...

[kevman3d]

Thought I would follow up on this thread, even though there's not a lot to report...

Mostly to say a big thank you to Alastair from Zigg.net who kindly contacted me and offered help! On another plus (for me at least), his advice has forced me to spend time picking up the black art of reading a circuit diagram and googling a few more electronic terms... Great education as I've said previously.

From what we can tell, the damage is likely not as extreme as I thought. Based on how the machine displays the screen when completely cleared of all ram, it looks like it may be a broken or shorting data line on the PCB, and he said its very likely that the ULA, CPU and ROM are actually ok... So the multimeter is going to come out again this weekend and I'll see if one of those not-so-nice scrapes/scratches is actually causing the issue.

If this is the case, I'll be trying my hand at fixing broken tracks and crossing my fingers I don't melt the poor old thing in the process...

Overall Its all feeling fairly positive. Will let you know after the weekend whether there's another update worth mentioning.

[kevman3d]

Holy Cow! I FIXED IT!

Who would have guessed I would eventually get this to come back to life after a lot of fudging my way through the unknown realm of electronics and scouring the net for information. Alastair from zigg.net suggested checking the lines from the lower 16 to the ULA. I did - he said it sounded strange that I had 0v on a data line, and then suggested testing the one pin on the ULA that shouldn't have been doing what it was doing.

Tested the resistance - yup, there was some level of resistance between pin 5 and the 0v. So there looked like a short. He suggested unplugging and testing - that time, no resistance.

The ULA was indeed faulty. Lucky for me, my less-then-happy second spectrum was handy for spares. Popped out that ULA, swapped it in. Powered on to make sure I was getting the pattern that usually occurs when there is no RAM. Yes.

Pushed the 16 replacement module back in. Closed my eyes (well, took a deep breath - actually closing my eyes wouldn't help seeing the power socket) - plugged it in. KAPOW! Up pops the Sinclair copyright!

I can't say how ecstatic I feel right now - mostly because a positive result from all those months fiddling...

I've written up a small bunch of notes from my noob experience to date, so I'll get that PDF'd up and online at some stage. Hopefully all those links, notes, thoughts, etc will all be of use to others in the same "no idea where to start" situation.

[kevman3d]

And here's a piccy - having a video capture USB on the PC (and that composit mod on the speccy) makes for a very useful "mini monitor" when the wife is watching Shortland Street...

I can't seem to get images to appear on these forums, but here's a direct link to the jpg.

https://www.dropbox.com/s/4qach788phkwapr/Igor_it_works.jpg

While its now working (well, without the keyboard back on at least), I'm still keen to get a few more speccies if possible. It would be nice to build the collection and be sure I won't burst into tears should this one die completely.

If anybody spots cheap Speccy gear going, then let me know.

[tezza]

Good ya Kevin. It's a great occasion when you've beavered away on a difficult repair project for a long time. and suddenly you see the machine spring to life. I know that feeling well.

Well done.

[kevman3d]

It's a great occasion when you've beavered away on a difficult repair project for a long time. and suddenly you see the machine spring to life.

Indeed! I think even more so, knowing just how little I started with and just how much knowledge I've managed to assimilate by going through the whole process.

The other nice side effect having this as an ongoing side project, is that it has made me follow-through on some of my retro 3D projects that I started before the repair-it-myself urge kicked in. Usually most personal art projects I tend to get sidetracked on, think of something much more interesting and go off do something instead.

If anybodies curious as to my other passion on the side (and my working profession as well) I posted up an overview of a scene I built with my retro collection... Perhaps its time to build a "virtual" collection for those machines I don't have. It'd be certainly a lot cheaper...

http://kevman3d.blogspot.co.nz/2013/12/the-8-bit-project-almost-completed-cg.html