Speccy repair

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IngoQ
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Speccy repair

Post by IngoQ » Thu Jan 11, 2018 3:24 pm

I have recently bought a non working ZX Spectrum 128 (toast rack) and I am struggling with the repair since. Maybe someone here has repair experience with these and has some ideas how to proceed...

It was pretty much dead when it arrived, but after changing the 5V regulator and TR4/TR5 it showed some life signs. To be safe I swapped all electrolytic capacitors as well.

From the symptoms it looked like some RAM chips were gone, to help in locating these I used the diagnostic ROM by Retroleum. It signaled a couple of malicious RAMs and I swapped them as well. It all worked well until I came to IC10 that should be responsible for Bit 3. I swapped it and nothing changed. On every test cycle I run it always shows this IC as faulty. Of course I swapped with (several) other chips, double checked the soldering even swapped the socket again just to be sure.

I checked continuity on the adress lines and looked at the data line, it all looks okay to me. Voltages are fine as well.

Two additional strange things:

1. Upper Ram tests completely fine.
2. a different diagnostic ROM shows bits 7,6,5,4 faulty... WTF?

The screen is garbled and barely readable, sound output looks fine. The ROM reacts as it should, but nevertheless I swapped the CPU, just to be safe. No change.

Right now I have swapped and socketed all RAM chips, the CPU and the ROM. All caps are replaced, as well as TR4, TR5 and the 7805.

What puzzles me, is that upper RAM works fine, only lower RAM seems to be faulty. When I look at the schematic, data busses are identical, so these are most likely not the fault. Has to be addressing then, and there seem to be two chips involved: the ZX8401 multiplexer and the ULA.

Both are hard to come by, so I hesitate to simply swap them without being sure.

So if anyone here has some ideas about this, it would be a great help :)
Ingo :geek:

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupéry

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exxos
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Re: Speccy repair

Post by exxos » Thu Jan 11, 2018 3:26 pm

All I can suggest is contacting Gadget UK... He does the hell of a lot of retro computer repairs.. So is probably the best person to ask about this sort of thing..

https://www.youtube.com/user/GadgetUK164/videos

I did mention this forum to a while ago in fact..
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IngoQ
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Re: Speccy repair

Post by IngoQ » Thu Jan 11, 2018 3:31 pm

Thanks, I know his videos, although I sometimes struggle in understanding him ;)
Ingo :geek:

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupéry

stephen_usher
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Re: Speccy repair

Post by stephen_usher » Thu Jan 11, 2018 9:34 pm

If you're going to send it away definitely use Mutant Catapillar:
http://www.mutant-caterpillar.co.uk/sho ... ts_id=3481

As for the problem, check all the resistors! I had a similar problem with my +128K and one of the problems was a resistor which looked OK but crumbled as soon as it was touched.

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IngoQ
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Re: Speccy repair

Post by IngoQ » Fri Jan 12, 2018 11:56 am

Thanks Stephen, I took your post as a challenge to recheck the mainboard very carefully for undiscovered faults.

When doing this I re-checked the address lines, and what can I say... found a nicked trace on A0 well hidden under a ceramic cap, separating IC10,11,12 and 13.

Lesson learned: Sometimes continuity check can be be wrong... supposedly when there is still charge in some caps :roll:

Now let's see why the AY is silent...
Ingo :geek:

“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.” - Antoine de Saint-Exupéry

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