Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

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1024MAK
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by 1024MAK »

ZTX parts are now made by Zetex Semiconductors, which is a U.K. company, which in turn is owned by Dallas-based Diodes Incorporated.

The ZTX651, ZTX653, ZTX751 and ZTX753 are all current production parts.

A ZTX651 or ZTX653 can be used in the TR4 position.
A ZTX751 or ZTX753 can be used in the TR5 position.

ZTX651, ZTX653, ZTX751 and ZTX753 are available in the U.S.A. Here’s a link to a Mouser listing for ZTX651 transistors.
Link for ZTX653.
Link for ZTX751.
Link for ZTX753.

They may also be available from other suppliers.

It is also possible to use other general purpose PNP small signal transistors for TR5 as long as they have a reasonable gain. But the lead-out is likely to be different. So a replacement will have to be fitted the opposite way round, or two leads may need to be crossed over.

Mark
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DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

Thanks for this. Sorry that I didn't respond sooner, but I didn't see the third page on this thread. This will be helpful!

I ended up ordering the transistors from Retro Spares Shop (a UK company) since they also had the most affordable ULA replacement, and my Spectrum wasn't going to work without it. So I figured that I'd just do it all at once. In any case, my parts arrived a couple of days ago and I installed the transistors. The -5v line is back, but the 12v is running concerningly high, beginning at about 12.88v before dropping to 12.66v. Then I tested the 5v line and found that it's beginning high as well. Starting at nearly 8v before dropping to a more appropriate 5.16v.

Admittedly, I haven't installed any RAM, or the new ULA yet, as I wanted to check the voltages beforehand to avoid damaging any chips. Still, is that normal? And if not, how do I bring these down? At this point, I'm thinking of replacing the 7805, but I don't know if that will resolve these issues.

I will be getting a second Spectrum (+) from eBay soon. It's a bit beat up, missing a hand full of keys, and I don't know the status of its membrane. Also, I don't know the board revision, but it was tested to boot. So hopefully, having a second working machine will help me troubleshoot this one. Of course, there is a possibility that I will have to recap it as well. So any continued advice, or ideas as to how to prevent this issue from repeating will be greatly appreciated.

ETA:
Just tested it again. The 12v line is snaking up now to around 14v, and the 5v line rose to about 5.5v and rising! I think I need a new 7805!?!
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1024MAK
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by 1024MAK »

The +12V line voltage is referenced to the +5V line. So if the +5V line is higher than normal, so will the +12V line.

The acceptable range for the +5V rail (line) is 4.75V to 5.25V. Operation above 5.25V may result in incorrect operation or if significantly higher, damage to some chips.

If the +5V rail voltage is too high, then either there is a poor/bad 0V/GND connection to the centre pin of the 7805 or it’s faulty. So first resolder this pin. If that does not help, replace the 7805.

For a replacement you can either use another 7805 or use a modern DC/DC converter (also called a ‘switcher’).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

1024MAK wrote: Mon Nov 08, 2021 9:52 am If the +5V rail voltage is too high, then either there is a poor/bad 0V/GND connection to the centre pin of the 7805 or it’s faulty. So first resolder this pin. If that does not help, replace the 7805.

Mark
Thank you! It seems you were right (fingers crossed). I resoldered the pins on the 7805 and now I'm getting 5.02v on the 5v rail, and just under 12.5v on the 12. I'm guessing that both are fine without the RAM or ULA as load? I still need to replace the socket for the ULA as I've been warned that the pins on the replacement are brittle and don't always play nice with the cheaper sockets Sinclair used back in the day. So my adventure is not over yet...
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1024MAK
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by 1024MAK »

The +12V acceptable range is 10.8V to 13.2V (+/-10%), so 12.5V is fine.

And your +5V is okay as well.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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There are four lights!
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Tiger
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by Tiger »

Perfect - in my third post I said: "Also the groundpin of the 7805 looks "cold" on the solder side." :roll:
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

1024MAK wrote: Mon Nov 08, 2021 9:34 pm The +12V acceptable range is 10.8V to 13.2V (+/-10%), so 12.5V is fine.

And your +5V is okay as well.

Mark
Excellent! I tried it with the old ULA, and as expected, it didn't boot. Still, it was worth a try. I still need to get a new 40-pin socket (the kind with the round holes so I don't break any pins).
Tiger wrote: Mon Nov 08, 2021 9:56 pm Perfect - in my third post I said: "Also the groundpin of the 7805 looks "cold" on the solder side." :roll:
I don't know what to say about that. I could have sworn that I resolder them several times. I guess the umpteenth time was the charm! :-)

Also, my new Spectrum+ arrived today. It's an issue 5A board in good condition and appears to have been recapped already! So the picture is perfect, but the keyboard membrane is trash. Any recommendations on a replacement? Also keycaps.

Question: Given all the voltage issues I've had, what other ICs should I be looking to check or replace? The way the new ULA is designed, it will cover several nearby chips. So if there's a chance they're bad, it might be a good idea to replace them before I fit the new ULA. Especially since, acording to the instructions for this thing, seem to indicate that this is a set it and leave it kinda thing.
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

So I finally worked up the courage to swap the ULAs between my two Spectrums. Screenshots below:

The first is of the new Spectrum+ with the old ULA
Screen Shot 2021-11-08 at 9.35.26 PM.png
As you can see, the ULA was damaged by the overvoltage, but not nearly as severely as I had initially feared.

The second is of the old 3B board with the confirmed to be working ULA from the new Spectrum+.
Screen Shot 2021-11-08 at 9.35.39 PM.png
Once again there appears to be little improvement. The lower RAM like the ULA is new, so I doubt that these are the problem (anymore). Maybe the CPU? I have a few spare Z80s, but I'm not sure what their speeds are?
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

OK, so I replaced ICs 3 and 4, which seemed to stabilize the bars seen in the previous post (the green seemed to flicker a bit) and made them solid grey/white. Then I tried piggybacking over the upper RAM. This resulted in a white or pink border depending on the chip, but no matter how many chips were piggybacked, it still would not boot. So I removed them and the black border returned. At this point, I'm running out of options, but the CPU is still a possible suspect. So I've removed that as well, with the intention of installing a socket in case it's not the problem. After that, I'm kinda left with four other ICs and the ROM. Unless someone has another suggestion?

ETA: I tested TR3 (ZTX313) and it seems fine
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