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

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

Post by DistantStar001 »

So I got a 48k Spectrum (Issue 3B) some years back and while for the most part, it works, it always had a lousy picture (noticeable vertical bars in the main field and diagonal wavy lines on the bottom). Basically, it needed a recap (also it was starting to randomly crash). So I recapped it. As near as I can tell, the system seems mostly good now. It boots more or less reliably (occasional glitch, but far less than before), no longer crashes, and the video artifacts from before the recap have more or less gone away. However, the colors are way off?

In testing the machine (I used the BASIC Border command), blue is black, green and cyan are indistinguishable from one and other (as are red and magenta), and yellow is light pink?

For info, I am in the United States, so this has been composite modded with a 100µf capacitor bypass to the RF, and I have been using both a Sharp Aquos LCD and a TCL 41in 4k LED TV as a monitor. Both TVs worked perfectly with the spectrum before the recap, as well as my PAL C64. Both have been tested with my PAL C64 since and give a perfect picture.

I've gone over this a dozen times. All caps appear to have the correct µf values (voltage ratings are rather high [some as high as 50v for what should be 10v], but I was told that shouldn't matter?) and are installed in the correct orientation (I don't think it would boot at all if they weren't). I used the kit from Console5 which is supposed to be correct for my revision.

Any ideas as to what I'm obviously missing?
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Tiger
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by Tiger »

If the color was ok before recapping, I would check if there is a shortcut by soldering anywhere. Crystal X2 disconnected (wire broken)? Wrong direction or wrong value of an electrolytic-cap (C25 = 22µF !)? Bad connection or shorten pins at the ULA?
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

Thanks for this, I checked X2, it seems firmly attached, and the ULA looks good too. I didn't find any shorts on the board either, but I did see some debris that I was better off without. I also replaced the cap at C25, which did result in a slight, but noticeable improvement in video quality. So maybe that was the problem? Unfortunately, I can't say yet, as the contacts on the keyboard membrane just disntigrated. It's a modern replacement, but apparently not a very good one. So I'm going to have to replace it. Any recommendations?
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Tiger
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by Tiger »

Is it possible to make pictures of your pcb? :geek:
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

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

Post by Tiger »

Ok - it looks perfect on the first view. Maybe there are some solderpoints with bad connection. I marked some which looks a little "pasted" (not hot enough to let the solder fluid well). Also the groundpin of the 7805 looks "cold" on the solder side. This points I marked in red. At the yellow marked area please look if there is any wrong connection (a wire seems to touch the ground - I marked it with a litte red circle -) or broken components.
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red marked solder
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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 membranes I recommend are those supplied by RWAP. E.g. this one on Sell My Retro. However, since brexit (where the U.K. left the European Union) and the change in how VAT (sales tax) is handled, many small U.K. businesses have stopped trading overseas. So I’m not sure if he still exports outside the U.K.

On one issue board, at least one electrolytic capacitor has the polarity incorrectly marked on the silk screen (printing on the PCB). Have you compared your board to pictures of others online, or did you make a record of the polarity of each capacitor before you started?

Is C65 the correct value (22µF) and polarity? Are the solder connections, pads and tracks all okay (visual inspection and continuity testing)?

As above for C25?

Note that as long as the DC voltage rating of a electrolytic capacitor is equal or greater than the component you are replacing, that’s fine. Small value electrolytic capacitors (e.g. 22µF and lower) are only available in a limited number of voltages unless you are a manufacturer buying in bulk.

Is the central area (paper area) white after power on / reset?
The colour output is determined by the analogue voltage on pins 15 and 16 of the ULA. With a white screen, what voltages do you get on these two pins?

What voltages do you get on pins 2 and 4 of IC14 (LM1889)?

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

Post by BarryN »

DistantStar001 wrote: Sun Oct 03, 2021 8:09 pm installed in the correct orientation (I don't think it would boot at all if they weren't).
The assumption that the machine wouldn't boot if some caps were reversed is a false assumption, although over time it would cause more damage to the caps. How quickly the damage occurs depends on the caps voltage rating and the applied voltage, and can range anywhere from the cap exploding to it working for years and eventually leaking.
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Tiger
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by Tiger »

As I have seen on the pictures are all values and polarities of the caps are ok for the Issue 3b. But some soldering points don't look good ...
The connection of C28 ("+" at the component side) looks very poor (dropped?)!
Keyboard membranes are also available from here:
https://retroradionics.co.uk/#!/ZX-Spec ... category=0
or
https://divide.cz/index.php?y=G_klav.en
DistantStar001
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Re: Recapped ZX Spectrum (Issue 3B) and now the Colors are wrong?

Post by DistantStar001 »

OK, I think I've gotten all the problem areas and "borrowed" a keyboard from a Timex Sinclair 1500. Unfortunately, there hasn't been much improvement. If anything, the pink that should be yellow is a bit pinker than before.
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