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Problem with colors on Issue2

M

McNerve

New Member
Joined
May 23, 2023
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7
Hey, I am new here so ... hello everyone :)

I have little problem with my speccy Issue 2. I can adjust colors using VR1 & VR2 and things looks good until ULA warms up (I mean turn hot). Then video loose color (goes monochrome). I can adjust color again using VR's but then, next time I start spectrum and ULA is cold video is monochrome till ULA warms up. (around 10 - 15 minutes)

I think it is ULA because in first case (VR's adjusted to cold ULA) when ULA warms up and video lost color and I put radiator on ULA effectively cooling it down then the color is back.

But maybe it is just side effect and something else is causing it ? Bad VR's ? IC14 ?

Many thanks.
 
NTSC or PAL?

Could be the capacitors around the 1889 composite encoder. You can check the ULA - it's signals are mostly digital. There's not many outputs that go to the video.

Also, take a feed from the line just before it goes into the video modulator, and see if it's color, and also check with a different monitor.

Check the 12v line is sufficient and stable.

And adjust -

Colour Adjustment; Sub-Carrier Oscillator; 14 Mhz Oscillator - Sinclair ZX Spectrum Service Manual [Page 18]

Sinclair ZX Spectrum Manual Online: colour adjustment, Sub-Carrier Oscillator, 14 Mhz Oscillator. Potentiometers Vr1 And Vr2 Are Used To Effectively Null The Voltages Between Pins 4 And 2 (For Vr1) And Pins 2 And 3 (For Vr2) On Ic14 (Lm1889). To Allow For Thermal Drift, The...
www.manualslib.com www.manualslib.com

Keep in mind that composite video isn't the clearest on a ZX SPectrum in color anyway.

David
 
Hey, thx for answer.

PAL, already using composite video

12V on 1889 (pins 14,15,16) reads 11.55V. On upper ram 12.02V

Cause I have no oscilloscope I set VR1 & VR2 to approximately get 50mV on pin 4 and -50mV on pin 2 relative to pin 3 of 1889

Overall image and color quality is quite good. Color is gone when unit warm up or cool down. I cannot adjust it to accommodate thermal drift

I replaced all electrolytical capacitors on that board.
 
Ok, bit more try & error with no joy.

Replaced C65 again (trayed both 22uF and 100uF). Removed socketed and replaced LM1889 ... still same result. Maybe 4.xxx crystal ?

On start:

good.jpggood2.jpg


After 10 - 15 minutes:
bad.jpg
 
Seems like the next step is to get an oscilloscope and see what is drifting, though you might need a vectorscope also. Dealing with PAL color issues is not going to be easy.
If it's thermally related, does it "reset" to normal when it cools? A thermal camera can also be helpful to see what is getting hotter.

otherwise, you can only really start measuring across every component hot and cold and see what starts changing when it warms up.

I've never heard of a 1889 doing that, but it is possible. Also, there's a lot of experienced Spectrum repairers on the https://spectrumcomputing.co.uk/forums/ site.

David.
 
Yup, it reset to normal when it cools. You're right it is not 1889 i replaced it with new one. ULA getting really hot.

One strange thing I noticed. Anytime I touch pin 13 of 1889 (chroma sub-carier) with multimeter probe, colors shows correctly and they stay correct as long as I keep touching that pin. Regardless of temperature. Though I didn't test it for too long.
 
Does it also come back with an "inert" probe like a scrap of cardboard or plastic to press on the pin? Maybe a cold/cracked solder joint that loses contact outside of a certain temperature range?
 
Hak Foo said:
Does it also come back with an "inert" probe like a scrap of cardboard or plastic to press on the pin? Maybe a cold/cracked solder joint that loses contact outside of a certain temperature range?
Click to expand...
Nope, need to be metal. Also though it might be dry joint, so I de-soldered 1889, installed socket and put new 1889. Problem remains

cj7hawk said:
Maybe connect a small capacitor (eg, 50pf?) from that pin to ground.

If you think it is the ULA, that's easy enough to replace - https://www.tindie.com/products/charlieingley/vla82-spectrum-48k-ula-replacement/

But a workaround is more satisfying than a proper repair - :) Though if the ULA is going, there's no telling if it might get worse.
Click to expand...

Yeah, I think I will purchase and swap ULA just to verify if it is source of that issue.
 
Think I fixed it by replacing TC2 with 15 pF capacitor. Will test it a bit more, but had 2h game session without the issues.
 
The ULAs in Spectrums always did get frighteningly hot, and that was when they were working normally. I remember hearing an anecdote about someone on the phone to tech support with a problem with a Spectrum he was having a problem with and he was asked to feel how hot the ULA was. He did and said 'Wow, that's really hot!' to which the response from the technician was 'Hmm, so no, that's not it'.

Joking aside, if the trimmer doesn't fix it after all, two suggestions:-

Do you have access to component freezer spray, which is useful for tracking down thermal faults? You let the fault develop and then you spray the component you think has a thermal fault - if spraying it clears the fault, you're right. Maybe you are already doing this?

Have you considered fitting a finned heatsink to the ULA? Maybe if you can keep the IC cool enough your fault will never materialise. You can get them in the exact size for 40-pin ICs and they usually come with a self-adhesive thermally conductive pad. If you buy one actually intended as a ZX81 or Spectrum ULA heatsink it will be low-profile enough to fit inside the casing of the computer.
 
It was definitely faulty TC2. I played with this spectrum for few days now and issue never come back. Installed heatsink on ULA anyway and replaced 7505 voltage regulator with TSR 1-2450 to reduce heat and prolong its lifespan :)
 
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