Issue 4A with no colour!

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Liveinabin
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Issue 4A with no colour!

Post by Liveinabin » Sun May 05, 2019 6:02 pm

Hi all. I’ve exhausted my wildly inadequate electronics knowledge and I’d appreciate some advice.

Found a Spectrum issue 4A, mostly working. Just needed a RAM chip changing at IC15. Did that. Worked beautifully.
So...for a lap of honour, thought I might use my nice new soldering iron to recap the whole thing and composite mod it.
Did that with one of the little sets from Retroleum (also replacing TR1 and 2).

Fired it up and..monochrome! Well, there’s intermittent colour but it’s VERY faint and fleeting. Initially, I had a capacitor as part of the composite mod but I removed that. I even tried going back to RF but that just gave me an unstable (and still mono) picture.
I’ve looked around for any damaged traces but I can’t see anything. Wondered if it might be the LM1889 video encoder chip. I only have a multimeter and I checked pin 14, getting about 12v so it’s alive. More than that, I can’t say.

With hindsight, I should have tested after each component change. Oh well, lesson learned :)
Any thoughts on what could be wrong with this? Thanks in advance.

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DouglasReynholm
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Re: Issue 4A with no colour!

Post by DouglasReynholm » Sun May 05, 2019 6:15 pm

There's some really dedicated Spectrum HW people on here that will be able to help you I'm sure, but as an EE myself, I can't help but point out that just because an IC has the right rail voltage, it doesn't necessarily follow that it is 'alive'! Good luck.
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Ast A. Moore
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Re: Issue 4A with no colour!

Post by Ast A. Moore » Sun May 05, 2019 6:17 pm

First, reseat the ULA. Then make sure C25 is installed correctly and soldered properly. Next, check TR1, TR2, TR8, and TR9.
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin » Sun May 05, 2019 6:57 pm

Will do. I can see myself having fallen at either of ‘installed correctly’ and ‘soldered properly’. :D
I’ll try that out and get back to you.
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1024MAK
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Re: Issue 4A with no colour!

Post by 1024MAK » Sun May 05, 2019 8:18 pm

Are the replacement transistors the correct way round?

Mark
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin » Sun May 05, 2019 8:29 pm

Well I wondered that. These are BC549C replacements (supposed to make the picture brighter) and the instructions were to install them the other way round from the original set.
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black » Sun May 05, 2019 8:34 pm

Same question from me to, about TR1 and TR2. If the transistors are the BC549C, they are fitted correctly (opposite way). But if they are the ZTX313 or the MPS2369 they fitted wrong.
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black » Sun May 05, 2019 8:38 pm

Ha! I was a bit late :)
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin » Sun May 05, 2019 8:39 pm

Definitely BC549C
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Re: Issue 4A with no colour!

Post by Liveinabin » Sun May 05, 2019 8:57 pm

Okay. Reseated the ULA, checked the polarity of all the capacitors and reflowed/resoldered. Transistors are in the right way round (and I reflowed solder and checked some continuity) but I don't know how to test transistors in situ.
Still mono :(
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