Issue 4A with no colour!

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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1024MAK
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Re: Issue 4A with no colour!

Post by 1024MAK » Mon May 06, 2019 9:43 pm

Yeah, but keep in mind that with any in-circuit testing, the results may be affected by the other components on the board.

Mark
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Muttley Black
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Re: Issue 4A with no colour!

Post by Muttley Black » Mon May 06, 2019 10:11 pm

That's true. But TR1 should give you this measurements ("open", both on collector and emitter with neg at base) in-circuit testing. I also test it on my spectrum's after your post.
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Re: Issue 4A with no colour!

Post by Liveinabin » Mon May 06, 2019 11:29 pm

Muttley Black wrote:
Mon May 06, 2019 8:40 pm
TR1 looks faulty to me. Cause with neg at base, you should get "open", both on collector and emitter if i am not mistaken. If you are sure that you measure TR1 as TR1 and TR2 as TR2 then replace TR1. I say that because your TR1 measurements fit more to TR2 transistor. Anyway, to be 100% sure replace both TR1 & TR2 with new BC549C transistors and tell as if the problem solved. The other transistors looks ok.

Also post your voltages just to have a picture about them. With neg probe to ground measure voltages on pins 1,8,9 and 16 from the IC6 and post them here.
I’ll check my measurements again and I’ll change both components (it’ll be a good chance to have a proper look at the tracks as make sure I haven’t shorted/damaged anything). I’ll report back tomorrow night with findings and more readings.
Thanks so much for helping me out here. Really appreciate it.
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Re: Issue 4A with no colour!

Post by 1024MAK » Mon May 06, 2019 11:44 pm

If you are getting a good monochrome picture. By that I mean good contrast and brightness, then there can’t be much wrong with TR1 and TR2, as the monochrome (‘luminance’) signal passes through both of these.

Also TR1 has nothing to do with the colour part of the signal. The encoded colour signal comes from pin 13 of IC14 (LM1889) via C65 (100nF) to TR2 base, where it is mixed with the ‘luminance’ signal.

An oscilloscope would be a great help with fault finding this problem.

Mark
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Re: Issue 4A with no colour!

Post by Liveinabin » Tue May 07, 2019 6:34 am

The mono image is perfectly fine (well, for a Spectrum).
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Re: Issue 4A with no colour!

Post by 1024MAK » Tue May 07, 2019 8:17 am

So, some questions.

First, have you used a magnifying glass to carefully visually examine the areas of the video section where you did any work. Both on the top side of the board and on the underside of the board. Do this under a good bright light, or better still, in good daylight. You are looking for any of the following: cracks or breaks in the PCB tracks/pads, dry solder joints (cracked, distorted, discoloured, dull), solder splashes (including thin solder threads, balls of solder stuck in flux), components fitted the wrong way round, components where a leg has been fitted in the wrong hole.

Tell me about the display/monitor/TV that you are using. Did this Spectrum work okay before with it? Have you tried this Spectrum on another display/monitor/TV? Does this display/monitor/TV work okay with other 8 bit computers?

I understand that you have a digital multimeter. Tell me about your digital multimeter. Does it have a frequency range? Does it have a 200mV AC range?

Do you have any other test gear?

Do you have another ZX Spectrum? If yes, which model?

Mark
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Re: Issue 4A with no colour!

Post by Muttley Black » Tue May 07, 2019 12:17 pm

All this "transistors" story happens because @Ast A. Moore ask him to test the transistors and friend didn't know the way to do it. So i drive him to JoulesperCoulomb video that shows the way. After that, he post the measurements and we found out a faulty transistor. For sure the first suspect for missing color is the LM1889 but we cant ignore the fact that is a faulty transistor on the board.

Anyway. I've been here, on this forum not much time, but for a reason. i have read ALL threads on hardware category and what i want to say is that both @1024MAK and @Ast A. Moore can be easily named "ZX Resurrectors". And it is not only that. They are both very well technical educated members of this forum and they are always happy to help. So you are in good hands my friend. :D
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Re: Issue 4A with no colour!

Post by Ast A. Moore » Tue May 07, 2019 12:31 pm

Aww, thanks, Muttley, but Mark is by far much more knowledgeable in this area than I am.
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Re: Issue 4A with no colour!

Post by Liveinabin » Tue May 07, 2019 6:59 pm

Thank you all. I’ll try and get some time tonight/poss tomorrow to go over the Spectrum carefully.
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Re: Issue 4A with no colour!

Post by Liveinabin » Tue May 07, 2019 8:11 pm

1024MAK wrote:
Tue May 07, 2019 8:17 am
So, some questions.

First, have you used a magnifying glass to carefully visually examine the areas of the video section where you did any work. Both on the top side of the board and on the underside of the board. Do this under a good bright light, or better still, in good daylight. You are looking for any of the following: cracks or breaks in the PCB tracks/pads, dry solder joints (cracked, distorted, discoloured, dull), solder splashes (including thin solder threads, balls of solder stuck in flux), components fitted the wrong way round, components where a leg has been fitted in the wrong hole.

Tell me about the display/monitor/TV that you are using. Did this Spectrum work okay before with it? Have you tried this Spectrum on another display/monitor/TV? Does this display/monitor/TV work okay with other 8 bit computers?

I understand that you have a digital multimeter. Tell me about your digital multimeter. Does it have a frequency range? Does it have a 200mV AC range?

Do you have any other test gear?

Do you have another ZX Spectrum? If yes, which model?

Mark
I’m busy with the magnifying glass right now. Aside from a tiny bit of messiness around TR1 and TR2 caused when desoldering them, I can’t see anything else..yet. I will desolder those transistors (carefully) to get a better look at the board underneath.

My monitor is an LG 24” LCD. I use it as it runs everything; the RF input likes ZX81s and Spectrums (and even Fairchild channel F and old pong machines) and the composite input is happy enough with everything else. It’s been fine with my other Spectrum’s (both now composite modded) and it liked this busted one back when it was RF.

I attached a picture of my multimeter.

As for other test equipment. Sadly no. I understand an oscilloscope would be a benefit but aren’t they astronomically expensive for a hobbyist? If there’s a halfway affordable model, I’ll happily pick one up.

As for other Spectrums. I have 2 more. An issue one 16K and a Samsung 3B 48K



Image
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