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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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin »

dfzx wrote: Sun May 19, 2019 10:09 am
Liveinabin wrote: Sat May 18, 2019 5:00 pm I’m glad it’ll belong to someone who can fix it or at least revive another Spectrum with it.
Having quietly followed the saga for 5 pages of comments, I'm now looking forward to Mark telling us all what's wrong with the flippin' thing!
Nothing before I started mucking about with it :)
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1024MAK
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Re: Issue 4A with no colour!

Post by 1024MAK »

Hi

I thought you all would like an update.

I’ve found two problems.

The cause of the loss of the video output, is a broken PCB track on the underside of the board. Between transistor TR1 collector lead and resistor R51. From R51 the track goes to TR2 base. Hence TR2 having no signal, is just turned fully on all the time due to resistor R51. Therefore there is no valid video signal going to the output socket (this machine has been composite video modified).

The cause of no colour is a missing PCB track between capacitor C65 + pad and a through-hole via under capacitor C40. This track is (well, was!) on the top side of the board. It links C65 + with R72, which in turn is linked to pin 13 of the LM1889 colour encoder chip. With this missing track, there was no way the colour video carrier signal could be mixed into the monochrome video signal.

But it appears that the system is running, as if I simulate key presses, the bleeper clicks, and if I fill up the input line, eventually I get the error rasp sound (which is BASIC saying, I’m not having that as an input!). And on my oscilloscope, I get a good video waveform at TR1 base.

So I fired up the soldering iron and set to repair these two problems.

Here’s some photos... :D

The relevant part of the schematic diagram:
Image

The damaged track causing the loss of the video signal:
Image

What, can’t see the break? Try this photo...
Image

Okay, so no you can’t really see in the earlier photos. So I cleaned the old flux and dirt off the board. Now you can see the track. It does not look like it is broken. But I get intermittent continuity when I test with the multimeter.
Image

Having confirmed that the track is the problem, I desoldered the relevant solder pads ready to fit a wire to replace the track. Look at the track now:
Image

And here is a shot showing the wire that replaces the broken track:
Image

This is the missing track that resulted in no colour in the picture (well, the bit of the board where the track used to be, you can just see the light green line where the copper track has been lifted away):
Image

And the fix (put on the bottom side so it does not spoil the top of the board and is easier to fit on this side anyway :mrgreen: ):
Image
A screen shot showing it now does colour (sorry, without a keyboard attached, I could only be bothered to poke wires in the keyboard sockets to type in BORDER 3)...
Image

So as you can see, it now works okay.

When I have got a keyboard membrane to hand, I’ll do a few tests to see if it’s running as a 48K machine.

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

Post by Muttley Black »

Well done Mark! :)
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Liveinabin
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Re: Issue 4A with no colour!

Post by Liveinabin »

Superb stuff, Mark! I'd never have found that in a million years. Glad I sent it to you. It's clearly gone to a better home :)
Barrakuda
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Re: Issue 4A with no colour!

Post by Barrakuda »

1024MAK wrote: Tue May 07, 2019 9:17 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
Have you tried this Spectrum on another display/monitor/TV?
This question probably saved me hours of troubleshooting!

My ZX Spectrum 3B was missing color, using a different LCD TV showed color.

Thanks Mark!
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