48k in black and white

[Aiden]

Hi all, just received my eBay find speccy 48k issue 3 and it’s displaying in black and white, I’ve tried swapping the video chip for a known working one, any other ideas please?
Thanks
Aiden

[hellraiseruk]

Which issue board is it? Issue 6 boards had a colour encoder chip which is prone to failure. Might also be a tv issue, have you tried composite modding it? Spectrums don't have a video chip as such as the display is generated by the ULA, are you able to swap this?

JD

[Aiden]

Cheers
I’ll try the ULA as I have a known good one and it’s an issue 3, thanks

[berarma]

As far as I know a bad 12v line can disable colour.

[1024MAK]

Pins 14, 15 and 16 of IC14 (LM1889) should all be about +12V.

Mark

[Aiden]

Thanks berarma and Mark

I’ll swap out the ULA and check voltages.

[Aiden]

So
pin 16 is 9.3
Pin 15 is 8.17
Pin 14 is 11.83,(ok I take it)

Thanks

Aiden

[1024MAK]

Um, pins 14, 15 and 16 are all connected together via the PCB track….

If the voltage is lower than about 11V, IC14 will no longer work correctly, as it’s supposed to operate at 12V. If IC14 does not operate, the video picture will lack any colours, so will be black, grey and white.

The +12V supply comes from the +12V rail produced by the DC/DC converter, which is based on TR4, TR5, the coil and the associated capacitors and resistors.

Note that with the design of the DC/DC converter circuitry (such as issue 3 boards) id the DC/DC converter stops (or fails to start) oscillating, whatever the +9V (nominal) rail voltage is, will leak through the coil, hence then the +12V rail will have a similar voltage.

If the DC/DC converter circuitry is not working correctly, the -5V rail will be out of specification, that can damage the 4116 (or equivalent) DRAM chips.

So, it’s important to check the voltage on the pins of the 4116 (or equivalent) DRAM chips (e.g. IC6). Pin out and voltages shown in this post

Mark

[Aiden]

Thanks Mark
It’s actually a 4b board, I’ve turned it over to follow tracks and noticed the mess someone has made while socketing the LM1889N, can you spot the two legs with no solder?
Where do I need to bridge these two please? I think it’s pin 8 and 13

[1024MAK]

@Aiden

When I click on your image, I get a blank web page. Can you please post a link to a better resolution image?

Mark

[Aiden]

Hope this works

[Aiden]

https://share.icloud.com/photos/0b9VYCB ... Tyt7AtQvlA

[1024MAK]

Gosh, that’s awful soldering. So, someone had had a go at this before. That’s not good. It’s possible that there is some PCB track or plated through hole damage.

How good are you at soldering and desoldering?

Did you test the voltages for the +12V and the -5V on IC6?

Mark

[Aiden]

Thanks Mark
Voltages are all good. I’m confident with soldering and have been messing with electronics for a while. I have a desoldering station duratool D0067, an engineer ss02 desoldering pump, lots of solder wick and a patient steady hand.

[mark8bit]

bit vague on my part but i seem to remember having a issue with lack of colour that turned out to be a track that went from the lower cap leg on C65 to a via under the axial cap C40 or in that area at least, but can't remember which issue this was .

[1024MAK]

ZX Spectrum, IC14 (LM1889) connections:

Code: Select all

Pin Function                To/From
1   Chroma Lead             C39
2   R-Y (V) input           R77, C53
3   Chroma Bias             R78, C76
4   B-Y (U) input           R76, C52
5   0V/GND
6   Not used
7   Not used
8   Not used
9   Not used
10  Not used
11  Not used
12  “Video input”           R42
13  Chroma Sub-carrier out  R72, C65
14  RF Supply (+12V)
15  Sound Tank (+12V)
16  Chroma Supply (+12V)
17  Chroma Osc output       R43, R44
18  Chroma Lag              R45, C37

Note, not all parts of IC14 are used. Pin 12 although labelled “video input’, is not actually used, but connected to +12V via R42.
The inputs that are used are pins 2 and 4.

The output is from pin 13. This is ONLY the colour carrier output. It goes via C65 to TR2, where it is mixed in with the monochrome video from TR1.

Mark

[Aiden]

Thank you so much Mark, I’ll report back eventually

[Aiden]

Cheers Mark, got it working, I checked all the pins and pin 13 had no connection so bridged it and after a ULA change we’re all good.

[1024MAK]

Good to hear that you found and fixed the problem

Thanks for letting us know.

Mark