Yellow screen

[98bishopmal]

Thanks spider,

It’s the same colours as before

[1024MAK]

I asked the question about the border, because one of the first things the ROM code does, is to TR6 to set the border to white. So if the border always turns white (yes, I know yours is yellow) then the ROM and the Z80 are likely okay.

Which you have proven by finding that with the upper DRAM disabled, you are now getting the copyright screen.

So as you have likely worked out, there are two faults; an upper DRAM fault (we will leave that for now) and a colour encoding fault.

I suggest next you renew IC14 (LM1889). But before you do, again with the screen displaying a clear screen, measure the voltages on all the pins of IC14 (with the black meter lead on any convenient 0V/GND point, such as the metal heatsink tab of the 7805 voltage regulator) listed here:
IC14 (LM1889)
Pins 2, 3, 4, 12, 13, 14, 15 and 16

List your results here on the forum.

Mark

[98bishopmal]

Pin 2, 4.95
3, 4.45
4, 4.97
12, 11.67
13, 11.67
14, 11.67
15, 4.31
16, 11.66

Chip has already been change and tested working on another board

If I disconnect a leg from C65 I get normal grayscale so white shows perfect then

[1024MAK]

Chip has already been change and tested working on another board

Ahh, yes, you said that you had changed it in your first post

If I disconnect a leg from C65 I get normal grayscale so white shows perfect then

Yes, that’s because C65 is the coupling capacitor that carries the colour carrier signal from pin 13 of IC14.

I’ll have a look at the voltages you posted tomorrow.

Mark

[1024MAK]

Actually, can you check those voltages again please. Pin 15 is definitely wrong.

Mark

[98bishopmal]

Sorry it’s as follows

Pin 2, 4.95
3, 4.45
4, 4.97
12, 11.67
13, 4.31
14, 11.67
15, 11.69
16, 11.66

Counted the pins wrong

[98bishopmal]

Hi Mark,

Do you have any ideas what I can try next?

[andywilliamsuk]

Interestingly I have the same issue - yellow screen - but I haven't got a composite mod (did have but reversed it out). I have got BC549 (reverse orientation) installed for TR1/TR2 perhaps I should revert to the original ZTX 313s when I get a chance. Will continue to follow the thread to see how you get on in the hope the fix when it comes will work for me too - I'm not so good on the analogue stuff!

[98bishopmal]

Hi [mention]andywilliamsuk[/mention]

Have you had any luck on yours?

I now have a Retroleum DiagROM and this picked up a bad IC17 so have swapped this and all tests run fine now, but still have the yellow tinted screen

I have also traced U and V to IC14 and these are fine as well

[andywilliamsuk]

Apologises. No I've been away from for a few days and not had a chance to play further. Back now so hopefully will take a look in the next day or so at the final output stages to see if I can figure out what is going wrong. I do have a good Spectrum that I can compare waveforms against so hopeful.

[98bishopmal]

Hi Andy,

Thank you, I have some new caps coming soon as i need to replace them but my guess is its still going to be the same but will keep you updated

If you do find anything then please let me know as it could be the same as my problem

Many Thanks

[andywilliamsuk]

Thank you, I have some new caps coming soon as i need to replace them but my guess is its still going to be the same but will keep you updated

If you do find anything then please let me know as it could be the same as my problem

Update: I'm getting close. After some careful probing around the board I've discovered that Pin 16 (V) and Pin 17 (YL) on the ULA are shorted together somewhere on the board. The righthand legs of the adjacent three small brown capacitors C71, C70 and C68 are all shorted together. I believe only C71 and C70 should be connected. I think this probably explains why the video output is so terrible, let alone having a yellow tint. Following the traces around the board is a nightmare! There are numerous vias some of which occur under other mounted components. I've left it for tonight and will take a look tomorrow with a fresh pair of eyes.

[andywilliamsuk]

Fixed!!!

Okay, utter nightmare trying to track down the short on the board. In the end discovered there was a little piece of FOD stuck in a via between two tracks which was shorting the U and YL signals together. Morale of the story - keep your desk clear of old solder and bits of wire. Cleaned the debris from the board and fully restored. Absolutely chuffed to bits. That's two completely dead Spectrum Plus from Ebay fully restored to working order. Just got one more to-do now which has an Upper Memory fault.

Have to say, if diagnosing faults - having an oscilloscope is a godsend because without it I don't think I'd have spotted the problem with Pin 16 and Pin 17 being shorted together. The YL signal was swamping the U signal. Now I know what they should look like normally for future reference.

If I get a chance later, I'll try and post some screenshots from the scope of what the signals should look like if that is helpful to folk.

[98bishopmal]

Hi Andy

That’s great news if you should post some screenshots that would be great

Many thanks

[andywilliamsuk]

Pin 17 (YL)



Pin 16 (V)



Pin 15 (U)



All of these signals are measured from power on, standard Sinclair copyright message being displayed. I've compared two Spectrums and the are showing the same outputs and known to have good video. In the case where I had a yellow screen and bad video, the signal on YL (Luminance) was also being plastered all over Pin 16 because of the short on the board.