Yellow screen

[1024MAK]

..and the modern replacement that is apparently the currently recommended one.

There is also the NebULA

Mark

[98bishopmal]

Think i have messed up

I removed the old ULA and put the new one in and now get yellow boarder and black paper, reboot a couple of times and get pure yellow screen and white cursor on bottom right of paper.

Any help would be great

[spider]

Think i have messed up

I removed the old ULA and put the new one in and now get yellow boarder and black paper, reboot a couple of times and get pure yellow screen and white cursor on bottom right of paper.

Any help would be great

It won't help (me) help you but just for clarity for 'the experts' what was the part number of both the "old" and "replacement" ULA's please ?

Unsure if a pic of "it now" with the "cursor" in the right hand (!) corner will help or not, if its convenient it won't do any harm to post a pic of the screen I guess, never know might give thoughts.

I was going to say try the "old" one back to see if there's any pattern change (as in something else amiss) but I'd say hold off for now until the tech experts appear hopefully.

It may sound stupid sorry but you were careful to not damage or 'fold under' any pins when you fitted it yes ? I have seen this on other machines where ROMS' were swapped over and its dead easy to accidentally get a pin bent under in a moments inattention, been there myself!

[1024MAK]

Yep, as Spider says, we need to know the part numbers of the original ULA and of the replacement ULA.

DO check that each pin is okay. And yes a photo of screen showing the symptoms would be helpful.

Remember, fault finding is a process of elimination.

Mark

[98bishopmal]

Thanks both,

ULA is the same as the old one 6c001e-7

All pins are connected, I have put the old ULA in and getting the same

This is what I see, sometimes the paper is just black



Many thanks

[98bishopmal]

Thanks both,

ULA is the same as the old one 6c001e-7

All pins are connected, I have put the old ULA in and getting the same

This is what I see, sometimes the paper is just black



I am also getting this sometimes



Many thanks

[1024MAK]

Is the border always that colour? Power down, wait ten seconds, power up gain. Do this at least five times. Does the border always end up being that colour?

Mark

[98bishopmal]

Hi Mark,

Yes the border is always that colour, I have done as requested and still doing the same.

Before the system was working fine but with yellow boarder and incorrect colours so did the following to try and fix it.

Replaced TR1 and TR2 (same result)
Replaced TV module (same result)
Replaced TR8 and TR9 (same result)
Replaced ULA (now having the problem where does not show copy write message and sometimes gets stuck at black paper)

Both ULAs are 6C001E-7

I have replaced the ULA with the old working one and getting the same.

Many thanks

[98bishopmal]

Ok so I have disabled the upper ram and this now shows the copy write message

But still has yellow boarder and colours are not correct

[spider]

Ok so I have disabled the upper ram and this now shows the copy write message

But still has yellow boarder and colours are not correct

That sounds positive to me. Until the tech members get back to you, now its showing the (c) screen, do the keys work ? Are you able to try any simple programs perhaps to type in or even perhaps load a tape ? If you do the latter it would have to be a 16K title.

But I guess anything would do for a basic "load" test if needed, I'd probably at this stage want to see if the keys worked and what you get with setting different colours (BORDER x PAPER x etc) again at this point.

[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.