Yellow screen

[98bishopmal]

Hi All,

I have a ZX spectrum 48K issue 4b

When powered on it loads and works fine but the screen is like a dull yellow, is there anyway to change this?

I have replaced the following, RF module + done the comp mod
replaced the LM1889N chip

Still the same

Any help would be great

[1024MAK]

Compare the voltages on each lead of TR8 and TR9 (both BC184) while it is supposed to be showing a white screen.

Also run this program:

Code: Select all

10 FOR a=1 TO 22
20 FOR c=0 TO 7
30 PRINT PAPER c; “    “;
40 NEXT c
50 PRINT
60 NEXT a
70 FOR c=0 TO 7
80 BORDER c
90 PAUSE 25
100 NEXT c
110 GO TO 70

Note that there are FOUR spaces between the double quotes in line 30.





Which other colours are wrong?

Mark

[spider]

Before I read the topic, from the title I was expecting this to be an Issue 2 (with varicaps and var resistors) with a case of 'Spectrum Jaundice' (mine has this!) , although I see you've said tis an Issue 4, hence automatic control of those.

One small thing you might want to add although I don't actually think it is relevant (but you never know) what type of composite mod did you do ?

> Straight through (in/out)
> With a capacitor (typically 100uf or so)
> With a transistor with or without a cap as well
> Something else (please say)

Also are you able to temporarily test the Spectrum on another display / TV etc ? I don't think it -would- make much difference but if its convenient it is worth trying once you've followed Mark's excellent answers above.

[98bishopmal]

Thanks mark,
I’m getting 4.9v on both
I am very new to spectrum so not sure how to run that

I have noticed that if I short R44 white shows correct but there is a lot of static on the screen, the resistor measures as 5.13 is that correct?
Comp mod was done with 100uf

[spider]

Are you able to type it in ? As I mean know how to use Caps or Symbol Shift to get the keywords / symbols ?

If not, for the immediate moment:

Reset the machine.

Following Mark's instructions on comparing colours (so you'll need to do this with numbers from 0 to 7 or at least 0 to 6) , the program would be better but this may do for the immediate moment:


At the copyright message press B once , "BORDER" should appear along with a flashing "L"

Press 0 (the number key zero)

Press ENTER

The border should turn black with an OK message, press ENTER again and you'll get a flashing "K"

Now hold down both Caps Shift and Symbol Shift, the flashing cursor should become a flashing "E"

Release both shift keys or Caps Shift at least, then press and hold Symbol Shift and while holding it down press the "C" key

"PAPER" should appear along with a flashing "L"

Press 0 (the number key zero) and then press ENTER a couple of times until the central area of the screen is black and you have a flashing "K"


You can now do these tests as above regarding colour comparison.

Repeat the above key entries with number 1 then 2 etc to 6 , 7 will give white again.

If you make a mistook press Sym Shift and the number 0 key to 'delete' or you can (as there's not a line to edit) press Sym Shift and 1 (edit) which will clear it too.

Hope that makes sense and I've not sounded patronising not intentional I don't know how much you know to type in etc as if you've not used a machine with token keyword entry it can be confusing if used to "letter by letter" entry.

[p13z]

If you make a mistook press Sym Shift and the number 0 key to 'delete' or you can (as there's not a line to edit) press Sym Shift and 1 (edit) which will clear it too.

The above post is nice and helpful -
but you'd want to use CAPS shift and 0 to delete
or CAPS shift and 1 to edit/clear a line
(as opposed to symbol shift in this case).
Just thought I clear up any potential typo confusion

[98bishopmal]

Hi Andy,

That's perfect thank you, I will give that a go later and update with the results, this was my grandads computer so will be nice if i can get this working, but this is what the screen looks like at the moment



Thanks for all your help so far everyone

[dfzx]

...this was my grandads computer...

Oh man, make us all feel old why don't you?

[stupidget]

.....this was my grandads computer.....

I good lord I feel old now

[spider]

The above post is nice and helpful -
but you'd want to use CAPS shift and 0 to delete
or CAPS shift and 1 to edit/clear a line
(as opposed to symbol shift in this case).
Just thought I clear up any potential typo confusion

Ah yes my fault, thank you. Can't edit it now but I gather the poster knows anyway.

Hi Andy,

That's perfect thank you, I will give that a go later and update with the results, this was my grandads computer so will be nice if i can get this working, but this is what the screen looks like at the moment



Thanks for all your help so far everyone

You could try different colours (no need to do them all) for instance set the border one colour then the paper another. I'm avoiding things like 'bright' or 'flash' etc as there's no need and as you're typing it in you need it as 'quick and easy as possible'

[98bishopmal]

Ok

So black is blackish
Blue is a very dark green
Red is fine
Magenta is a little lighter red
Green is bright green
Cyan is bright green
Yellow is yellow
White is also yellow

I have replaced tr9 and tr8 from a donor bored and still the same and have also now replaced tr1 and tr2 still the same

[1024MAK]

Time to change the ULA.

Mark

[98bishopmal]

Thanks mark

Is it worth change x1 and x2 as well or will this not help?

[98bishopmal]

Also where can I get a replacement?

[Sol_HSA]

Also where can I get a replacement?

Unfortunately, from another speccy. There are modern reproductions, though, but then you're getting into the whole Theseus's ship thing.

[98bishopmal]

Thanks Mark,

Managed to find one on eBay will let you know how it goes

[1024MAK]

Is it worth change x1 and x2 as well or will this not help?

I don’t think that will help.
If X1 was faulty, you would not get any stable screen display.
If X2 was faulty, as in not working at all, you would be no colours at all, just a monochrome picture. And if it was slightly off frequency, all the colours would be wrong.

It’s the analogue outputs from the ULA that control IC14. But this chip is normally fairly reliable.

Mark

[spider]

Regarding replacement (original types not 'modern replacements') see:

Here , please scroll down to the " What ULA works in what machine ? " section

and also Here


As I see it, in a brief sentence, as per the first link)

... The 6C001E-7 ULA is backwards compatible with every 48K machine from the Issue 1 right up to the Issue 6A ...

[Sol_HSA]

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

[1024MAK]

Regarding replacement (original types not 'modern replacements') see:

Here , please scroll down to the " What
As I see it, in a brief sentence, as per the first link)

... The 6C001E-7 ULA is backwards compatible with every 48K machine from the Issue 1 right up to the Issue 6A ...

Each ULA version can be used in the board issue it was intended for and any earlier issue boards. In other words, it’s backwards compatible with earlier boards.

The 5C102E ULAs were used in issue one and some issue two boards. The ‘dead cockroach’ daughter board is required. The transistor ‘spider’ should also have been fitted, but at the time the problem that this fixes had not been discovered.

The 5C112E ULAs were used in most issue two boards. The ‘dead cockroach’ daughter board was no longer required, the design of the ULA having been modified. The transistor ‘spider’ was however required.

The 6C001E-5 ULAs were used on some late issue two boards. The transistor ‘spider’ was however still required. May also be found in some issue three boards.

The 6C001E-6 ULAs were used on issue three boards. An additional transistor ‘spider’ was no longer required on issue three and later boards, as it became a permanent part of the PCB circuitry. This ULA version may also be found on some issue two boards. All issue two boards still need the ‘spider’.

The 6C001E-7 ULAs were used on issue four and later boards. An earlier ULA version is likely to cause unreliable operation in issue four and later boards due to a design change with both the ULA and the circuitry on the boards. However the 6C001E-7 is still backwards compatible with earlier boards.

Mark

[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