Gray color, a second after power up

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Post Reply
User avatar
RMartins
Manic Miner
Posts: 776
Joined: Thu Nov 16, 2017 3:26 pm

Gray color, a second after power up

Post by RMartins »

Hi, guys

Long time no see. Life has kept me busy with other stuff.

I'm working on reviving another ZX48, which is already working, after a few required fixes (Power transistor, damaged high memory chip, etc ...).
But it still has this strange glitch, where it loses color after a second of power on.

I'm using one of these test interfaces, that provides a colorful start screen, and I see the color, but then it goes away.

I have checked the video signal, and it shows the color burst were it is supposed to, and the color frequencies over the pixel data, on power up and after.

I think the problem might be related with the the video frequency of the burst signal, getting slightly of mark, as soon as something worms up, or some capacitor charges up.
I'm considering replacing, the small caps that connect directly to the crystal, since they influence the oscillating frequency.

Have any of you found a similar situation ?
Any tips ?

Thanks, in advance.
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Gray color, a second after power up

Post by 1024MAK »

Obvious first “stupid” question, does your display work okay with other ZX Spectrums of the same type?

Which issue board does your colour shy ZX Spectrum have?

And I presume you have looked at the video signal with an oscilloscope? So have you checked the frequency that the colour encoder chip is running at?

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
RMartins
Manic Miner
Posts: 776
Joined: Thu Nov 16, 2017 3:26 pm

Re: Gray color, a second after power up

Post by RMartins »

1024MAK wrote: Sat Dec 29, 2018 1:59 am Obvious first “stupid” question, does your display work okay with other ZX Spectrums of the same type?
Yes
1024MAK wrote: Sat Dec 29, 2018 1:59 am Which issue board does your colour shy ZX Spectrum have?
Issue 3
1024MAK wrote: Sat Dec 29, 2018 1:59 am And I presume you have looked at the video signal with an oscilloscope?
Correct.
1024MAK wrote: Sat Dec 29, 2018 1:59 am So have you checked the frequency that the colour encoder chip is running at?
Are you referring to input frequency or the output frequency ?
I don't remember what it was, but I can re-test, when possible.

Any particular pin to reference ?

I remember the output frequency to be identical to what comes as signal out (colour burst signal), since it seems to be an adder circuit.

Which frequency (Hz) should it be ?

Is there a public standard reference for PAL colour freely available for reference ?
( There are too many PAL standards variations, already :shock: )
Last edited by RMartins on Sat Dec 29, 2018 3:51 am, edited 1 time in total.
User avatar
1024MAK
Bugaboo
Posts: 3104
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: Gray color, a second after power up

Post by 1024MAK »

Quickly, I can give this link for the last question, the rest will have to wait for later...
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
User avatar
RMartins
Manic Miner
Posts: 776
Joined: Thu Nov 16, 2017 3:26 pm

Re: Gray color, a second after power up

Post by RMartins »

Thanks

So from that source
When used with PAL, the colour subcarrier is 4.43361875 MHz ...
That's exactly the frequency of the crystal used for generating the PAL output (4.4336MHZ) on the Spectrum.

So I'll try to measure if that's what I'm getting, although is not very easy to measure since it's not a constant signal.
I'll try to use the Math and TV features of the Oscilloscope to see if I can get a decent reading of it.
Post Reply