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.
Gray color, a second after power up
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Re: Gray color, a second after power up
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
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
Looking forward to summer later in the year.
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Gray color, a second after power up
Yes
Issue 3
Correct.
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 )
Last edited by RMartins on Sat Dec 29, 2018 3:51 am, edited 1 time in total.
- 1024MAK
- Bugaboo
- Posts: 3123
- 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
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
Looking forward to summer later in the year.
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Gray color, a second after power up
Thanks
So from that source
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.
So from that source
That's exactly the frequency of the crystal used for generating the PAL output (4.4336MHZ) on the Spectrum.When used with PAL, the colour subcarrier is 4.43361875 MHz ...
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.