ZX Spectrum+ video signal problem

dizzy33
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Joined: Mon Jan 13, 2020 3:24 pm

ZX Spectrum+ video signal problem

Post by dizzy33 »

Hello,

I own a ZX Spectrum+ which is working fine but it has a small video output problem :
I did the composite mod on it, I don't know how was the picture before (never tried using the RF modulator), but the picture is a bit blurry compared to my ZX Spectrum issue 2 with composite mod, plus the colors are not colorfull enough (sorry for my vocabulary, I am french), I mean the yellow is too pale. Colors need a boost ...
On the issue 2 motherboard, you can adjust these settings but not on issue 4S.

Unless this is a normal display for an issue 4S, do you know which motherboard component may require to be replaced ?

For information I have already replaced :
- Z80
- ULA
- TR4 and TR5
- 7805 by a TRACO
- all capacitors
dizzy33
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Re: ZX Spectrum+ video signal problem

Post by dizzy33 »

I answer to myself ... I have read that TR8 and TR9 are replacing the issue 2 potentiometers for the colors ... So maybe TR8 or TR9 is the culprit.
I will try to replace them.
Lardo Boffin
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Re: ZX Spectrum+ video signal problem

Post by Lardo Boffin »

Or get one of these. Rock steady output with no dot crawl!

http://www.fruitcake.plus.com/Sinclair/ ... erface.htm
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1024MAK
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Re: ZX Spectrum+ video signal problem

Post by 1024MAK »

It’s VERY rare for TR8 and TR9 to fail so it is unlikely that these are the cause.

What voltage do you get on pins 14, 15 and 16 (all three should be linked together) with respect to 0V/GND?

Do you have, or have access to an oscilloscope?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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dizzy33
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Re: ZX Spectrum+ video signal problem

Post by dizzy33 »

Mark, which component are you talking about, when you ask me to check some pins ? ULA ?
Yes, I own an oscilloscope.
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1024MAK
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Re: ZX Spectrum+ video signal problem

Post by 1024MAK »

ULA (IC1) pins:
  • 17 = /Y monochrome video signal (connect to channel 1 on your 'scope and trigger on this signal, then use another channel to test the other signals)
  • 15 = U colour difference signal
  • 16 = V colour difference signal
IC14 (LM1889) pins:
  • 2 = R-Y input
  • 4 = B-Y input
  • 13 = chroma output
  • 17 = crystal oscillator
Transistors
  • TR1 base / R48 / R49 / R50 junction (monochrome video signal at a lower level)
  • TR2 base - inverted version of above with the chroma signal mixed in
  • TR2 emitter - composite colour video signal
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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dizzy33
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Re: ZX Spectrum+ video signal problem

Post by dizzy33 »

Mark, please remember that I am a noob ... On my oscilloscope (FNIRSI 5012h, the one you recommanded from Amazon), there is only one main probe (so only one channel I guess ?). To do my measurements, I put the main probe on the component pin, and a small secondary probe (connected to the main probe) to the ground of the motherboard.
To test pins 15 / 16 / 17 of ULA, I have put the ground probe on the modulator, and the main probe on pin 15, then 16, then 17.
I have then compared with the ULA of another Spectrum, and I have quite similar results.

For IC14, with my oscilloscope, the only pin where I could find a signal is pin 17 (pin 2/4/13 does not show any signal). I compared this result with another Spectrum and the result is exactly the same.

For TR1 and TR2, the oscilloscope graphs are the same on both Spectrum. But for the one with the "poor" video color quality, the voltages on both TR1 and TR2 are missing 0,20v compared to the good Spectrum. I don't know if it's in the tolerance window.
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1024MAK
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Re: ZX Spectrum+ video signal problem

Post by 1024MAK »

dizzy33 wrote: Sun Apr 19, 2020 3:21 pm On my oscilloscope (FNIRSI 5012h), there is only one main probe (so only one channel I guess ?).
Sorry, I try to help many, so sometimes forget who has what equipment :oops:

The signal on IC14 pin 17 should be a 4.43 MHz AC waveform.
The signals on IC14 pins 2 and 4 should be between 0V and 75mV (0.075V).

With respect to TR1 and TR2, are you saying that the signals are 0.2V different to the signals on another ZX Spectrum?

Can you please test the DC voltage on IC1 pin 13 please (expect between 3V and 3.5V here).

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: 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.
dizzy33
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Joined: Mon Jan 13, 2020 3:24 pm

Re: ZX Spectrum+ video signal problem

Post by dizzy33 »

Mark, no problem, I understand, thanks a million for your time to help me !

- IC14 pin 17 shows a very regular 4.43 MHZ waveform (2.80V)
- IC14 pin 2 and 4 show no waveform, 0V.
- IC1 pin 13 shows 3.20V

About TR1 and TR2, yes, on this Spectrum with poor colors on display (especially the yellow which is very poor) I confirm the signals are 0.2V different ( 0.2V lower) from another ZX Spectrum (which has a very good video signal).
I measured around 1.40V for my working Spectrum, only 1.20V for this "defective" one.
dizzy33
Posts: 108
Joined: Mon Jan 13, 2020 3:24 pm

Re: ZX Spectrum+ video signal problem

Post by dizzy33 »

Mark, for TR1 and TR2 on this Spectrum with poor colors on display (especially the yellow which is very poor) I confirm the signals are 0.2V different ( 0.2V lower) from another ZX Spectrum (which has a very good video signal).
I measured around 1.40V for my working Spectrum, only 1.20V for this "defective" one.
Do you think I should replace TR1 and TR2 ?
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