VLa81 noisy video signal
Re: VLa81 noisy video signal
Meanwhile, yesterday I exchanged the 7805 by this regulator
and the result is a (slightly better to the eyes) thick diagonal moving bars Re: VLa81 noisy video signal
I tested and it mesure 0.5 Ohm
- 1024MAK
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Re: VLa81 noisy video signal
That’s okay.
Well, that’s me exhausted of all the simple suggestions…
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Amp
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
Re: VLa81 noisy video signal
I noted that, as I do testing the ground track around the board, the noise is amplified.
The far is the ground from the regulator, the bigger is the noise.
The far is the ground from the regulator, the bigger is the noise.
Re: VLa81 noisy video signal
Change the capacitor C2 (located close to the modulator) into a bigger one, 100nF or 1µF.
- 1024MAK
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Re: VLa81 noisy video signal
Because there are lots of digital signals constantly changing state, there will be switching noise on the 5V and 0V/GND points and it will be worse the further from the 7805 voltage regulator that you test.
Try adding an extra capacitor. Add a ceramic type (or better yet, a multilayer ceramic type) and connect it between the existing connections for C2 on the underside of the board, and/or add a capacitor (again a ceramic type) between pins 34 and 40 of the ULA socket (IC1) again on underside of the board.
Ceramic capacitors are not polarity sensitive, so can be fitted either way round.
The actual value is not critical, but suggested values are 100nF, 220nF, 470nF or 1uF.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
Re: VLa81 noisy video signal
I recently installed a composite mod per Mark’s design in my TS1000 which had terrible RF. I got a nice sharp picture but terrible vertical jailbars in the display. I had previously tried a 33uF electrolytic between the 5V and GND rails near the VR per a suggestion elsewhere but it made no difference. I also replaced the VR with a modern, high efficiency switching unit but also no change. Today after reading Mark’s post above I installed .47uF ceramic caps in the locations indicated and it did wonders to clean up the noise. I might do the same for the Vcc and Vss pins of the Z80 and SRAM chips too to see if that clears it up completely.1024MAK wrote: ↑Sat Aug 14, 2021 3:00 pm Try adding an extra capacitor. Add a ceramic type (or better yet, a multilayer ceramic type) and connect it between the existing connections for C2 on the underside of the board, and/or add a capacitor (again a ceramic type) between pins 34 and 40 of the ULA socket (IC1) again on underside of the board.
…
The actual value is not critical, but suggested values are 100nF, 220nF, 470nF or 1uF.
Re: VLa81 noisy video signal
Hi Mark, thanks for helping.1024MAK wrote: ↑Sat Aug 14, 2021 3:00 pm Try adding an extra capacitor. Add a ceramic type (or better yet, a multilayer ceramic type) and connect it between the existing connections for C2 on the underside of the board, and/or add a capacitor (again a ceramic type) between pins 34 and 40 of the ULA socket (IC1) again on underside of the board.
Mark
I will try that, but for now, as I had no solution I already close the computer.
Have to wait some days until I could dispend some time to open it again and try the incoming solutions.
- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
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Re: VLa81 noisy video signal
Fitting extra multilayer ceramic capacitors near the 7805 voltage regulator or DC/DC converter / switching regulator will also help. In this topic I posted photos of where I fit them when I install a DC/DC converter / switching regulator. But they may help even if you continue to use the existing 7805 voltage regulator.
Mark
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.