VLa81 noisy video signal

[AlfaRomeo]

Meanwhile, yesterday I exchanged the 7805 by this regulator

and the result is a (slightly better to the eyes) thick diagonal moving bars

[AlfaRomeo]

With the computer disconnected from the power, test between the outer ground/0V connection of the phono output connector on the modulator and the heatsink tab of the 7805 voltage regulator using the 200 ohm resistance range on your multimeter.
Mark

I tested and it mesure 0.5 Ohm

[1024MAK]

With the computer disconnected from the power, test between the outer ground/0V connection of the phono output connector on the modulator and the heatsink tab of the 7805 voltage regulator using the 200 ohm resistance range on your multimeter.
Mark

I tested and it mesure 0.5 Ohm

That’s okay.

Well, that’s me exhausted of all the simple suggestions…

Mark

[AlfaRomeo]

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.

[zx-heinz]

Change the capacitor C2 (located close to the modulator) into a bigger one, 100nF or 1µF.

[AlfaRomeo]

Change the capacitor C2 (located close to the modulator) into a bigger one, 100nF or 1µF.

Thanks, will try it

[1024MAK]

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.

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

[DrVenkman]

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.

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.

[AlfaRomeo]

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

Hi Mark, thanks for helping.

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]

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