Unknown circuit find on Issue 4A.

[Muttley Black]

Hello all.

I find an unknown circuit on issue 4A and i want to understand why was there. Maybe was an external modulator? I don't really know.




Black cable was soldered on modulator case.
Yellow cable was soldered on composite video cable.
White cable was soldered on bottom speaker pin.


All this on a 4A issue as i said.






ZX Spectrum post, but very dark, no matter if this circuit was plugged or not. I decided to move to composite mod and that fix the dark issue. But still didn't find the "use" of this circuit, and why was fitted there.

Any help?

Thank you.

[1024MAK]

Based on the connections you describe, and the inductor plus the number of capacitors, I would speculate that it modulated the audio from the Spectrum onto the video signal. So that when tuned in to channel 36 via RF (UHF), instead of noise and hiss, the TV would reproduce the beeper sounds from the Spectrum.

Not seen anything like this as an add-on before though.

The loading of this on the composite video signal may have reduced the video signal level, hence why you got a dark picture.

Mark

[Muttley Black]

Thank you Mark.

I had dark picture on RF mode even with this circuit off the board. I haven't test the sound on TV, but i am gonna test it with other none composite modded unit and i report the results here.