Should there be SOUND on RF?

[gehtjanx]

Hi,

I have a Sinclair Spectrum +2 (Grey model, 128K with tape drive inbuilt)

I am relatively new to Spectrum.

The TV Mod was snapped off and some solder Joints were gone. I repaired that.

The RF Output VIDEO does work, but there is NO AUDIO sent to the TV (via RF Modulator).

I know there is a SOUND jack but I would like to know if that is normal to not have Audio via RF or of there is anything broken?

My friend said it _should_ have Audio on RF (but was not sure), but the Modulator only has 3 contacts:
- +5 V as far as I know (cable)
- Video Signal (cable)
- GND (modulator's case soldered to the board)

So in theory what I understand, there can't be Audio as there is not Audio in signal.
I would like to know if you can confirm that?!?!?

Thanks!

[RetroTechie]

Welcome to this forum, gehtjanx!

You already gave the answer: to have audio in the RF signal, you'd need an RF modulator that has inputs for both video and audio, so that it can include sound in the RF signal along with video.

Checking a few schematics it appears that's not the case (as you confirm), therefore: no audio in the RF signal. Indeed it's a simple case of looking what connections the RF modulator has.

[gehtjanx]

oh cool, thank you so much! Just wanted to be sure!

I now made a Composite cable, but I wanted to repair the mod someone had done to it and make it fully working again. I also gave new rubber feet, screws (were missing), a new silver paint onto the logo and glued the PAUSE key, because it was broken. I am not sure if I Need that Cassette Thing or if I can repair the button or Exchange the drive sometime later.

I have also a DivMMC Enjoy with a 3d printed case that I will use / learn more from Sinclair (I am a commodore/Amiga/Atari guy until since I got my 48 and no +2 sinclair)

So a lot to learn new for me. Maybe I will have some more questions later.

There is my Sinclair - you sure know what it Looks like but I am proud of Now I Need the rest of the series or a ZX80/81, too

[balford]

Your friend was correct - on the 128K models the audio is modulated onto the composite signal before feeding to the RF modulator (this wasn't the case on the 48K models).

So you should have audio over RF, if you don't then L5 may be out of adjustment, or someone may have removed C31 to remove the modulated audio carrier from the composite output without reconnecting it to the collector of TR2 to keep it on RF.

There's an excellent video which covers this subject towards the end here: https://www.youtube.com/watch?v=nLCi4da3f1g.

B

[1024MAK]

Yes, on the 128k models, they did not include an internal L/S. But instead modulated the audio and mixed it into the composite video signal. That's why they could use the cheaper video only modulator. Details of this and other problems (including fixes) here on WoSF

Mark

[dr beep]

Never understood why this wasn't default on the ZX Spectrum. It is an easy fix and saves a speaker.

[RetroTechie]

@balford and Mark: you guys are right, I stand corrected! Learning something every day. Schematic shows audio passes through IC11 (MC1376P, some sort of modulator IC?) which feeds it into the luminance signal through C31.

Don't understand why it was done that way though. Wouldn't it have been easier to just use an RF modulator that comes with audio input?

[1024MAK]

It's the same in the black Sinclair 128k "toastrack". I suspect it was done that way due to either cost, or because it was developed for the Spanish market first (or maybe both). Then the same system was used in later machines.

zx128a.png

(86.16 KiB) Downloaded 832 times

Mark