Tecnimagen 14TXS - Can't change sound from 5.5MHz to 6MHz

[JamesQB]

Hi,

It's been a while since I last did sound conversion on a TV set. Living in Spain, I used to get asked to alter the sets people had brought from the UK (Pal I) to work with Spanish signals (Pal BG), and although rarely, to convert the other way around, if someone had bought a set in Spain to work with the UK equipment. Normally, I'd just change all the sound filters (5.5MHz for Spain) to the ones used in UK sets (6.0MHz), and then tweak the relevant inductor core to bring up perfect sound. Haven't done this on anything modern though.

I've now been given a 14" set to change from Spanish to English sound. I changed the 5.5MHz filter to a 6.0MHz one, but could find no inductor to alter. A look at the service manual (all in Spanish I'm afraid, it's manufactured here) shows that the IC STV2246 internally handles separating the audio from the video signal. They do a multisystem version of the set with the difference being that STV2248 is used instead, and a handful of components would be present where the PCB shows spaces for them. I think the menu option for system (Euro, UK and another) causes the IC to switch between the internal filters.

I can't see an easy way around this, because even if I changed the IC and added the parts missing from the PCB, the menu still wouldn't allow me to change from Euro to UK, as it doesn't at present. So the IC governing the menu would need changing or re-programming?

I wish I understood this part of TVs better, as I'm not sure how it works. It seems that the IF signal is split into video and audio and fed into the IC STV2246, and then an output named as CVBS uses the sound filter (5.5MHz or 6.0MHz now I've changed it) to remove the sound carrier. So CVBS is the video output from the chip? If that's the case, why is there an audio signal which must be removed using the sound filter crystal, if the sound and audio are separated before going into the IC?

If anyone else gets around these sound conversion problems somehow, I'd like to know what I could do.

Thanks to anyone who can offer advice.

James