question Clansman PRC351 Squelch

Another PRC351 question, I'm afraid. Having got my misbehaving PRC351 to fire up, the squelch seems over-enthusiastic and kiils the receiver entirely. I know that the unit is working as defeating the squelch in the * position it receives OK, but is silent in L and W. Does anyone know where the squelch adjustment is? I know that these units sometimes suffer from sticky potentiometers and it seems possible that a quick twiddle might clean it off.

Andy

Andy

It is a 150Hz tone squelch system (CTCSS) - a locally generated 160Hz tone is also injected to the receiver (unless in rebroadcast mode) so strong signals without 150Hz tone are supposed to be modulated locally and also open the squelch. EMER F582 says that the squelch sensitivity pot is 9R2 in squelch module 9 but the 160Hz injection level will also affect the end result for signals lacking 150/160Hz CTCSS.

The 160Hz source is part of transmit audio assembly 13 - the EMER shows frequency and level adjust pots but the partial circuit diagram in the EMER does not give their numbers. The layout diagram shows it as R9 OSC LEVEL on module 13. The output is module 13 pin 6 if you want to check the level and frequency with a scope.

I have noticed similar interworking issues with amateur 6M and 4M FM transceivers to the point that one of my 351s cannot hear a 1W station a few yards away in L or W mode but can in * mode over miles and I suspect the 160Hz injection is either turned down or there is a switching issue that disables it as if in rebroadcast mode.

Out of curiosity is the problem set a 351 or 351M? - I am fairly sure the one I noticed this problem with was a 351M which has modified audio circuitry for a BID300 speech scrambler requiring wider audio bandwidth (I measured below 200Hz to above 8KHz rather than the usual 300Hz-3KHz) - if so it may be deliberate to stop the 160Hz injection interfering with the BID

Regards

Iain

73 de G0OZS

Thanks Iain - it's a standard 351, for reference.

Unfortunately tweaking 9R2 to full scale in either direction has no effect, so I think it's bust. :-(

The comment about 13R9 is interesting as I thought that turning it fully anti-clockwise was a "standard" mod for 4m use to prevent the 150Hz CTCSS tone annoying civilian users. I had my ear bent about the tone this morning on the 4m Portsmouth "TedNet"! Turning 13R9 fully anti-clockwise doesn't seem to have affected the squelch on my other 351s.

Andy