UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Kenwood TM322. (Actually a TM-733).

SiriusHardware · 27th May 2020, 3:48 pm

Well, as you seem to know, CTCSS actually is 'Hum', with a frequency of between about 67Hz and 250Hz, depending on the actual tone in use. It's injected onto your carrier at about 10% of the maximum voice audio level.

Since you have this problem, presumably equally, on both 70cms and 2m it sounds like the CTCSS is being injected at much too high a level / percentage. That's going on the evidence of your spectrum readings. If it was only excessive on 2m that would just suggest that the CTCSS input level on 2m needed to be reduced in similar fashion to the audio deviation.

Another thing to be aware of is that there are two types of receiver in the world, one which 'knows' about CTCSS and the other type, usually older, which doesn't. Receivers which are designed with CTCSS in mind shave off the audio frequencies below 250Hz before they are passed on to the audio amplifer and the speaker, but other receivers pass audio all the way down to 50Hz through the receiver and so anyone who is transmitting a CTCSS tone will appear to be humming. If working a net, with a wide selection of receivers listening, some of those users will complain you are transmitting a hum, others will not be able to hear it.

27th May 2020, 3:48 pm	#13
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,575	Re: Kenwood TM322. (Actually a TM-733). Well, as you seem to know, CTCSS actually is 'Hum', with a frequency of between about 67Hz and 250Hz, depending on the actual tone in use. It's injected onto your carrier at about 10% of the maximum voice audio level. Since you have this problem, presumably equally, on both 70cms and 2m it sounds like the CTCSS is being injected at much too high a level / percentage. That's going on the evidence of your spectrum readings. If it was only excessive on 2m that would just suggest that the CTCSS input level on 2m needed to be reduced in similar fashion to the audio deviation. Another thing to be aware of is that there are two types of receiver in the world, one which 'knows' about CTCSS and the other type, usually older, which doesn't. Receivers which are designed with CTCSS in mind shave off the audio frequencies below 250Hz before they are passed on to the audio amplifer and the speaker, but other receivers pass audio all the way down to 50Hz through the receiver and so anyone who is transmitting a CTCSS tone will appear to be humming. If working a net, with a wide selection of receivers listening, some of those users will complain you are transmitting a hum, others will not be able to hear it.