Kenwood TM322. (Actually a TM-733). - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

SiriusHardware · 28th May 2020, 8:36 pm

Page 6 of this Kenwood manual - not the same radio, but the same KRR-C001 part and a good description of it.

http://www.catlog.net/_Descarregas/M...d%20TM-231.pdf

Presumably the discrete external pull up resistors and the complex ladder network inside the package combine to produce the desired voltage output for each possible binary input value.

Maybe the choice of value for the external resistors determines the size of the output voltage swing?

Jon_G4MDC · 28th May 2020, 10:32 pm

Thanks for swelling my Kenwood "gen" folder even more!
Never lose a .pdf...hehehe.

I'm no expert on this but I suppose you could mux the output lines in a way which generates the wanted tone frequency as a piece-wise sine wave with a few bits up to the maximum of 7bits resolution?

In that way there could be some level control from the uP, but then lower output levels would have more THD? Y/N?

SiriusHardware · 28th May 2020, 11:12 pm

The waveform resulting from less bits being used would be rougher/ lower resolution, but you are right, instead of utilising the full output swing of the DAC you could output a reduced range of values and thereby control the size of the output waveform to some extent. Imagine if it got to the point where you were only using one bit. The output wouldn't be much of a sine wave.

The 'official' way to change the size of the output swing without degrading the resolution of the signal would be to change the value of those fixed resistors and continue to output a 7-bit sinewave to the resistor ladder.

If the aim was to reduce the level of CTCSS tone going out to both RF PCBs, probably the simplest practical method would be to insert a fixed or variable divider after the three-component filter.

John-39 · 2nd Jun 2020, 9:11 am

Solution found! As so often, the answer was not as technical as feared. The clue was in the advice that there aren’t separate controls for setting Tone and Audio levels. I noted the position of the Max Dev control RV2 before experimenting with it and found it had been turned to maximum. This would explain why the transmitted CTCSS spectrum was twice as twice as wide as that from the Baofeng. Turning RV2 down then reduced the audio deviation. Perhaps it had been turned up because of a fault in the mic. circuit, but applying an audio generator at the level given in the manual showed good deviation. The mic. was then inspected carefully. I found that what was probably once a bit of plastic gauze behind the hole in the front of the mic. had turned to a thick paste. Poking this out solved the audio problem and the set is now working fine. Although some users say they can still hear a slight hum, it is obviously heaps better and quite usable. (Incidentally the TO line showed a well-shaped sine wave but with considerable noise on it.) Thanks everyone for your time and thoughts. Another bit of experience learnt the hard way!

Jon_G4MDC · 2nd Jun 2020, 10:06 am

Thanks for a nice ending to the story. Enjoy using it!

SiriusHardware · 2nd Jun 2020, 10:51 am

Yes, we like happy endings.

A footnote: Where I work we get door entry panels, among other things, returned for repair and sometimes we get 'no microphone audio' as a fault description. Very often we find the microphone aperture filled solid with what looks like cotton wool, but is actually an insect nest which we then have to clear out, although I always feel quite bad about that. Just goes to show, odd things happen to microphones.

28th May 2020, 8:36 pm	#21
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Kenwood TM322. (Actually a TM-733). Page 6 of this Kenwood manual - not the same radio, but the same KRR-C001 part and a good description of it. http://www.catlog.net/_Descarregas/M...d%20TM-231.pdf Presumably the discrete external pull up resistors and the complex ladder network inside the package combine to produce the desired voltage output for each possible binary input value. Maybe the choice of value for the external resistors determines the size of the output voltage swing? Last edited by SiriusHardware; 28th May 2020 at 8:43 pm.

28th May 2020, 10:32 pm	#22
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Kenwood TM322. (Actually a TM-733). Thanks for swelling my Kenwood "gen" folder even more! Never lose a .pdf...hehehe. I'm no expert on this but I suppose you could mux the output lines in a way which generates the wanted tone frequency as a piece-wise sine wave with a few bits up to the maximum of 7bits resolution? In that way there could be some level control from the uP, but then lower output levels would have more THD? Y/N?

28th May 2020, 11:12 pm	#23
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Kenwood TM322. (Actually a TM-733). The waveform resulting from less bits being used would be rougher/ lower resolution, but you are right, instead of utilising the full output swing of the DAC you could output a reduced range of values and thereby control the size of the output waveform to some extent. Imagine if it got to the point where you were only using one bit. The output wouldn't be much of a sine wave. The 'official' way to change the size of the output swing without degrading the resolution of the signal would be to change the value of those fixed resistors and continue to output a 7-bit sinewave to the resistor ladder. If the aim was to reduce the level of CTCSS tone going out to both RF PCBs, probably the simplest practical method would be to insert a fixed or variable divider after the three-component filter.

2nd Jun 2020, 9:11 am	#24
John-39 Triode Join Date: Mar 2016 Location: Chelmsford, Essex, UK. Posts: 11	Re: Kenwood TM322. (Actually a TM-733). Solution found! As so often, the answer was not as technical as feared. The clue was in the advice that there aren’t separate controls for setting Tone and Audio levels. I noted the position of the Max Dev control RV2 before experimenting with it and found it had been turned to maximum. This would explain why the transmitted CTCSS spectrum was twice as twice as wide as that from the Baofeng. Turning RV2 down then reduced the audio deviation. Perhaps it had been turned up because of a fault in the mic. circuit, but applying an audio generator at the level given in the manual showed good deviation. The mic. was then inspected carefully. I found that what was probably once a bit of plastic gauze behind the hole in the front of the mic. had turned to a thick paste. Poking this out solved the audio problem and the set is now working fine. Although some users say they can still hear a slight hum, it is obviously heaps better and quite usable. (Incidentally the TO line showed a well-shaped sine wave but with considerable noise on it.) Thanks everyone for your time and thoughts. Another bit of experience learnt the hard way!

2nd Jun 2020, 10:06 am	#25
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: Kenwood TM322. (Actually a TM-733). Thanks for a nice ending to the story. Enjoy using it!

2nd Jun 2020, 10:51 am	#26
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,560	Re: Kenwood TM322. (Actually a TM-733). Yes, we like happy endings. A footnote: Where I work we get door entry panels, among other things, returned for repair and sometimes we get 'no microphone audio' as a fault description. Very often we find the microphone aperture filled solid with what looks like cotton wool, but is actually an insect nest which we then have to clear out, although I always feel quite bad about that. Just goes to show, odd things happen to microphones.