Kenwood TK-880 UHF - European version 440-470MHz - tune for 430MHz?

[g8tzl2004]

I have a European version of the TK880 UHF mobile which officially covers 440 - 470MHz (the USA version covers 450 - 470MHz, I think).

I have successfully programmed the radio down to 430MHz but its very deaf at 430MHz. Sensitivity at 435MHz is just about OK but declines significantly at 434MHz and below.

I wonder if anybody has modded a TK880 to get better RX sensitivity below 440 MHz (or 450MHz)?

Front end RX alignment is done at 440.05 (25), 455.05 (93) and 469.95 (250). Tuning is very broad with no RX improvement achieved for in-band frequencies.

In-band RX performance of the UHF TK880 also appears to be relatively poor. Official narrow band sensitivity is quoted as 0.35uV (-116dBm). My "optimized" UHF TK8180 (after realignment) has much better weak signal performance than the TK880...around 7dB better!!

Is it possible to manually adjust the TK880 front end BPF's to get better RX performance?

Is it possible to hex edit the "low" 440.05 tuning frequency to 430.05 to get better out of band sensitivity?

The RX VCO is in lock at 430MHz (the TK880 does RX at 430 MHz) but does the RX VCO voltage level effect RX sensitivity? The service manual states that the RX VCO should be at 1.5v or above.

Overall I'm disappointed with the "in-band" 440 - 470 MHz TK880 RX sensitivity compared with the TK8180....but maybe its just my radio??

Thanks

 

[wd8chl]

You can add test channels that would let you tune down where you want to be. Just add whatever you want to the 'test frequency' list and reprogram the radio. You'll be able to select those channels in the alignment section.
Whether it actually does what you want is hard to say. It very well may lose sensitivity on the top end, or it may just not do it at all.

 

[g8tzl2004]

I already tried changing the 440.050 test frequency (channel 2) to 430.050 BUT it always defaults back to 440.050 when the sensitivity alignment is performed. Other alignment procedures do use the "new" channel 2 430.050 frequency but not with the sensitivity test. It is also not possible to select different test channels when using the sensitivity test. It appears that the sensitivity test uses fixed default values...but maybe there is a way to hex edit the fixed values??

 

[g8tzl2004]

SUCCESS!!! It looks like the TK880 went completely deaf at around 434MHz because the interpolated softpot tuning value went below 0 or 1. As a result the tuning value either wrapped around to "255" or was negative which resulted in alignment being well off at 434MHz.

The TK880 is now RXing OK at slightly above 430 MHz!!

The original tuning values were: 440.050 (25) 455.050 (93) and 469.950 (250).

Based on the original tuning values of 25, 93 and 250, the theoretical interpolated tuning value for 430MHz is around (negative) -20.

The minimum tuning value has to be positive ie at least 0 or 1. This can be calculated using linear interpolation. The minimum tuning value for 440.050 has to be 38 to give an interpolated "positive" value of around 0 or 1 at 430 MHz.

Tuning is very broad so a tuning value of 38 rather than 25 at 440.050 MHz does not make much difference.

I checked out the optimal values for in-band alignment. The value for 455.050 was changed from 93 to 75. This resulted in in-band optimal alignment of 440.050 (25). 455.050 (75) and 469.950 (250).

So to allow RX at 430 MHz, the following values were used 440.050 (31) , 455.050 (75) and 469.950 (250).

The optimum value at 440.050 is 25 but alignment is very broad and using a value of 31 (to allow RX at 430MHz) results in practically identical RX sensitivity

!!

I'm still a bit disappointed with the TK880 "in-band" 440 - 470 MHz RX sensitivity compared with the TK8180....its around 5dB down...but at least the TK880 will now RX at 430 MHz