Modifications for the Kenwood TS-770

Modifications for the Kenwood TS-770
Modification Kenwood TS-770E for 9600 baud (G3RUH)

If you remove the small plate on the back of the unit you will find an easy way to mount an 9 pin sub d connector (at least on the "E" version) which has enough room for rx, tx and ptt.

You can get the signal output from the discriminator unit, plus side of C163 (on my rig this capacitor was mounted the wrong way round, I corrected this). The voltage is very low, so you may need some amplification - 10 in my case. C165 (22nF) on the output of the discriminator has to be replaced by 1.5 nF or 1nF.

The transmission signal goes to diode D47 in the modulator stage, a resistor of 10k in series. Check the output adjustment of your modem by using a second receiver. Adjust it that the modem noise is somewhat lower than the noise of the empty channel.

Use shielded cable for these connections.

The easiest way to find the ptt line is by looking at the microphone connector.

If you want to suppress "man made noise" over the possibly connected microphone while transmitting packet, you can short the microphone by using a reed relay. Then you have to connect the packet-ptt to the rig-ptt by using a diode to prevent shorting the mike on normal transmission.

Cause I experimented a lot with my rig I also replaced the IF-filter with a wider one. The built-in type CFW455E may work ok.

I didn't make a BERT, but there are very few retries using filter characteristic #1 on the 9600 modem of my TNC3S.