Cybernet PTBM121D4X Improvements
There is always a lot of discussion about which is best cybernet or uniden ssb radios. Personally I like cybernet stuff but I admit uniden ssb export radios do work well on ssb.
A lot of the issues people have with cybernet stuff is a result of the way that they left the factory. Common complaints are scratchy ssb audio and awful fm transmit audio.
Below are a few things you can do to the 121 chassis. This is used in radios like the Hy-gain 2795,Tristar 747, Concorde 2 and lots of others including some Colt Excaliburs and the rare Major 688.
These simple mods will cure the ssb and fm audio
To improve the audio limiter performance change R150 to a 5.6k ohm
Also change R166 to a 5.6k ohm.
Also add a 1000pf (102) Capacitor One side to the junction of R28 and C32 and the other to ground.
Check the FM deviation level and adjust if needed using RV1
Change R40 to a 33 ohm to improve the bias on the driver transistor.
Change R44 to a 15 ohm and set the bias to the output transistor using RV2 by keying up the radio up on ssb with no audio and set the voltage on the base of Q10 to 0.7 volts.
To stop the popping squelch remove R132 and add a link wire as shown below
Posted on February 25, 2015, in cb radio, modifications and tagged cb radio, cubwolf, cybernet, fm audio, ham international, hy-gain 5, Leicester, mods, PTBM121D4X, rig doctor, Squelch Mod, ssb, tristar 747. Bookmark the permalink. 2 Comments.