Jumbo TX audio problems.

Problems.
The early version of the "Ham International Jumbo" radio (PTBM059) had problems with poor transmitted audio on FM or SSB. These problems are easily cured, and the details are included on this page.

SSB fault.
The early Jumbo had distorted "gritty" sounding audio on the outgoing audio. This was caused by a low bias in the RF output stage, which caused the outgoing signal to distort.
As a transistor requires a minimum of around 0.6 volts to conduct, signals below this level are clipped unless the transistor is biased to conduction with a constant DC current. This is not a problem for FM use (the output is constant), but SSB output level varies with the audio signal, and distortion will occur.

SSB cure.
The cure for this problem is to increase the bias. As the bias adjustment (RV1) will not usually take the bias high enough, a resistor will need to be changed.
  * Adjust the bias to minimum using RV1.
  * Replace the 10 Ohm bias resistor (R38) with an 18 Ohm.
  * Adjust bias to 50mA using RV1.

FM fault.
The FM audio problem (disgusting outgoing audio) was a design problem in the FM modulator circuit. Not only was the audio response poorly shaped, but also the audio insertion point was poorly chosen.

FM cure.
To cure the FM audio problem you must tackle both design faults... change the audio insertion point (this will increase the possible deviation), and reduce the low frequency response of the circuit.
  * Replace C32 with a 22nF capacitor.
  * Short out R26.
  * Connect one end of a 680K resistor to TP2.
  * Remove L3, and connect a wire from the point where L3 was connected to C30/C31, to the open end of the 680K resistor on TP2.
  Adjust RV3 for correct deviation.
The 680K resistor and its joints can be insulated with heatshrink tubing. Do not cover the whole TP2 pin, as this is used for tuning the VCO.

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