The ever popular Ham International jumbo and its mobile counterparts the Multimode 2,Concorde 1 and Major 588 and not a bad radio for their age but they do have some niggling faults.
One of these is the awful FM modulation. Most modern radios apply the fm tx audio to the vco using a varicap diode.
In these earlier radios cybernet modulated the crystals to get the deviation. This results in poor and quiet audio which when turned up makes the radio wander about on transmit frequency.
The squelch is also renowned for popping as it opens and closes. My good friend Paul Michelin Man in Leicester worked out a mod which cures this and details are at the bottom of the page.
The other is a lack of the correct bias voltage on the output transistor. This can be fixed by changing R38 to a 15 ohm resistor.
You will then need to measure the bias voltage on the base of Q10.Key the radio up on SSB with no modulation to measure this. It needs to be 0.7 volts and can be adjusted with RV1.
The way i do them is.. Remove L3 and C30. Link out R26 Connect a 10000pf (103) cap to Pin 2 of IC2. To the other end of the cap connect a 10k resistor and the cathode (with the line on) of an ITT310 or 410 Varicap diode. The other end of the varicap and resistor goes to ground.
Then solder a 220k resistor to the cathode of the varicap and connect the other end of this resistor to the point on the board where you removed L3 and C30 from (R25)
Replace C32 with a 100000pf (104) capacitor.
You will just have to adjust the deviation using RV3 and check that the vco locks on all the bands. I have tried using other varicaps but these ones sound the best.
Squelch Pop Mod
Jumbo Owners Manual