Ham International Jumbo / Multimode 2 Modifications.

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

059 pop mod diagram 059 pop mod PCB side

Jumbo Owners Manual


About Cubwolf

Im Dave Smith also known by my cb radio handle Cubwolf. I live in Leicester in the United Kingdom

Posted on February 25, 2015, in cb radio, modifications and tagged , , , , , , , , , , , . Bookmark the permalink. 7 Comments.

  1. Hiya Dave, do you by any chance do work on multimode 2 radios, ie fm modulation, power etc?

  2. Hi Dave, thanks very much for the instructions. Since doing the FM modification on the jumbo though I’ve noticed the RX audio has changed tone. The Hi-filter switch on the front either makes it very bassy or very sharp, there’s no in between. Is there any reason this would be? Thanks for any advice!

    • Hi. There isn’t anything in the fm tx mod that would affect the incoming Rx audio. Bit of a strange one that and not something I have come across before.

      • Ok, thanks for the reply. The only other thing I did was move the position of the roger beep board (someone had taped it to the loom for some strange reason). I’ll have another look and see if I’ve did something daft.

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