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Author Topic: Icom ic-735  (Read 1512 times)

Posts: 27

« on: February 18, 2004, 06:00:01 PM »

I have a problem with a ic-735. The finals were blown and I replaced them along with resoldering the circuit board to so239 connector. The problem now is the relays on the filter board aren't working. There is supposed to be 12 volts to the band filter relays but there isn't. I pulled the plug to the relays and supplied 12 volts to the relalys and they do work. The tx-rcv relay also works. When the filter relay is powered the output is around 125 watts and is not controlable, the output is the same on SSB,AM,and FM with no modulation. Also the output frequency is not on the selected band, it's in the 33mhz range. The receive works fine and the transmitt works with no power out when the filter relays are not powered up. I can hear the radio transmitt on another radio on any of the modes.
Has anyone had this same problem? If so what did it take to fix it?


Posts: 21764

« Reply #1 on: February 19, 2004, 01:22:40 PM »

I don't know why the relays aren't keying, you need to trace that circuit and determine why; but by the other symptoms you describe, your newly installed PA transistors are now officially a 33 MHz oscillator.  That you have "full output power" on SSB with no modulation is a sure clue to that!

Sounds to me like something's been "lost" during the re-work process: Possibly a bypass capacitor is no longer in the circuit, or an RF choke is no longer functioning, or the PA ground has been compromised, or maybe the new PA transistors aren't the same as the original ones and simply have too much gain at too high a frequency.  But obviously, something's different.

It is unfortunate that in some cases, manufacturers (including Icom, in some rigs) actually selected PA transistors having certain characteristics *not* listed on their data sheets, rendering random field replacement impossible without some additional engineering.  I know a lot of Motorola (and other) power transistors have been sold with various suffix letters indicating their high-frequency gain range, and Icom (and others) have chosen specific suffix devices to work in their circuits.  The same exact transistor P/N with a different suffix letter may not work, and if it has too much high frequency gain, it can oscillate rather than amplify in the same circuit.

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