Icom IC-7300 Over Voltage Repair

Parts of this repair could also be valid if you reversed polarity to the radio, though the survival for reverse polarity should be better as a diode (marked C further down) should just blow the fuses on your DC cable.

Now to the over-voltage bit….

A linear power supply without a crowbar protection circuit is a bad idea, I supply kits on Tindie to avoid this situation. A high current linear PSU with output pass transistors (usually some 2N3055) going short circuit from Emitter to Collector could possibly feed your radio with a high current supply anywhere from 20 to 30 volts ! OUCH ! – Get a crowbar for linear PSU or go SMPS. Click HERE for Crowbar kits.

Now to this poor fellow who went through it and survived, with minor injuries, and fully repaired.

This is quite an easy fix though some care must be taken. Not all radios may fail the same, so your mileage may vary, maybe you have fewer components damaged, maybe more. The parts are cheap and easily available, I would just change them all.

The main symptom will usually be blown in-line fuses and no power. Make sure your PSU did really go high voltage. Disconnect the radio from this PSU!

Do NOT replace the internal 5 Amp fuse and connect to a power supply.

“Blowing” a fuse takes quite a bit of current (usually more than it’s rating) and quite a bit of time, the fuse is the “last-resort”, so if fuses have blown, (i shall say this in CAPS AND BOLD) THERE IS A SERIOUS SHORT CIRCUIT, DO NOT ATTEMPT TO “JUST CHANGE THE FUSE”, more damage could surely happen, semiconductors could burn out in microseconds with microamps, do you think keep putting 10’s of amps for nearly a second is going to keep them working long ? Fuses don’t blow for fun, take note !

1st, the “wash my hands” bit – DISCLAIMER

Any mistake can cause serious damage to your radio. So if you are not experienced in SMD soldering or using a soldering iron in a very FRAGILE environment – do not try to perform this fix yourself, buy the parts and a 6 pack of beer and give them to someone who can do it for you ! I will not be held responsible for any damage you cause to your radio based on this article.

On with it…..

A typical check is a multimeter on resistance/continuity/diode test mode, “beeper” will do. Check across the DC input connector on the radio, you will probably find it shorted or very low resistance, good ! hopefully the fast diodes at the DC input side have probably gone short circuit and acted before the CPU and PA transistors went bang !

Damaged Components
Schematic Reference
My MarkingPart NumberUsage
ADZ2J18018.9v Zener diode, this would probably pop first, but can’t carry enough current to blow the main fuses, it will probably go short then open circuit with high voltage at high current
B1/2/3470μF 25v3 pcs of 470μF 25v electrolytic capacitors, if they blew or not just change them ! This radio got 30v and they out-gassed. Yes ! LOTS OF MAGIC SMOKE !
CDF30SC4M40V 30A Schottky diode, this is the reverse polarity protection in fact, the surge of current still made it go short circuit in this failure
DFuse 5AMini blade flus protecting the low current side, i.e. anything not the PA, so CPU, accessories etc
E50S06I used a 50N05, a pretty common MOSFET, this going short blows the 5A fuse. This MOSFET “powers” on the main radio circuits, it is a “software controlled” switch as the radio by design is in sort-of “standby” let say, the PA is always “ON ” across the input power source, as is with most radios.
*FTPSMD15AVoltage transients suppressor, this can take quite a beating and is what probably saves the rest of the radio in this situation. I have marked it with a * simply because it is a star! well, strangely it doesn’t show up anywhere in Icom’s service manual !

All parts can be obtained from common sources, I used Mouser.

Following this repair, apply power the radio using a current limited power supply, start at 50mA to make sure there are no shorts, take it up to 500ma, then power on the radio, it should turn on and work normally, then you can move to use a high current PSU and test it on transmit, luckily the PA transistors are fine.

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