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Icom 756pro III got zapped

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Walkin' the dog
Sep 12, 2009
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Chilliwack, BC
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Anyone familiar with Q25, D22, D23 on the CTRL unit?
Mine got smoked by a close lightning strike.

I repaired it but its only partially fixed.
Apparently there are some shorted diodes on the RF-B unit.



History: The radio wouldn't make power, only a 50w carrier on all modes.
It did 100w with only 18db att. on oscillated at 0,6 and 12db, receive was 50% what it should be with 0 Att on the 24mhz up bands..

After fixing the CTRL unit, it would only make power in 6, 12 or 18db att. oscillated 50w on 0db.
Receive still low, nothing on the lower bands, no wwv, no broadcast stations at all, deaf.
Digging further I found shorted diodes on the RF-B unit. Mainly D104, D851 but others are leaky too.
(maybe just do them all?)

Anyone dealt with this before?
I've scoured the net I'm looking for some first hand knowledge.

Thanks.

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Sounds like maybe the first RF amp is gone and possibly one or more bandpass filters. Check for open inductors in the different bandpass filters and make sure they are being switched when they should be.
 
Well the 756 was still having trouble.
The low band, 0.3-1.5999, it was deaf.

Everything else worked. I started chasing the schematic and determined I didn't replace D802 and figured it was popped.
Under further inspection I seen this.


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That was after pulling the preamp board off to get to anything. A real chore with a soldering iron.

I looked online and found a mod that allows full signal in the low band and it effectively jumpers out the burnt R801 so I thought thats half the battle right there.
Now these chips are under the bpf board and I just did't want to chance removing it again.

When I first read this mod I thought it said jump R802 and R803 to 0 ohms so I swapped in 0 ohm smd's and put it all back together and the low band was still dead.
I fucked up and was supposed to bridge 802 and 803 and after going over it all again, tore back into it again today.

I put in 2 new 100ohm's and Replace C802 that the mod said to remove also.

Now this all was s u p e r difficult.

Maybe Yvo and TRX and Mike have all the fancy equipment to make this easier but I dont so some stuff is crooked and difficult to get to.

Some plastic got nicked but in the end a miracle happened and the radio works top notch now.

Dont try this without flux. The flux is the secret to not vaporizing parts.
I basically hold the part down with a small screwdriver and tack them down with the iron and lots of flux.

It was pure luck I was able to fix this radio.




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All finished. Dont forget this is on a Imax for the antenna.
In the end D802 was fine and it was R801 that smoked and killed the MW band.
Then I almost killed it more and then got it working in the end plus the MW "mod".

Time for the papers and scissors, its a wrap!


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