Icom IC-751 (IC 751 IC751) transceiver mods reviews software and diagrams

   
 
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26 February 2021, 4:29 UTC 


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Icom IC-751 (IC 751 IC751)
IC-751
Programming interface schematics for Icom:
Programming software for Icom IC-751 (IC 751 IC751) :
PDF User Manual for Icom IC-751 (IC 751 IC751) :
PDF Adjusments Procedures Manual with schematics for Icom IC-751 (IC 751 IC751) :
Schematics for Icom IC-751 (IC 751 IC751) :

Mods for Icom IC-751 (IC 751 IC751) :
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    Icom IC-751 (IC 751 IC751) RAM Card Backup Battery Replacement Instructions
    Icom IC-751 (IC 751 IC751) Allow CW Xmit & USB recieve split mode
    Icom IC-751 (IC 751 IC751) to 37 MHz
    Icom IC-751 (IC 751 IC751) Bad Cap Problems
    Icom IC-751 (IC 751 IC751) General coverage mod
    Icom IC-751 (IC 751 IC751) Switch Matrix
    Icom IC-751 (IC 751 IC751) How to replace the lithium battery in your ICOM radio

Icom IC-751 (IC 751 IC751) technical specifications :  Icom IC-751 (IC 751 IC751) technical specifications



Icom IC-751 (IC 751 IC751) Bad Cap Problems

This info may be helpful to ICOM-751 owners with units approaching five years of many operating hours; two UNRELATED problems occurred, both due to a similar 10uf electrolytic opening up. The first was the input cap.
to the 5-volt regulater chip which supplies PLL board. Proper by-passing is required, probably to prevent internal oscillations in the chip.

Replaced it with a tantulum. (Also arbitrarily replaced electrolytic on the output side of the 5-volt reg. chip). The symptoms were 3 or 4 minutes delay before unit would function. Also the screw holding the reg-chip to its heat-sink was a little loose...probably due to COLD-FLOW characteristics of aluminum. This can also occur in pheof aluminum. This can also occur in phenolic P-C boards where the groundfoil is supposed to be bolted tightly to the chassis. It would probably be a good idea to use small split-ring lock-washers (along with flat-washers) in these cases to maintain tension over time.

The second malfunction was failure of the display digits etc to appear until a warm-up time of perhaps 10 minutes or more (after which you must power-down up to generate a new reset-pulse). It also turned out to be an open 10uf electrolytic on the -5volt line which supplies IC-1 IC-2 on the Display Unit Board. This board attaches to the front assembly near the display digits...parallel to the floor. The front assembly DOES unscrew with 4 screws and move forward an inch or two, but this is actually not necessary. The board has a 3-position connector attaching, but only the two outside ones have wires....red is 13.8v outside ones have wires....red is 13.8v dc input, and purple is -5v output.

The purple (-5v) wire, but rather is on the opposite side of the inductor (L1) which does connect to the purple wire. A -5v zener-to-ground appears here, and the culprit 10uf is directly in parallel across the zener. I didn't have a 10uf tantulum handy but a 39uf did the job. I also arbitrarily shunted C-17 (easily visible) with a 33uf tantulum, and C-25 which is another 10uf (easily visible) was shunted with a 2.2uf tantulum. (Tack-soldered on foil-side).

I didn't have a 10uf tantulum handy but a 39uf did the job. I also arbitrarily shunted C-17 (easily visible) with a 33uf tantulum, and C-25 which is another 10uf (easily visible) was shunted with a 2.2uf tantulum.
(Tack-soldered on foil-side).

With such a trend developing...it would appear that if you are experiencing other kinds of malfunctions, open electrolytics might be under suspicion....
it should not be necessary to remove them in most cases...just tack-solder a tantulum in parallel on the foil-side.

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