Modifications for the ICOM - RogerK

19-07-1998(IC-751) IC-751 Bad Cap ProblemsThis info may be helpful to ICOM-751 owners with units approaching five years of many operating hours; two UNRELATEDproblems 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 inthe chip.Replaced it with a tantulum. (Also arbitrarily replaced electrolytic on the output side of the 5-volt reg. chip). The symptoms were3 or 4 minutes delay before unit would function. Also the screw holding the reg-chip to its heat-sink was a little loose...probablydue to COLD-FLOW characteristics of aluminum. This can also occur in pheof aluminum. This can also occur in phenolic P-Cboards where the groundfoil is supposed to be bolted tightly to the chassis. It would probably be a good idea to use small splitringlock-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 (afterwhich you must power-down up to generate a new reset-pulse). It also turned out to be an open 10uf electrolytic on the -5voltline which supplies IC-1 IC-2 on the Display Unit Board. This board attaches to the front assembly near the displaydigits...parallel to the floor. The front assembly DOES unscrew with 4 screws and move forward an inch or two, but this isactually not necessary. The board has a 3-position connector attaching, but only the two outside ones have wires....red is 13.8voutside 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 zenerto-groundappears here, and the culprit 10uf is directly in parallel across the zener. I didn't have a 10uf tantulum handy but a39uf did the job. I also arbitrarily shunted C-17 (easily visible) with a 33uf tantulum, and C-25 which is another 10uf (easilyvisible) 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 mightbe under suspicion....it should not be necessary to remove them in most cases...just tack-solder a tantulum in parallel on the foil-side.This modification is read 1246 times.top of page19-07-1998(IC-751) General coverage mod for the ICOM IC-751A1. Remove top and bottom covers from rig.2. Remove cover for RF unit (left side).3. Locate J2 near center of RF board (multi-pin connector).4. Clip brown wire (pin 1). Maybe the wire is black5. Rig will now transmit in Gen. Cov. mode above 1600 khz.This modification is read 1467 times.top of page19-07-1998(IC-751) IC-751 Switch MatrixThis following chart was generated by shorting out the matrix points and watching the function that is performed. Some areintuitive already from the schematic. There are only a couple of "extra" functions in the matrix (ie, USB and LSB, Reset 100&10Hz [The TS switch does this as well]). The KEY? GENE switches (Y0) are accessed by the RC-10 controller, but I noticed thatsome "removed" circuitry in the rig was designed to switch the digit switches on the RC-10 controller to the KEY? HAM array(Y3), effectively making the switches directly access each band. However, the removed circuitry (a set of switching transistors) isreplaced by a solid jumper to Y0 :-(. The "empty" slots in the following table showed no perceivable function.The Y5 matrix, acted like it was shorting some other behavior out causing the rig to switch to the specific memory channels (youcan observe a binary + 1 selecting behavior) temporarily.Have fun, Ciao, 73 de VE6MGS/Mark -sk-IC-751A Switch Matrix (W/ Function)