IC751A
SM7OVK-Maintenance & repair


Introduction

The IC751A has, as the FT225, been around for several years now. My transceiver is probably manufactured 1985, since most of the components are marked 84 or 85.
There is also a IC751 type, ie without an A. I donít know much about the differences, but Iím sure there must be some! A big difference ICOM vs. Yeasu and other hamradio manufacturers is that ICOM has a page on the web with FAQs! Take a look at ICOM and youíll find information about some adjustments. Some of these are also shown below and hopefully this will clarify if you have any doubts. The idea is to show what I have done with my IC751A and encourage other hams to do some maintenance on their IC751Aís. Iím of course interested to know if you have any suggestions or if you have made any modification or maintenance not mentioned here, (please always put IC751A in the subject).
If you are interested to do any repair or maintenance on your IC751A I also recommend a visit at mods. Itís a good idea to download schematics, service manual etc from this sight before you have any problem!

Maintenance

As with the other maintenance-pages I have divided them into groups each marked with a number of + and *. These marks should be read as follows;
  • * = easy with no instruments
  • ** = can be done with no RF-instruments, but a receiver or DVM.
  • *** = can maybe be done with no RF-instruments, but a receiver, DVM, oscilloscope.
  • + = easy with some basic RF-instruments as powermeter or spectrum analyser
  • ++ = must have some basic RF-instruments as powermeter, spectrum analyser, frequency counter
  • +++ = must have good RF-lab
  • ++++ = only RF-pro
Capacitors

Electrolytic capacitors dry out and if they are placed in a hot environment this process goes even faster. If the capacitor gets dry the capacitance will be lower, and after some time it will be close to 0. In my IC751A several capacitors are ďbrownĒ, ie they have been very hot and are all a ďticking bombĒ. Sooner or later these overheated capacitors will be totally destroyed. I have measured the capacitance on some of the capacitors that I have exchanged and the capacitance was sometimes down at about half of what the capacitors was marked with, some capacitors would work better as a resistor and could also result in a hard to find short circuit! It has been shown that a decrease in capacitor working-temperature of ten degrees would double the lifetime!
The only way is to exchange the capacitors and my recommendation is to exchange all capacitors! This takes time off course, but itís certainly worth the time and money! My suggestion is to make a paper-copy of the IC751A-schematic and whenever you replace a capacitor Ė mark this on the special schematic-copy! In this way you will be sure to exchange all capacitors. (One doesnít have to exchange all capacitors at one occasion.)
Electrolytic-capacitors are most often marked with either 85 degrees or 105 degrees, (there are 125 degree types as well but these are not commonly available). Try only to use the 105-type since this will increase lifetime. Always use capacitors that can handle voltages with a factor of 1,5 to 2 times the maximum voltage present at the capacitor. It isnít any advantage to increase this factor more since this can result in a capacitor with higher internal resistance and this will slow down the capacitor.
Itís important to get new capacitors of good quality, do not use old capacitors from your junk-box they could be older than the capacitors youíre about to exchange! Also beware of cheap stores with large quantities of capacitors where you can by 100 to the price of one at a normal store Ė there is a reason for this low price. Transistors, diodes etc survive but electrolytic capacitors dry out if not stored properly.
When exchanging the capacitors also pay attention to the voltage-regulators. Many of them have originally too large capacitors close to them. Normal voltage-regulators, as the 78XX-types, must have small capacitors mounted close, 0,1-1uF, to prevent oscillations. Close in this case is about 10mm from the voltage regulator, ie often soldered directly on the regulator.
Tantal-capacitors most often have longer life than electrolytic capacitors and they can often replace electrolytic capacitors with same or better result.
These small tips will increase the capacitor-life and therefore also the life of your IC751A. I would like to point out that normally no adjustments are needed after exchanging an electrolytic capacitor, but be careful when you are inside the IC751A and soldering!

Temperature

As mentioned above many components in the IC751 get very hot and heat shortens lifetime for many electrical components. I have heard about hams installing fans in their IC751 and that might be a good idea. Just donít forget that you have installed the fan! The MTBF of many small fans is only 30000 hours. This sounds as a lot, but after a year or two you have probably forgotten about the fan and suddenly your radio is broken since the fan is broken. Mounting of a fan inside the IC751 is a good idea, but first you should give the radio ďspace to briefĒ.

Repair

Some modes don't work *** ++

(To to this you need a VDM and a oscilloscope(***) but also a frequency counter (++) if you have to adjust the BFO-frequencies.)
I had problems with LSB reception. I removed the upper coverage of the radio. Control that the control-voltages are present at J1 on the mainboard - the large board with the crystalfilters. J1 is nearly in the middle of the board.
Since all control-voltages were all ok I looked at CP1, a testpoint close to Q50 and Q51. If there isn't any signal on CP1 the BFO-oscillator doesn't operate. The two crystals X5 and X6 are used depending on chosen mode.
My problem was LSB and therefore I looked more closely at X5. This crystal had a grounding wire soldered between the crystal and shielding-box. I just bent the wire a little and after this the oscillator is running. I don't know if this is a common fault but it's quite rare that crystals are faulty. Icom could of course have had quality problems with it but I haven't heard of any. It can be that I have a bad connection inside the crystal and when I bend the wire this put some tension to the crystal connections. I realize that this is not a complete repair, but the radio has worked for several months now.

No reception on some bands ++

(To to this you need a frequency counter ++ )
One day I didnít had any reception between 8 and 15 MHz. In the Icom FAQs this was mentioned and four trimmers should be exchanged on the PLL-board. Itís also quite obvious since there are four oscillators and these are used depending the received frequency-band and nothing much in the radio can give the same behaviour.
First both top and bottom cover must be removed. The PLL-unit is located in the bottom area and with the radio upside down and the display against you, the PLL-unit is the right unit. One can also see shielded boxes on this unit. Unfortunately one of these boxes must be removed. The easiest way is probably to unsolder the box. (I couldnít get the top off.) The box of interest has four holes, below C78, C88, C97 and C107 are located. Itís these capacitors that must be exchanged. Two types are used originally, but I used ceramic 10pF for all four. The type available at my local store were a Murata TZ03Z100E.
Now when the board is handy itís a good idea to change all electrolytic capacitors. There are only thirteen so this is made easy!
The variable resistor R43, 470ohm, should be exchanged to a fixed resistor of 220 ohm.
When I had made these things I also put some small SMD-capacitors at the in and output of the IC8, 7805. These small capacitors should be 0.1 to 1uF and will prevent the voltage regulator from oscillate. Check the fastening of the IC8 since itís important to get all the cooling available with this little aluminium plate.
The PLL-unit is fastened with six screws. Before you mount the unit back into the IC751A check the brass bolts, inside the radio. This is the earth contact. Itís also a good idea to put some solder on the PLL-unit where the board meet the brass bolts. Also bend the contact tongue from the board so these make good contact at the main chassis.
When this is made you have to go through the adjustments of the PLL-unit according to the service-manual. This manual is found on several places on the net.


To be continued...


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