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ICOM IC-751A  EXPANDED RF
1- Remove power and antenna.
2- Remove screws open case.
3- Locate noise blanker board.( Near upper left hand of the front panel ).
4- Locate and CUT resistor R-34. (far left end of the noise blanker board ).
5- Reassemble the radio.

 

Modifications for the Icom IC-751 / A


 

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

 

 

  Allow CW Xmit & USB recieve split mode for IC-751  
     

Allow CW Xmit & USB recieve split mode to still use the CW VOX. This allows one to operate with limited privileges when communicating with SSB stations above 10MHz. Below 10MHz, the CW receiver is actually LSB so split operation in that case is effortless and does not require the split operation to be invoked. The original bogus operation can be confirmed by placing the rig into CW Tx and USB Rx split operation and operate the key. A more `graphic' example of this operation is CW Rx and USB Tx split and operate the key (scary eh? :-).

PARTS REQUIRED:
1 4.7Kohm resistor and 2 1N914/1N4148/1N4448 diodes
 
SCHEMATIC:         +8V (Pin 14 of IC1)
                     |
                      >
                     <
                      > 4.7Kohm
                     <
                      >
                     |
                     +-------------//-----------------+
                     |                                |
      +-------+------(----------+                     o
      |       |      |          |                    /
      |       |1N914 | 1N914    |                   /
      |      _|__   _|__        |               +--o   Split Switch
      |        /     /        |            ___|___
 D11  |     __/__ __/__       |            / / / /  o-----> to `split' display
 | /| |       |      |          |     |                       segment driver
 |/ |_|_/____|______|______/__|____ |
 | |   /    |      |      /       |
 | |         >      >                |
             < R29  < R26
              >5.6K  > 4.7K
             <      <
             |      |


DESCRIPTION:

On the Main PC board, there is only one cut trace because R29 & R26 are both fed from 1 trace and isolated from the rest of the circuitry. One of the diodes is surface mounted on the solder side of the board over the cut. R29 is unsoldered so that a thin wire (from w/w or pulled from a ribbon cable) is then subsequently inserted with the R29 lead back into the hole to bring R29+R26 common point up to the top of the main board.
The 4.7Kohm resistor is soldered to pin 14 of IC1, the other end of the resistor is soldered to the anode of the other diode's anode and another long wire to be routed to the split switch. The cathode of the diode is attached to the wire inserted earlier with R29.
All the components may be surface mounted if desired on the bottom of the main board, but I chose to perform this mod on the top of the board to allow easier removal of the main board for servicing. The long wire that was attached to the second diode and the resistor is routed to the empty pin on the split switch that is driven to ground when the switch is in the OFF position.

BUGS:

The CW VOX will be active when using split mode, even if neither the transmit or receive mode are the CW mode.

Enjoy, 73 de VE6MGS/Mark -sk-

  IC-751 to 37 MHz  
     

From: N5OFF@W5DDL.AARA.ORG

With a great amount of help from Mark, VE6MGS, I was able to modify my Icom 751 to operate at an extended frequency range of up to 37 MHz. This is how it was done.

Mark developed a clever way to read and interpret the contents of the Icom RAM chip (the old battery backed-up one) and alter its frequency limits in the GENeral coverage and HAM modes. This was done in conjunction with the modification of his 751A to operate six meters. My interests are in listening to the low band skip present above the ten meter band during band openings, so I asked Mark for simply a new upper frequency limit, leaving the six meter mods to the very ambitious.

First, I changed the battery on my RAM board since mine was eight years old, and I didn't want Icom messing up my custom program if my battery died later.

I then shipped the board off to Mark, and he changed my GEN mode limits from .10-30 MHz, to .01 to 60 MHz (subject to VCO and RF limits, naturally). He also customized my HAM mode limits as shown:
 
Before                  After
1.8-2.0               27.98-30.62   Default HAM mode selection
                                    changed from 160M
3.45-4.1              31.98-33.02
6.95-7.5              33.98-35.02
9.95-10.5             1.78- 2.02
13.95-14.5            3.48- 4.02
17.95-18.5            6.98- 7.32
20.95-21.5            9.98-10.17
24.45-25.1            13.98-14.37
27.95-30.0            18.05-18.18
none                  20.98-21.47
none                  24.86-25.01

These new HAM mode limits provide tighter control while hamming.
Note the tight 12M and 17M limits. Much more realistic.

One problem encountered during the mod was that the rig's processor (IC-14) instructs the rig to select the proper VCO and band pass filter for a given frequency, and it uses the highest BPF up until 31 MHz, then must be further instructed as to what to do at frequencies > 31 MHz.

IC-14 has several pins which turn on and off to do this instructing. Mark found that pin 18 (originally not used) of IC-14 turns on at >31 MHz. This could be used to turn on the highest BPF and VCO, thus allowing operation to higher frequencies. This was done physically by cutting the trace from pin 17's normal route, adding a diode to each pin 17 and 18 of IC-14, and joining the anodes of the two diodes to the circuit originally driven by pin 17, but now isolated by the trace cut. The effect is the use of the highest BPF and VCO until 31 MHz (pin 17) , and then the same BPF and VCO from 31 MHz and beyond (pin 18).

As expected, VCO4, as originally adjusted, dropped out at a low frequency (32.33 MHZ). Since the VCO has a range of about 15 MHz (and was originally employed to cover only 8 MHz with 3.5 MHz of lagniappe on each end), and kicks in at 22 MHz, I adjusted it for a comfortable low limit of 22 MHz (with little margin), which lifted the upper range to a maximum observed limit of 37.175 MHz. This was done by giving C107 on the VCO unit 1/4 turn CW.

I haven't yet done any sensitivity measurements, as I'll have to borrow the equipment from my employer when its convenient for him. I don't know where, or to what extent the band pass filter will cut off on the way to 37 MHz, if it does at all.

I believe the rig "hears itself" on a few certain frequencies, with strong signals at 30.715, 31.33, and 30.95. I believe this is normal as Icom probably moved these birdies to >30 MHz purposely. There are few other spurs.

CONCLUSIONS: I borrowed an R-7000 from a friend some months ago, and found that my favorite listening was in the lower 30's. Now, my 751 is as good to me as having my own R-7000 for what I would use it for. In the ham mode, its much easier to stay in the ham bands (especially WARC bands) with the new program, and the default band of 10M much more suits my tastes than the old 160M default band.

Thanks to VE6MGS [mark@ve6mgs.uucp] for his great help, without which I could not have accomplished this.

 

  IC-751 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 regulated chip which supplies PLL board. Proper by-passing is required, probably to prevent internal oscillations in the chip.

Replaced it with a tantalum. (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 ground foil 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 tantalum handy but a 39uf did the job. I also arbitrarily shunted C-17 (easily visible) with a 33uf tantalum, and C-25 which is another 10uf (easily visible) was shunted with a 2.2uf tantalum. (Tack-soldered on foil-side).

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

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

 

  General coverage mod for the ICOM IC-751A  
     

 
  1. 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 black
  5. Rig will now transmit in Gen. Cov. mode above 1600 khz.


 

  IC-751 Switch Matrix  
     

This following chart was generated by shorting out the matrix points and watching the function that is performed. Some are intuitive already from the schematic. There are only a couple of "extra" functions in the matrix (ie, USB and LSB, Reset 100&10 Hz [The TS switch does this as well]). The KEY? GENE switches (Y0) are accessed by the RC-10 controller, but I noticed that some "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) is replaced 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 (you can observe a binary + 1 selecting behavior) temporarily.

Have fun, Ciao, 73 de VE6MGS/Mark -sk-
 
                IC-751A Switch Matrix (W/ Function)
      DB0     DB1     DB2     DB3     DB4     DB5     DB6     DB7
   +-------+-------+-------+-------+-------+-------+-------+-------+
   | KEY0  | KEY1  | KEY2  | KEY3  |       |       |       |       |
Y0 | GENE  | GENE  | GENE  | GENE  | SCAN  |       |       |       |
   +-------+-------+-------+-------+-------+-------+-------+-------+
   | MHZ/  | MHZ/  |       |       |  VFO  | VFO/  | HAM/  |Disable|
Y1 | BAND  | BAND  |       |       |  A/B  |Memory | GENE  |RIT/XIT|
   |__Up___|_Down__|_______|_______|_______|_______|_______|_______|
   |       |       |       | Clear |  A=B  |       |       |       |
Y2 | FUNC  |       |       |RIT/XIT| (B=A) |       |MWrite |M->VFO |
   |_______|_______|_______|(->VFO)|_______|_______|_______|_______|
   |  KEY0 |  KEY1 |  KEY2 |  KEY3 |       |       |       |       |
Y3 |  HAM  |  HAM  |  HAM  |  HAM  |       |       |       |       |
   |__BAND_|__BAND_|__BAND_|__BAND_|_______|_______|_______|_______|
   |       |       |   AM  |  CW   | RTTY  |  SSB  |       |       |
Y4 |  LSB  |  USB  |  (FM) | (NAR) | (NAR) | (REV) |  FM   |  LSB  |
   |_______|_______|_______|_______|_______|_______|_______|_______|
   |  M2   |  M3   |  M5   |  M9   |  M17  |  M33  |  M65  |  M29  | Danger
Y5 | Prio. | Prio. | Prio. | Prio. | Prio. | Prio. | Prio. | Prio. | Will
   |or_M1__|_______|_______|_______|_______|_______|_______|_______| Robinson
   | MODE  | Reset | Change|       |       |       |Disable| Scan  |
Y6 | Scan  |100&10 |Memory |       |       |       |RIT/XIT| OFF   |
   |_______|__HZ___|On_Dial|_______|_______|_______|offset_|_______|
   |  I/O  |  I/O  |  HOLD |Altern-|Band W/|       |       |       |
Y7 |Option | Option|  Scan |ate VFO|DB6&DB7|       |       |       |
   |___RP__|___WP__|10_Sec.|_______|or_TS__|_______|_______|_______|


 

  How to replace the lithium battery in your ICOM radio  
     

Note: All this information is based upon the IC-751; if your radio is a different model the board references may be different.
 
  1. Go to Radio Shack or your favorite supplier and buy a lithium battery to replace the one in your radio. The actual type used is a BR2325 (3.0 volt 165 mAh), which Radio Shack does not carry. You can order this one from Digi-key. I bought a CR2450 at Radio Shack (3.0 volt 500 mAh), which was the largest capacity unit they had in about the same size. I had to solder some short leads to this battery (quickly to avoid overheating), before I could attach it to the RAM board.

     
  2. Find a 5 volt power source to run the RAM board during the transplant. I used a modular plug-in unit rated at 4.5 volts @ 60 ma. I connected a 500 Mfd. capacitor to this supply and then added a resistive divider to reduce the voltage to 5 volts. You may need to re-adjust this divider to supply 5.0 volts after connecting it to the RAM board. Connect several feet of wire to the divider output, which will go to the RAM board.

     
  3. Remove the covers from your ICOM radio to expose the plug-in RAM board, which I found on the bottom of my radio.

     
  4. Turn the power on and locate the 5.0 volt and ground pins, which bring power to the board. In my ICOM radio service manual I found a board overlay for the 'RAM Unit', which shows the two connectors and labels the connections. While looking down at the board note two connectors on opposite sides of the board. Both connectors are closer to the same end of the board. The pin closest to the end of the board on the longest connector (J1) is pin 12, which is the 5 volt input. The pin closest to the end of the board on the shorter connector (J2) is pin 1, which is ground. If you connect a voltmeter between these two pins you should measure 5.0 volts. These are the pins you will connect your external power to.

     
  5. Turn off the power to your radio and remove the one small screw that holds the RAM unit to the main board (you need a jewelers' screwdriver). Now you can remove the RAM board from the radio; remember from now on you do not want to touch any tools or other metal items to the board or it's components.

     
  6. With the board removed you now need to solder the two wires from your 5 volt power source to the board. You can solder to the small metal piece that is on the side of the connector toward the inside of the board; this will keep solder out of the connector pins. First make sure that your soldering iron does not have any direct ground connection to the board power source.

     
  7. After the external power source is connected, turn it on and measure the voltage being supplied to the board. You may need to adjust the values of the dividers to get between 4.5 and 5.5 volts. Disconect the power while making modifications.

     
  8. Now, with the 5 volts connected and on, you are ready to remove the old battery. Use an insulated tool to pry the battery off while heating the solder connections on the backside of the board. Only touch one of the battery connections at a time. Here you need to be careful not to touch any of the board traces or component leads.

     
  9. Once the old battery is removed, you can solder the new one in place, being sure that the polarity is correct. Now carefully inspect your job to be sure that the battery is connected to the correct pads on the board.

     
  10. Now you can disconnect the external power and unsolder the leads from the board.

     
  11. The final step is to re-install the board in your radio and continue to use it for many more years.


 

  RAM Card Backup Battery Replacement Instructions  
     
Author: ICOM


In the early 1980s Icom America marketed a group of transceivers and radios that utilized the latest state-of-the-art computer technology to enhance the operation of the radios. This technology improvement allowed hams to afford a reasonably priced high performance transceiver which until then was only available on units costing much more. Features now available to the hams included: higher frequency stability, better frequency resolution, digital frequency display, almost instant recall of saved frequencies (memory channels), quick mode signal processing and built in tones and offsets for the rapidly emerging repeater operation.

Implementation of all these features required a CPU with associated logic circuitry. In the early 80s, the most cost effective way to implement data processing and memory functions was through a DRAM (Dynamic Random Access Memory) to control the CPU. This DRAM was a volatile memory integrated circuit which required a lithium battery to retain its instruction set. The following receivers and transceivers had such RAM units:

IC-271 ; IC-471 ; IC-1271 ; IC-745 ; IC-751/A ; IC-R71A

The lithium batteries in these units typically lasted about 5 to 7 years before replacement was required. This was specifically noted in each of the Owners Manuals for the products.

This lithium battery can be replaced by the radio owner if care is used. The following is a recommended procedure to accomplish this:

Procedure

  1. Disconnect the power cable from the radio, and take the cover off.

     
  2. Unplug the RAM board and remove it from the radio.

     
  3. Temporarily solder a 3 Volt DC battery source across the existing battery terminals (see suggested connection points on the circuit board pictures).

     
  4. Unsolder the old lithium battery and replace it with a new one. (BR2325 1HC, ICOM stock number 945 03112)

     
  5. Unsolder your temporary 3 Volt DC source.

     
  6. Reinstall the RAM board into the radio.

Cautions

  1. Do not use an AC powered 3 Volt DC source, your grounded soldering iron tip could short out the battery (+) terminal. Use a 3 Volt battery DC source only (2 alkaline cells for example).

     
  2. Do not solder the external DC wires directly to the lithium battery tab pads. If you do so, you will not be able unsolder the battery without having the wires drop off.

     
  3. If by accident you lose power to the RAM unit it must be sent to Icom America for reprogramming.

     
  4. Be careful not to damage or bend the connector pins on the radio side while removing or reinstalling the RAM card.

Pictures

There are two versions of the ICOM RAM boards. Both are identical in operation, and they are interchangeable. The differences are in the circuit board layouts only. Note that the black and red wires visible on these images are the suggested way of connecting the external DC backup voltage while the battery is being replaced. The wires are not a part of the RAM card, and should be removed when the battery replacement procedure is complete. Refer to the replacement procedure outlined above.

 

RAM card version A images:

 


RAM card version B images:

This modification can also be found at ICOM's own homepage on the following URL: "RAM Card Backup Battery Replacement Instructions"