modifications_for_icom_ic_751_921

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Modifications for the Icom IC-751Picture(s) of Icom - IC-751

19-07-1998 add a comment

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 tooperate with limited privaledges when communicating with SSB stations above 10MHz.Below 10MHz, the CW receiver is actually LSB so split operation in that case is effortlessand does not require the split oepration to be invoked. The original bogus operation can beconfirmed by placing the rig into CW Tx and USB Rx split operation and operate the key. Amore `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 diodesSCHEMATIC: +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< 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__|_______|_______|_______|

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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 theboard references may be different.

1. Go to Radio Shack or your favorite supplier and buy a lithium battery to replace theone in your radio. The actual type used is a BR2325 (3.0 volt 165 mAh), whichRadio Shack does not carry. You can order this one from Digikey. I bought a

CR2450 at Radio Shack (3.0 volt 500 mAh), which was the largest capacity unitthey 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 amodular plug-in unit rated at 4.5 volts @ 60 ma. I connected a 500 Mfd. capacitorto 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 theRAM board. Connect several feet of wire to the divider output, which will go to theRAM board.

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

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


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5. Turn off the power to your radio and remove the one samll screw that holds theRAM unit to the main board (you need a jewlers' screwdriver). Now you canremove the RAM board from the radio; remember from now on you do not want totouch any tools or other metal i tems to the board or it's components.

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

7. After the external power source is connected, turn it on and measure the voltagebeing supplied to the board. You may need to adjust the values of the dividers toget 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 onthe 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 componentleads.

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

Date: 21-02-2003 User comment From:Yuriy Belov, UA4FCO

Subject: ic-751

I need of technical parametrs trcv IC-751 and IC751A.TKS,73!

Date: 10-06-2003 User commentFrom:richaerd bundy

MO.493

Subject: manual download Icom 751

you can download the service manual so you can see the ram board here on dk mods,Just look up the radio and click manuals, find radio and start printing. enjoy.

This modification has been read 3573 times.

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RAM Card Backup Battery Replacement Instructions

Author: ICOM

In the early 1980s Icom America marketed a group of transceivers and radios that utilizedthe latest state-of-the-art computer technology to enhance the operation of the radios.This technology improvement allowed hams to afford a reasonably priced highperformance transceiver which until then was only available on units costing much more.Features now available to the hams included: higher frequency stability, better frequency


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resolution, digital frequency display, almost instant recall of saved frequencies (memorychannels), quick mode signal processing and built in tones and offsets for the rapidlyemerging repeater operation.

Implementation of all these features required a CPU with associated logic circuitry. In theearly 80s, the most cost effective way to implement data processing and memoryfunctions 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 retainits 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 replacementwas 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 arecommended 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, ICOMstock 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 couldshort out the battery (+) terminal. Use a 3 Volt battery DC source only (2 alkalinecells for example).

2. Do not solder the external DC wires directly to the lithium battery tab pads. If youdo 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 forreprogramming.

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

Pictures

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


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complete. Refer to the replacement procedure outlined above.

RAM card version A images:The image cannotbe displayed. Yourcomputermay nothave enough memory to openthe image, orthe image may have beencorrupted. Restarty ourcomputer, and thenopenthe file again. If the red x stil lappears, you may have to delete the image and theninsertit again.

The image cannotbe displayed. Yourcomputermay nothave enough memory to openthe image, orthe image may have beencorrupted. Restarty ourcomputer, and thenopenthe file again. If the red x stil lappears, you may have to delete the image and theninsertit again.


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RAM card version B images:

This modification can also be found at ICOM's own homepage on the following URL: " RAM

The linked image cannot be displayed. The file may have beenmov ed, renamed, or deleted. Verify thatthe link points to the correctfile and location.



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Card Backup Battery Replacement Instructions"




ICOM IC-751 Dual RAM back-up Battery Modification

Author: Stuart - KI6QP - [email protected]

This modification adds two lithium cells and diode steering to provide future fail -safe cellreplacement. You will have to build a little breadboard with a couple of Schottky diodesand battery holders but the cost is way less than the shipping charge to send your rigback.

Stuart Rumleyki6qp20 January 2002

The ICOM IC-751 is my all time favorite radio. I haven't personally owned a lot of radiosbut I have worked on quite a few including Collins, Drake, Yaesu, Kenwood, Alinco,Heathkit, Icom, and many others.

What makes the IC-751 a nice radio for me is its clean, mostly discrete design with only afew high integration integrated circuits. It has a robust receiver with high input IP3 andwell balanced gain. The 751 uses real crystal filters and doesn't rely on any phony-baloneylast-IF-stage-in-radio-pseudo DSP for selectivity.

The front panel is intuitive and well laid out with a real meter, nothing I hate more thanand LCD or plasm meter. There are real medium sized knows and push-button switches

designed for real people's hands and fingers that make operating this radio a joy.

The transmitter is also an excellent design with separate, discrete and robust poweramplifier section.The 751 does not include an antenna tuner which I think to some degree is an advantage.For antennas that are not well matched, an external antenna tuner must be used.Generally, external antenna tuners are more efficient and always more reliable thaninternal antenna tuners and are very simple to maintain.

RAM battery scheme is a bit dubious. The original design requires the owner to send theradio back to ICOM every decade or so and have the lithium cell replaced. Failure to do so

has left a number of owners with a brain dead rig and a conviction to never by anotherICOM radio again.

Clearly it was a manufacturing expedience to use a battery backed SRAM instead of amore expensive EPROM for the program code plus an SRAM for the user memory. One canimagine that they could have socketed the lithium cell so that user could replace it whilethe radio was powered on. I suspect this idea was discarded because of possible productliability issues with having the operator change the cell with the power on.

This modification adds a small, home brew breadboard containing two replaceable 20mm


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lithium cells. Each cell can be replaced one at a time so that one cell is always poweringthe RAM. In addition, this modification leaves the original cell in place which greatlyreduces the possibility of accidentally clearing the memory.

Modification instructions

This is a pretty simple modification and I think considerably safer to perform than the

battery replacement method that Icom recommends. However, you do have to use aboveaverage care due to the risk of losing the program memory. If this happens the radio willnot be able to tune the synthesizer and you will have to return the RAM board to ICOM forreprogramming.

You will not need any additional power supplies or batteries to power the RAM boardduring this procedure. The existing RAM board cell will keep the memory alive during thisoperation. You should be using a grounded soldering iron, however, to minimize the risk ofstatic discharge altering the memory.

You will be working with rather close spaced traces on the RAM board and the onlyprecaution you need to be aware of is to not allow the battery board leads, soldering iron,or tools to short the RAM board cell.

Step 1: Assemble the dual battery board as shown below and in the schematic shown inpdf. Use approximately 10 inches of twisted black and red AWG 26 hook-up wire to makethe connection to the RAM board. Prepare the RAM board ends of the black and red wireby stripping back about 1/8" of insulation an tin the leads.

Note that the board has two 2mm mounting holes spaced approximately 35mm. This willfacilitate mounting the board in the 751 chassis on the voice module mounting holes.

Test the completed board with a VOM to ensure that the diodes have be correctly installed.

Note: Do not insertthe lithium cells at this time. The cells will be inserted as the last step.

Step 2: Remove the bottom cover from IC-751 and pull out the RAM memory board.

Step 3: Carefully solder the negative lead (black) of the dual battery board to the RAM



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board ground trace at IC2 pin 7.

Step 4: Carefully solder the positive lead (red) of the dual battery board to the RAM boardat the junction of diodes D1 and D2.

The RAM board connections should look like the photo above.

Step 5: Re-install the RAM board back into the 751. You can leave the dual battery boarddangling outside temporarily.

Step 6: Monitor the voltage at the red wire on the dual battery board. You should seeapproximately 3.0 volts coming from the RAM board. Now, power on the 751, the voltageshould now be approximately 4.8 volts if everything is ok.

Step 7: Install the dual batter board in the 751 bottom chassis. I used the mounting holesreserved for the speech module. If you have the speech module installed you will have tolook for a suitable location.Fortunately, there is plenty of room in the bottom chassis to mount the board. I can'trecommend using double sided foam tape as the adhesive usually fails after some time soits best to use screws.



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Step 8: Turn the radio off and install the 20mm lithium cells in the holders. That's it,you're done.

You can now check conveniently check each cells voltage including the Icom RAM cell withyour VOM. Just connect the negative lead of the VOM to the chassis and touch the positivelead to the top (+) side of each cell. You may notice that the memory RAM cell voltage isslightly higher than the new 20mm cells on the dual battery board. This is because theSchottky diodes have lower forward voltage than the diodes used on the Icom board. The20mm cells are now powering the RAM and the Icom cell is floating.

In the future, when cell replacement is need, simply replace one cell at time and you willnever have to worry about losing the RAM program.

73sStuart

Thanks to Stuart for the modification.

Date: 15-06-2003 User comment From:KG4OHH

Subject: about the batt. back up ram

Just wondering:

Could you not solder a resistor a 5 watt resistor to the + 12 volt supply and solder somerechargeables or even a cordless phone battery to the ram board via wires and add adiode to the 4.3v + to prevent voltage leakage back in to the radio..



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This way the rechargeable are always charged up and ready even if the power is cut off?

I would use 3x 1.2v Li-On for long-term life or you could use some cord less phonebatters @ 3.6 volts from radio shack @ $7 to $973 De KG4OHH

Date: 18-06-2003 User comment From:Jim Horn, WB9SYN/6

Subject: Rechargeables vs. Lithium

Yes, any number of backup supplies may be used. A lithium ion rechargeable wouldrequire more extensive charge and discharge protection circuitry and has a lifetime aboutas long as a CR2032 will power the RAM anyway. Same is true of NiCd and NiMH cells.Alternatively, if you want long life between battery changes, the dual-cell approach withlarger cells (Li primary cells come in AA size, for example) should last over a decade withno problems. And with only diode(s) needed to use them, that's pretty hard t o beat!

A final alternative - if you use your rig frequently, a supercap (double layer capacitor) ofseveral farads should work as well with no limit to charge / discharge cycles. And nofancy circuitry needed either. I just picked up some 5.5V 1F caps that are about CR2032sized for only US$2 each so that's another possibility.

Best to all!




IC-751 Cooling, remote jacks

Author: KV5R - [email protected]

The posting of the capacitor problems led me to accidently discover a bad solder jointwhich had killed my IC-751 display more than a year ago. THANK YOU!

Mod: IC-751: Put a little 12-vdc computer fan in the bottom of the 751 and this willeliminate all those heat-related problems with trimmers and caps getting too hot from the8-v regulator sink. This one mod will could save a great deal of expensive repairs. Itsolved my overheated PLL trimmer distortion problem.

Mod: IC-751: I put 1/8th-inch mini-jacks in the rear panel connected to mic-in, AF out,and PTT, to allow easy connection to computer interface. The AF and PTT wire to the ACCjack. The mic is harder, requiring shielded line to the mic connector PCB. This also putsphantom power on the rear jack, allowing use of inexpensive computer headsets withelectret boom mic and one earpiece.

Mod: IC-751: I installed the Piexx UX-14PX interface, and am very happy with it. It was

$145, delivered very quickly. It provides BOTH CI-V AND RS-232 on the rear panel,without any additional external converter. It adds PTT and S-meter functions (and severalother features) that the original ICOM part (UX-14) did not have.http://www.piexx.com/cgi-bin/piexx.cgi?p=ux14

Mod: Icom SM-6 microphone. I put a tiny switch in the side of the SM-6 base to kill themic audio line while sending SSTV and other compu ter modes. With the rear panel jacksconnected to the computer (with a simple homemade interface), and the switch on themic, I don't have to (1) swap jacks, or (2) buy a WAY overpriced rig interface.


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73, KV5R



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