Modifications for the ICOM - RogerK

28-01-2002(IC-751) ICOM IC-751 Dual RAM back-up Battery ModificationAuthor: Stuart - KI6QP - stuart@valontechnology.com.MODIFICATION.NETThis modification adds two lithium cells and diode steering to provide future fail-safe cell replacement. You will have to build alittle breadboard with a couple of Schottky diodes and battery holders but the cost is way less than the shipping charge to sendyour rig back.Stuart Rumleyki6qp20 January 2002The ICOM IC-751 is my all time favorite radio. I haven't personally owned a lot of radios but I have worked on quite a fewincluding 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 a few high integration integratedcircuits. It has a robust receiver with high input IP3 and well balanced gain. The 751 uses real crystal filters and doesn't rely onany phony-baloney last-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 than and LCD or plasm meter. There are realmedium 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 power amplifier section.The 751 does not include an antenna tuner which I think to some degree is an advantage. For antennas that are not wellmatched, an external antenna tuner must be used. Generally, external antenna tuners are more efficient and always morereliable than internal antenna tuners and are very simple to maintain.RAM battery scheme is a bit dubious. The original design requires the owner to send the radio back to ICOM every decade or soand have the lithium cell replaced. Failure to do so has left a number of owners with a brain dead rig and a conviction to neverby another ICOM radio again.Clearly it was a manufacturing expedience to use a battery backed SRAM instead of a more expensive EPROM for the programcode plus an SRAM for the user memory. One can imagine that they could have socketed the lithium cell so that user couldreplace it while the radio was powered on. I suspect this idea was discarded because of possible product liability issues withhaving the operator change the cell with the power on.This modification adds a small, home brew breadboard containing two replaceable 20mm lithium cells. Each cell can be replacedone at a time so that one cell is always powering the RAM. In addition, this modification leaves the original cell in place whichgreatly reduces the possibility of accidentally clearing the memory.Modification instructionsThis is a pretty simple modification and I think considerably safer to perform than the battery replacement method that Icomrecommends. However, you do have to use above average care due to the risk of losing the program memory. If this happensthe radio will not be able to tune the synthesizer and you will have to return the RAM board to ICOM for reprogramming.You will not need any additional power supplies or batteries to power the RAM board during this procedure. The existing RAMboard cell will keep the memory alive during this operation. You should be using a grounded soldering iron, however, to minimizethe risk of static discharge altering the memory.You will be working with rather close spaced traces on the RAM board and the only precaution you need to be aware of is to notallow 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 in pdf. Use approximately 10 inches oftwisted black and red AWG 26 hook-up wire to make the connection to the RAM board. Prepare the RAM board ends of the blackand red wire by stripping back about 1/8" of insulation an tin the leads.Note that the board has two 2mm mounting holes spaced approximately 35mm. This will facilitate mounting the board in the 751chassis on the voice module mounting holes.Test the completed board with a VOM to ensure that the diodes have be correctly installed.