http://groups.yahoo.com/group/AR902Mhz   From the description text: "This listserv is used to further experimentation and utilization of the 902-928Mhz amateur radio spectrum using voice and data communications. Please limit your posts to information relating to these subjects ONLY. Want and For-Sale ads allowed. Do not post EBAY links. Do NOT list non-900Mhz applicable items."

http://groups.yahoo.com/group/900mhz   From the description text: "For those interested in utilizing the 900 Mhz amateur radio band. This group is here to discuss modification of commercial radio gear to amateur radio use, repeater construction for 900 Mhz, and other related topics relating to the 900 Mhz amateur radio band."

The EFJ 8640 is a 10 ch. 15w 900mhz radio with full PL/DPL capabilities. The 8644 is a 4 ch. version of the same radio and exists only because the EFJ sales team needed a lower end radio than the 8640, so the EFJ programmers stripped the firmware from ten channels to four (that is the only difference between the two models). The 864x series can a talk only to a repeater, meaning it has no talkaround (simplex) circuit in the VCO. Since the replacement 900mhz firmware was based on the 8640, any converted 8644 will end up as an ten channel 8640.

The 8655 is a 5-30 watts (adjustable) radio, does both repeat and talk-around (simplex), and has either ten or sixteen banks of 10 channels each depending upon the vintage of the display board... therefor the "low end" 8655 has only 100 channels...

The factory mobile power cable has a 15a fuse in the positive lead and Anderson Powerpoles on the end that plugs into the back of the radio - yes, the radio has chassis mount powerpoles on the rear panel right from the factory - a nice touch. The only problem is that both are colored white(!) plus there is no marking on the back of the chassis. The top one is positive, the bottom one is negative, and the radio is negative gound only (the negative one is grounded to the chassis). I suggest a Sharpie pen for adding "+" and "-" marks next to the chassis mount Powerpoles.
The official factory mic for these radios was the 250-0740-300 "Amplified Dynamic" or the 250-0751-021 12-button DTMF or the 250-0742-010 Desk microphone.
Microphone pinout: (pin 1) Mic shield and PTT return, (2) Mic hot, (4) PTT

The "old" (ARRL endorsed) 900mhz amateur band plan was designed with no consideration to the fact that ham radio is secondary in the band, and no consideration to the available equipment. That plan specified -12mhz offsets and is used in very very few areas. The 900mhz band is shared with many other services, and they all tend to leave the bottom and top megahertz alone as guard bands. The 12mhz plan has the ham repeaters right in the middle of the commercial users (and they are primary in that allocation) and they do not take kindly to hams. Due to hostility from the primary users the ham repeaters have migrated to the outer megahertz at each end - 902 to 903 as repeater inputs, and 927 to 928 as repeater outputs. The N2MCI firmware was written for the modern 25mhz offset amateur environment and will NOT do the 12mhz offsets, or the commercial 900mhz frequencies.

Any of the above three models will go on the amateur 900mhz band by swapping the main microprocessor (it's socketed) and one SMD chip cap, and loading new frequencies into the radio! Pete Dziomba N2MCI developed the modification technique and sells complete kits.

The N2MCI kit consists of:
    a) one replacement microprocessor chip with new embedded firmware (available only from N2MCI).
    b) one surface mount capacitor, size 1206 (the value varies depending on if it's an 8640/8644 or an 8655 radio). You unsolder one cap on the RF board and replace it with the new one from the kit.

The kits cost US$8 each. Shipping is US$5 for any number of kits...
To purchase the conversion kit you will need to Pete an email first at n2mci //at// hvc //dot// rr //dot// com and then send a money order or a bank check.

Installation information and more details at http://home.hvc.rr.com/n2mci/efj864x.html.

Kyle Yoksh KØKN has a web page that has full modification information and procedures for the 8640, 8644 and 8655, complete with photos. In addition to the primary site he has an alternate location at qsl.net. His site also has info on converting the 800mhz model 8615 for use as a 902 MHz repeater receiver.

Here's the The 8640 / 8644 Service Manual   (9.9mb PDF).   It offers 5-12 watts (adjustable), 4 channels (8644) or 10 channels (8640), and is repeat only (no simplex/talkaround.
Here's the The 8655 Service Manual   (12.9mb PDF dated Dec 1987).   It offers 5-30 watts (adjustable), does both repeat and talk-around (simplex), and has either ten or sixteen banks of 10 channels each depending upon the vintage of the display board.

The microphone connector wiring is pretty EFJ-standard:       (courtesy of Steve KB3FSR)
Pin 1 : Ground = shield of pin 2
Pin 2 : Audio = White Wire
Pin 3 : Hang-up clip = Black Wire
Pin 4 : PTT = Red Wire
Pin 5 : 13.8v = Blue Wire
Pin 6 : Not connected

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Page created 27-December-2005 by Mike Morris WA6ILQ