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  E. F. Johnson
900 MHz Information

Compiled by Mike Morris WA6ILQ

The "old" (ARRL endorsed) 900 MHz 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 -12 MHz offsets and is used in very very few areas. The 900 MHz band is shared with many other services, and they all tend to leave the bottom and top megahertz alone as guard bands. The 12 MHz 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 interference from our operations. 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 E. F. Johnson 86xx mobile radios are a natural for the 900 MHz amateur band, and the mod to put them there is simple (we have no info on the 98xx series yet). Since the 86xx are not as popular as the Motorola Spectra the radios are less in demand, therefore a lot less expensive. The programming software is readily available, doesn't have the computer speed limitation quirks that the Motorola software has and there are no EFJ software police to worry about. EFJ just doesn't care... you can download the software from a dozen web sites here in the USA.

If anyone would like to put together a programming article (with screen shots? they are not hard to do) we'd be happy to put it here.

There are two 900 MHz Discussion Lists and the first one listed below has 95% of the traffic. Note that these are 900 MHz lists (any radio, not just Johnson) and do not take kindly to off-topic discussions. I'd subscribe to both.   From the description text: "This listserv is used to further experimentation and utilization of the 902-928 MHz 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."   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 topics relating to the 900 MHz amateur radio band."

Below we have information on making the EFJ 8600, 8615, 8640, 8644 and 8655 models useful on the 900 MHz amateur band. If anyone has any info on any other 900 MHz Johnson mobiles or handhelds, please contact the maintainer of the E.F.Johnson equipment page Robert Meister WA1MIK.

The EFJ 8600 is a 10 ch. 15w 800 MHz radio with full PL/DPL capabilities. The 8615 is also 800 MHz, and while neither transmitter can be converted to 927 MHz the receiver section can be converted into a 902 MHz receiver (i.e. repeater input or link). Note that the deviation at 800 MHz is 5 kHz and at 900 it's 2.5, so the receiver audio recovery level will be lower (it won't deliver as much audio). However due to the wider IF filters it will not be as tolerant of adjacent channel interference, and it will have a bit more tolerance of mobile transmitter drift than the average made-for-900 MHz receiver. But if your mobile transmitter is drifting you need to fix that, not get a wider repeater receiver.

The 8640 is a 10 ch. 15w 900 MHz radio with full PL/DPL capabilities — basically a 900 MHz version of the 8600 radio.

The 8644 is a 4 ch. version of the 8640 radio and exists only because the EFJ sales team decided that they 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 900 MHz firmware was based on the 8640, any converted 8644 will end up as an amateur-band ten channel 8640.

The 8655 is a 5-30 watts (adjustable) 900 MHz 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...

Any of the above three models will go on the amateur 900 MHz band by swapping the main microprocessor (it's socketed) and one SMD chip cap, and loading new frequencies into the radio!

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). So take a VOM on the resistance scale and determine which one is grounded, and use a Sharpie pen to adding "+" and "GND" 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.

The microphone connector in the 864x or 8655 radio is an eight pin jack. The microphone plug is a six pin plug that makes contact with the center six pins. The wiring of the six pins on the mic plug is pretty-much EFJ-standard: (courtesy of Steve KB3FSR)
8-pin jack
8-pin jack
6-pin plug
Description / Notes
Wire colors mentioned are from the four Johnson mics I've been inside. Yours may be different.
Pin 1Pin AUsed only for programming (serial data from radio to computer). There is no pin on the microphone plug for this connection.
Pin 2Pin 1Mic Audio Ground, PTT return and coax shield of mic audio pin
Pin 3Pin 2Mic Audio = White Wire
Pin 4Pin 3Hang-up / Monitor clip = Black Wire
Pin 5Pin 4PTT = Red Wire
Pin 6Pin 513.8v = Blue Wire (in some radios this pin is dead, in others it is hot if an internal jumper is installed). This was intended to power a DTMF mic.
Some programming interfaces regulate this voltage to 5v to power the programmer.
Pin 7Pin 6Not connected. Some conversions route COR or RUS to this pin.
Pin 8Pin BUsed only for programming (serial data to radio from computer). There is no pin on the microphone plug for this connection.
So again, take a VOM on the resistance scale and determine which second-from-the-end pin is grounded, and that's pin 2. Then verify that pin 6 is floating and that will let you know how to wire the programmming cable.

Besides the pin numbering, there is one little quirk about the EFJ 86xx programming cable... The plug that goes into the microphone jack of the radio is a special 8 pin RJ-12 made by Vriginia Plastics and not the normal 8 pin RJ-45... and note that a standard RJ-12 is 6 pins. You can use a modify a standard 8-pin RJ-45 plug but you have to take off a few thousands of an inch off each side. I use a grinding wheel to take most of the excess off, and then a hand flat file to fine tune the fix... If you take off a little too much it's not critical as the spring tab latch centers the plug... If you don't have a grinding wheel you just to get to do some more filing. I've found it useful to tape a bubble level to the top of the file.

Kyle Yoksh KØKN has a web page that has full modification information and procedures for the 8640, 8644 and 8655, complete with photos. His site also has info on converting the 800 MHz model 8615 for use as a 902 MHz repeater or link receiver. Note that 800 MHz radios are 5 kHz deviation, and 900 MHz radios are 2.5 kHz deviation - the audio recovery is a little low on the converted receiver.

Here's the The 8640 Operating Manual (1.4 MB), and the 8640 / 8644 Service Manual (2.9 MB PDF). It offers 5-12 watts (adjustable), 4 channels (8644) or 10 channels (8640), and is repeat only (the radio does not have the simplex / talkaround ability).
Here's the The 8655 Service Manual   (3.6 MB 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.

Repeater-builder received four Field Advisory (service) notes applicable to the EFJ 8640, 8644 and 8655 (and some others):
1) FA#95-005 - Replacing U601 on 86xx mobile radios.
2) FA#95-007 - 86xx Desense and Microphonics.
3) FA#96-006 - Revised CMOS microprocessor in 8600/50 radios.
4) FA#98-003 - Installation and Modifications for new U801 prescaler in 8605, 8606, 8610, 8615, 8616, 8620, 8622 and 8655 mobiles.

N2MCI came up with a COR buffer for the Johnson radios. Click here for a schematic.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

This page split from the main page 16-Nov-2011.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by multiple originating authors and Kevin Custer W3KKC. All Rights are Reserved, including that of paper and web publication elsewhere.