Maycom EM-27 / EM27 CB Radio Mods & Fixes

Remote controlled Maycom EM-27!

Frequency Mods:

Important Information: On the European models the display shows "Mhz" instead of ".25" and "Am/Fm" instead of "Uk/Eu". When you do any of these frequency mods on a UK Maycom the radio will be on frequency, but will still display the ".25" UK shift in the display. There is no (known) easy way to remove the ".25" from the display window (other than to buy a knackered Stabo and change the display panel).

UK Stealth Mod: (Devised by Norm of normsweb.com)

This mod will allow you to mod your Maycom EM-27 to 400 channels mode "secretly". When turned on, it looks and works exactly the same as a non-modified 80ch UK EM-27. However, if M4 is held whilst turning on, the radio will be in 400ch mode (25.165 MHz - 29.655 MHz).

How to mod:

Near the main processor, on the Front-Board, you will see 8 solder-links labeled 2,1,4,3,8,7,6,5. Link 2,1, & 3 then clear all other links. (except 5 & 6 if you've done those mods). Make sure the back-up battery is installed (if it isn't then the radio may lock up). Momentarily short the battery to clear the memory

Non Stealth Mod:

This mod will allow you to convert your radio to 200, 400 or 600 channels mode.

200 channels is 26.065 to 28.305 MHz (A,B,C,D,E)
400 channels is 25.165 to 29.655 MHz (-D,-E,A,B,C,D,E,+A,+B,+C) Also known as 10 meter mode
600 channels is 23.815 to 30.555 MHz (-A,-B,-C,-D,-E,A,B,C,D,E,+A,+B,+C,+D,+E)

It is not possible to get the radio transmitting from 23.815 all the way up to 30.555 as the VCO won't go that far. (any ideas on expanding the VCO are welcomed!) I recommend doing the 400ch Stealth mod, as the radio will tune to cover 25.165-29.655 easily. For advice on tuning the VCO and some photo's to help you, check out Jon's site.

How to mod:

Frequency Adjustment:

How to Retune for 80ch mode:

Connect radio to a suitable Frequency Counter.
Tune to 27.601.25 MHz (ch 1 UK band)
Adjust the white trimmer until you obtain 27.601.25
Tune to 26.965 MHz (ch 1 mid band)
Adjust the red trimmer until you obtain 26.965 MHz

How to Retune for 200, 400, or 600ch modes:

Connect radio to a suitable Frequency Counter.
Tune to 26.965 MHz (ch 1 EU band)
Adjust the white trimmer until you obtain 26.965.
If you can not obtain the desired freq, then move on to the other trimmer.

AM/FM Mode Switching: 

Repeater Shift Operation (aka split frequencies):

5kc Shift:

Change Emergency from 9 to 19:

Tweak 'n' Peak:

Microphone Wiring:

Lights come on, but radio is dead:

This is a known manufacturing defect with these radios and a whole load of the green screen V2's were re-called. It is just possible that your radio slipped through the net. If you have plugged in the wrong microphone or got the wires mixed you will also have the same symptoms. (likewise, if you have tried to expand your radio and cock'd it up)

To Fix, you need to locate the surface-mount 5v regulator transistor (IC 300 - KIA78L05) and replace it. Its located roughly in the center of the board near the back, surrounded by small electrolytic capacitors.

Main-Board to Front-Board Voltages:

NOTE: There are four (known) revisions of this radio. The first and second has an Orange display (known as version 1). While the newer third and fourth revisions have a Green display (known as version 2). There are differences in the voltages, so be sure you read the correct data.

All readings taken with radio connected to power supply at 13.5v, but with the radio switched off. The digital voltmeter is grounded at the power supply and set to DC.

Orange Screen (v1):

Green Screen (v2):

Intermittent TX leading to TX Failure: (no tx or no rf output etc.)

Sometimes when microphone is keyed the radio fails to enter transmit mode. This is a common fault in both older and newer radios. Usually, once it appears, it gets progressively worse over time and eventually the radio will fail to key-up at all. This problem seems to be accelerated by using a non standard microphone or having a high SWR. You should obviously check your microphone first, for maybe a dirty switch or bad connection. I have seen several EM27's with this fault and so far all of the microphones tested fine, so chances are that your mic is fine.

If you get absolutely nothing happening when you key the mike, then I'm afraid you'll have to wait as I'm currently working on the solution for this.(see my notes below). If however, when you key the mike you're getting the signal meter lights, but no output, try this solution by "absorbentgnome" & Rick (Euro Radio Co.):

The conditions it is under must cause it to go intermittent then fail o/c. Hope this is the answer you are looking for.

Unfortunately, this solution didn't help in my case, but it may help in yours.

My Notes: First, spray the mic switch with contact cleaner. I have recently had a radio where the mic tested ok on the mic tester, but failed to key the radio. After spraying the mic switch (out of sheer desperation!) it is working fine again. This will not be the case with most rigs, but it may help!

Check the voltage on pin 16 of IC 001 (front left corner of radio, labled NJU3713), when the mike is keyed, the voltage should be around 5vdc. If you're getting 5vdc every time, but the radio only transmits occasionally or not at all then "absorbentgnome's" solution should help you.

Try squeezing the mike key very slowly. you will probably find that every time you key the mike slowly the radio will transmit, but if you key rapidly it will not...

If you are getting no voltage at all on pin 16 when radio fails to TX and 5vdc when it works, then you have got the same fault as me. Next check the voltage on pin 65 of IC 901 (main processor) on the front panel of the radio. Should read about 4vdc in receive mode and drop to about 200mvdc when radio is transmitting. when mike is keyed and radio is not transmitting the voltage will drop right down to nearly nothing. this info was worked out on a green screen v2 maycom, but should be the same on v1's. I'm currently working on a solution for this. If you already know the solution please contact me.

Schematic:

Battery Worries:

Its only if you have got an early orange screen Maycom that you really need to panic about replacing the battery. The early radio's had a "barrel shaped" backup battery that was very unreliable and more often than not, leaked, causing untold and usually un-repairable damage to the PCB. Later radio's have a flat "disc type" lithium battery which (so far) seems to be very reliable. This "disc type" lithium battery is, by some, mistaken as a capacitor and does not (usually) need replacing.

UPDATE: I just opened a new Maycom and there was actually a small capacitor in place of the battery (5.5v .22f). To upgrade, simply find a suitable cappy and swap it for the battery. (be sure to observe polarity!!!)