Maycom EM-27 CB Radio Mods & Fixes

(aka Radioshack TRC-1080, Stabo XM-7xxx)

IMPORTANT NOTICE: You definitely do not need to put any relays inside your radio for any of these mods to work. Anyone that says you need to put relays in your Maycom is an ass. Do these mods at your own risk! Most of the information on this page is copyright to me as I have spent a lot of time working it out, if you wish to re-produce anything and/or put anything from my site onto another website then please be courteous and e-mail me first (I don't bite!).

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:

Link-Pad Layout
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:

Near the main processor, on the front board, you will see 8 solder-links labeled 2,1,4,3,8,7,6,5 (as above). For 200ch link 8, for 400ch link 7, for 600ch link 7&8. 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.
Frequency Adjustment:

On the main board, just behind M7 there are 2 Variable Capacitors, a RED and a WHITE (or sometimes green). The white Vari-Cap adjusts the main frequency of the radio, while the red Vari-Cap is used for offset frequency. In UK radio's (and maybe others?) the UK band is the Main frequency and the EU (mid) band is the offset. therefore, you must first tune the radio on the UK band with the white and then offset the mid band from this with the red.

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 vari-cap with a plastic tool until you obtain 27.601.25
Tune to 26.965 MHz (ch 1 mid band)
Adjust the red vari-cap 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 vari-cap with a plastic tool until you obtain 26.965.
If you can not obtain the desired freq, then move on to the red vari-cap.

AM/FM Mode Switching:

In 200, 400 or 600ch mode, Pressing M2 with FUN not flashing (LCR) toggles AM/FM mode, the screen indicates UK for FM and EU for AM. (There is no outgoing modulation on AM)

Repeater Shift Operation (aka split frequencies):

In 200, 400 or 600ch mode, Pressing M7 with FUN flashing (blank) puts radio in repeater mode; the screen indicates RPT above the channel number. This allows you to transmit on one frequency and receive on another. To use, select the desired TX frequency, press M7 so RPT is displayed, and then select the desired RX frequency. When you press the mike the radio will jump to the TX freq then return to RX freq when you release the mike.

5kc Shift:

Link option 5 on the above solder-links to enable 5kc shift. To use, hold M1 and press M6. In 200, 400 or 600ch mode the radio will display the change, however, in 80ch mode the display does not change but the frequency does.

Change Emergency from 9 to 19:

Link option 6 on the above solder-links to change the emergency channel from 9 to 19. To use, Press M7 with FUN not flashing and instead of jumping to channel 9 (as usual) the radio will now jump to channel 19 (some have reported a slight rf power boost by doing this - I'm not so sure about that!).

Tweak 'n' Peak:

VR1 = TX Power
VR301 = RX Meter
VR501 = Squelch Threshold
VR850 = FM Deviation

Microphone Wiring:
PIN FUNCTION COLOUR (original mic)
1 Modulation White
2 Receive Black
3 Transmit Red
4 Mic Functions Blue
5 Ground Braid
6 8Vdc Supply Green
Lights come on, but radio is dead:
The chances are that you are looking at a rig that won't do a heck of a lot. The lights behind the display will light, but nothing else happens. Pressing buttons won't do anything, but if you key the mike, you may (or may not) notice the lights get slightly dimmer.

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.

The legs on the regulator, with the display facing you are from left to right: Out (5v), Ground (0v) & in (12-14v). the forth leg at the top is also Ground (0v). If you are careful, you can replace it with a standard 7805 regulator (on wires, bolted to case) and it works fine.
You should also check/replace R310 as required.
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.


Front to Main Pin-Outs

Orange Screen (v1):

1
2
3
4
5
6
7
8
9
10
11
12
A
12.6
0
0
0
8.0
5.0
0
5.8
0
0
8.0
NC
B
0
2.3
0
0
2.5
2.3
5.0
0
0
0
5.4
0

Green Screen (v2):
1
2
3
4
5
6
7
8
9
10
11
12
A
0
0
0
0
7.8
5.0
0
5.6
0
0
7.8
NC
B
0
2.2
0
5.0
2.5
2.1
5.0
0
0
0
5.0
0
 

Intermittent Transmit leading to Transmit 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.):

On this radio, Q306, a KRC110S is o/c base to collector. It receives the TX CTRL B+ signal from pin 16 of IC001 and drives Q300, a KTA1505S which switches on the VDD TX B+ supply for the TX electronics. This supply is separate to the multitude of main ones (TX REG B+, MAIN REG B+, VDD B+ etc) and only exists for small part of the circuit so is easy to miss, and when missing everything appears to work except the TX power.

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:

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

So, the fault would appear to be on the front panel of the rig, somewhere between the mic socket and pin 65 of ic901. Either (if i read the schematic correctly) C888, L853 or C883 are the main suspects (possibly a capacitor overloading and shorting). I'll update the site as soon as I crack it. (which may be a while, as I'm not always able to work on rig)

I'm currently working on a solution for this. (if you already know the solution please contact me)

 

Schematic:

To download the schematic, right click on the link below and click on "save target as.."

Maycom em 27 Schematic 200kb

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: 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!!!)