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Maycom EM-27 EM27 CB Radio Mods and Fixes

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

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Frequency Mods

Also known as: expand the channels, more channels, 10 meter conversion, wideband modification.

On the non-UK models the display shows "Mhz" instead of ".25" and "AM/FM" instead of "UK/EU". When you do these frequency mods on a UK EM-27 the radio will be on frequency, but will still show ".25" 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).

Maycom Solderlinks - Click for larger pic

How to do it:

Near the main processor, on the Front Board, you will see 8 solder-links (as pic) 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 reset the CPU.

Depending on which country your radio was from originally, there will be tiny SMD capacitors on some of the links. (EG. on link 3 for a UK radio) If the capacitor is on a link which you want to use in the modification you can just leave it there and do not need to solder that link, however if a capacitor is on a link which you do not want to use for modification then it MUST be removed or your mod won't work correctly.

Maycom Battery

When you have done all your solder-links for your modification, the backup battery MUST be shorted momentarily to reset the CPU. With the radio turned off and unplugged from the power supply, use a short wire to link the 2 points shown in the photo for about a second.

Your battery may be different to this one, or it may even be a small capacitor. To my knowledge there are at least 4 different types of battery / capacitor used in this radio, but you still need to short these same 2 points whatever battery / capacitor your radio has.

Non Stealth Mod:

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

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's not possible to get the radio transmitting from 23.815 Mhz to 30.555 Mhz without major VCO mods.

Remote Controlled EM-27 Mod

I did this mod several years ago and have only recently re-found the video.

What do you do if you have a broken 5 inch TV? You take the remote control circuit out and fit it in a radio! I can't remember the exact model of TV, but the remote control circuit (as it turns out) was on a seperate little board, which made this mod SO MUCH easier... The RC board needed some minor modification to make it work with the EM-27 and I ended up with a board that would ground three different pins when the remote buttons were pressed.

In the EM-27, all of the switching on the front panel is 'ground switching', so to activate a function, that switch simply grounds the connection. Perfect! So to wire the remote board in, I just had to decide which switches to activate and figure how to mount the IR receiver behind the display. (this probably took the longest time!) Here is the video of the mod in action:

If I get time soon, I may open the radio and take some hi-res pics of the ir board and stuff (and maybe a better quality video too!), but until then this video will have to suffice :-)

Frequency Adjustment

On the main board, just behind M7 there are 2 trimmers, a RED and a WHITE (or sometimes green). The white trimmer adjusts the main frequency of the radio, while the red trimmer 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. This must be done using a plastic trim tool (screwdriver), if a metal tool is used you will not be able to tune it correctly

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 then tune to 26.965 MHz (ch 1 mid band) and 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 frequency, set the white trimmer to roughly mid position and then move on to the other trimmer. You may need to go back and adjust the first trimmer to obtain the correct frequency (try not to have either trimmer maxed)

AM/FM Mode Switching

In 200, 400 (including Stealth) 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 for AM on UK Radio's)

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.

5 Khz (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 Channel 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:

Also known as: tune up, adjustment, retune.

Here is a picture showing the adjustment points within the radio. It is advisable to mark the pots before adjustment, so you can put them back to their original positions if you need to.

Another good rule to stick to is: If you are not sure what it does, dont turn it!

Pot Layout Diagram

Microphone Wiring

Pin 1: Audio (white)
Pin 2: Receive (black)
Pin 3: Transmit (red)
Pin 4: Mic Functions (blue)
Pin 5: Ground (braid)
Pin 6: 8v Supply (green)

Lights Come on, but Radio is Dead

The chances are that you are looking at a radio 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.

Loud Howling or Squealing Noise from Speaker

If when turn on your radio it all lights up fine and appears to work ok, but you get a loud howling or squealing noise coming from the speaker then you should remove or replace the back-up battery (or capacitor). In this radio, when the back-up battery goes bad the radio makes the loud howling noise. Unplugging the microphone disconnects the speaker so the howling will not appear when mic is not plugged in.

Update: This solution will usually stop the squealing noise, but I have since found that there may be another bad component in the radio causing the noise, so after you replace the battery the noise *may* come back within a few days/weeks. As soon as I can find out exactly which component it is, I will update this section with a fix.

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

If you are occasionally getting nothing happening when you key the mic, try keying the mic slowly. You will probably find that when you key slowly the radio will transmit every time. This indicates that the mic switch needs cleaning. Open the mic and squirt some switch cleaner or contact cleaner spray into the switch and key it rappidly a few times (do not use WD40!!!). This will fix it 90% of the time.

If you're getting signal meter lights everytime, 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.

Schematic

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

Maycom EM 27 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 opened a new EM27 and there was actually a small capacitor in place of the battery: 5.5v .22f

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