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Convert EM27 CB to 10Meters Amateur Radio

Do this entirely at your own risk, if things go wrong I am not liable ! This is a one way modification to turn a Maycom EM27 CB into a 10 meter FM amateur radio.


Step one is to connect a CB style SWR meter and a dummy load. Most CB style SWR meters don't have a proper power reading so before going any further transmit into the dummy load and set the meter to read full scale. This will be a reference level for 4w - useful during step3. If you're new to 10meters dont assume your 2m/70cms SWR meter will do the job, a VHF/UHF meter will do if nothing else is to hand, but any power readings will be miles out.

Step two is to relink the CPU board. The CPU has a number of lines with links to tell the computer what modes it should use. With the radio face down and unplugged from power set 4th and 6th and 8th link from the left only, clear the other pads.
Viewed from above thats O=Open C=Closed. The pins are numbered (though not on my PCB !) the rightmost pad pair isnt used hence the X

     2 1 4 3 8 7 6 5 X 
     O O O O O C O C O 

This is what other articles call the "400 channel" mode. The CPU will try and tune 25 to about 30 Mhz, the VCO is not capable of this swing but will normally do 26.2ish to 29.7Mhz. Tune from one end to the other and check you get what is expected.

Step2, Front board links
CPU Board
Step3, Retune These Inductors

Step three is to retune the inductor pair to peak output for 10 meters. Tune the radio to 29.6ish and transmit. Adjust the coils for peak output. Dont worry if you cant get full scale, but try and get as close as possible. Anything >3.5w (80% of the original reading) is ok, but the real cost will be to receiver performance if the bandpass filter is poorly aligned.

Step four, the final job is to realign the radio so that it sits on frequency for the 10Khz channels of the FM bandplan. Before I found the article at Normsweb.com I was pulling the PLL 5.25Khz to the left (using a 33pf capacitor soldered across the 3.6Mhz XTal). This turned out to be pretty dumb as the radio is cabable of tuning in 10Khz steps with the correct alignment. Thanks to Norm for his help and emails.

The righmost link that was set in step2 on the CPU board enables 5Khz frequency shifting. Turn the radio on and Hold M1 - then press M6, you may need to try a couple of times - the radio should now step in 10Khz steps with the correct alignment. It still displays the small .25 symbol but a frequency counter reveals it correctly aligned.


XX

If for some reason its off tune then adjust the inductor directly in front of the 3.6Mhz XTal. The XTAL is used to clock the CPU on the front board and as the reference to the PLL. The inductor (coloured green on my board) is used to finetune the clock, use the correct plastic tool as sticking anything metal in the inductor causes the oscillator to collapse and the CPU to crash.


Notes on using the radio

Repeater Shift. Press M7 with FUN flashing (blank) puts radio in repeater mode. The RPT symbol should appear. 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.

Some useful internal pots (thnaks again to Norm)
VR1 = TX Power
VR301 = RX Meter
VR501 = Squelch Threshold
VR850 = FM Deviation

Some notes about 10m.

Let's be honest, its close to CB and works about as well. If you're used to 2meters then you're in for a bit of a shock. For mobile comms its a pretty poor choice, 3 miles down the road can be a poor copy, almost regardless of the amount of power you run. The good news is that it tends to be pretty empty, so if you want a quiet place to chat then this is it. During the top of the sun spot cycle the band tends to open up and long distance comms is not only possible but often unavoidable. Some 10 meter repeaters exist, the maycom radio can work with a repeater offset, on the down side you would need to add CTCSS encode to use them.

The current(2007) RSGB UK Bandplan shows (Power upto 400w):
XXXXXX> 29.210MHz & 29.290Mhz Unattended UK Internet voice gateway
XXXXXX> 29.520 - 29.550 FM simplex channels - 10kHz spacing
XXXXXX> 29.530 UK unattended internet voice gateway
XXXXXX> 29.560 - 29.590 FM repeater inputs RH1 - RH4
XXXXXX> 29.600 FM calling channel
XXXXXX> 29.600 - 29.650 FM simplex channels - 10kHz spacing
XXXXXX> 29.600 - 29.700 FM repeater outputs RH1 - RH4
XXXXXX> 29.630 UK unattended internet voice gateway



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