EM27 CB on 10 Meters
The following is a step by step guide to modifying the EM27 CB to work on 10 Meters as well as EU and UK CB. The radios vary somewhat and you may find yours in not capable of working from 26.5 to nearly 30Mhz. When modified the software will try and tune from 25 to 29.650Mhz, the VCO and RF filters are not capable of this. Frequencies below 26.5Mhz will always be unusable for transmit even after the modification.
STEP 1) Test the radio! Make sure it works correctly as a CB, plug in an SWR meter and dummy load. Check the radio produces round about 4W of TX power and mark this position on the SWR meter. If your meter is the type with a sensitivity control then set it to read full scale.
STEP 2) 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 the 1st, 2nd and 8th from the left only. Link number 3 contains a capacitor(4th from left), do not remove it. The rightmost pad pair
isn't used hence the X. On most boards the links are labelled, but some board revisions are completely blank.
| | . C . . . | .
2 1 4 3 8 7 6 5 X
STEP 3) The CPU link pattern is known as the Stealth mod here at Normsweb.com
Thanks to Norm for some great information. Assuming the links are correctly set its time to test its working OK. Power the radio on in the normal
way, check that pressing F1 now displays UK/EU in the F6 Position. Press F6 and check that the UK/EU toggles correctly. The radio should work as a standard
STEP 4) Tune for TX power over the maxiumum bandwidth. Refer to the larger image above marked "PCB View". Start by removing the inductor slug from
T3 completely. This is not a nice thing to do, the designer added two filters here not one. My spectrum analyser indicates this has negligible effect on
the TX output, if you have the facilities to check look for the 2nd and 3rd harmonics of the transmitted frequency.
Optional LED Modification) Remove the lightbulbs from the front panel, replace them with 3mm white LEDs with 1k series resistors. This gives a white with a blue tinge to the display, I like the look but some people don't.
Power + F4 Multi Channel Mode
F1 + F6 10Khz Channel steps
Press M7 with FUN flashing 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 (thanks again to Norm)
VR1 = TX Power
VR301 = RX Meter
VR501 = Squelch Threshold
VR850 = FM Deviation
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