Direct FM of an FM828 or FM814 A band radio
Please Note: conduct any modifications to these radios with
care. The author of this page will take no responsability for any damage
caused by use of this or other information provided.
Modification of FM828 or FM814.
Step 1 tune the radio up in the normal way in accordance with the manual.
Step 2 remove power from the radio to be modified.
Applying the modification to Osciallator channel 1:
Step 3 Remove C3 33pF , R6 1.5k
Components on the underside of the exiter PCA
Step 4 fit the varicap with both anodes soldered to the wide earth
trace adjacent to C3
Step 5 Fit the 560pF ceramic capacitor from the cathode of the varicap
to the pad of C3
Step 6 Fit the 100k resistor to the non earth side of C8. This
allows pin 36 to be used as a connection point on the top side of the PCA
Components on the top side of the exciter PCA
Step 7 Fit the 6.2k resistor to the outside of the VCO shield near
to pin 4
Step 8 fit a 10 turn 5k trim pot with the CCW end connected to the
6.2k resistor. If nessecary glue the trimpot to the PCA with a drop
of super glue.
Step 9 run a wire from the CW end of the trim pot to pin 8 at the
other end of the PCA (make sure this +10V supply is always turned
on)
Step 10 Connect the 10k resistor from the wiper of the trimpot to pin 36
Step 11 connect one side of the 10uF bipolar electrolytic capacitor to pin
36. Connect the other side to a nearby feedthrough if using an FM814
with the Exciter shield. If not attach a suitable piece of wire.
Step12 Install a peak limiter PCA into the radio (details of a suitable
circuiut will appear else where on this page at a later date) Connect the
output of this PCA to the 10uF electrolytic.
Testing
Step 13 With no audio power up the radio
Step 14 Set the voltage on the wiper of the pot to 8.00V DC
Step 15 Key the transceiver up into a dummy load with a frequency counter,
and set the Slug L1 for correct frequency.
Step 16 Apply modulation and set the deviation control on the output of
the Audio PCA
27/02/2003