MICOR® High-Band RF Preamplifier - TLD8422
Conversion to 2 Meters

By Kevin K. Custer  W3KKC

Make 150.8 - 174 MHz. MICOR RF Preamp TLD8422 properly tune the two meter ham band.

The helical coils in the preamp are physically too short to tune the 2M ham band, even adding longer tuning screws may seem to make the preamp tune, however the circuit "Q" is lessened, and therefore selectivity will not be as good. This modification allows the original tuning screws to be used.

The preamp will properly tune the two meter ham band, Circuit Q is as original.

Remove the 8 screws holding the circuit board to the casting. Heat the connections that are close to the input and output connectors, with soldering gun or iron to release the board. Suck solder from these holes to allow reassembly. Clean excess solder from the connecting wires from the connectors to allow easy reassembly.

With a sharp pair of diagonal cutters, like the Radio Shack "nippy cutters", cut the coil form away to expose the open end of the helical resonator. Expose just enough of the coil to allow soldering.
Scab an additional 1¼ turn of approximately the same size wire onto L1, the input resonator. Scab an additional 1 turn of approximately the same size wire onto L2 the other resonator.

C1, a 2 pF cap that goes across L3 (the slug tuned coil), needs changed to a 4 pF as per the Motorola manual.

Reassemble the preamp, by making sure the wires from the connectors thread back through the pc board, replace screws at 8 locations and resolder the wires going to the two connectors.

Retune the preamp, as per the Motorola manual. You will enjoy the way the preamp tunes and works in the 2 Meter band!

While you are at it, you might want to consider having your 150.8 to 174 MHz receiver preselector redone to properly tune the 2 meter ham band. Here's a spectrum trace of the frequency response of a modified preselector.

If you are interested in this service, go here: Receiver preselector modification

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