FPM-300 Mods
Jim Zellmer, KA0VSL

Editor's Note: Rig pictured is not actual rig restored

I was going through a manual for a FPM-300 and found an old copy of a list of mods for the FPM-300. The list originated from a letter a communications engineer who worked at the Hallicrafters Commercial Products Division. 

The list reads as follows:

"Here are mods I felt worth putting into the FPM-300, in order of their importance..."

1. ALC 

On the bottom of pre-selector board add parallel resistors to R518 and R516. Shunt R518 with a 8.2K watt and R516 with a 22K watt.

2. HUM 

a. Make up a shield between the VFO and PS boards. Cement a piece of aluminum foil to underside of fiber spacer. Make aluminum to cover just inside screw holes. Do not attempt to ground the foil, it is just a static shield to isolate the switching transistors from the audio circuits. 

b. Solder in on back of VFO board a 15 pf (disc or mica) between the base of Q407 and the junction of R442 and R441. Trace board wiring and use spot closest to Q407.

c. Change C437 from 0.1 to .01 mfd. 

d. Open up on end of C432 and put 4.7K watt resistor in series. (This is optional, I don't always do it). 


I solder in short #24 bare wire jumpers on the bottom of the preselector board to join the front and read lands of the switch wafers. This is tricky so be careful or you can mess up the works. (14 jumpers on preselector board) 

The same can be done on VFO board switches, but these do not usually cause trouble. 

4. AGC 

A faster time constant can be obtained by shunting R511 with 10K watt. The same applies to R204. This seems to smooth out the audio due to better AGC attack., ie., less overload to the detector.

5. MISC 

Locate nuts on spade lugs used to attach PA cage to chassis. Invariably these are loose and should be tightened. If dial drags on panel, bend back chassis lip or add shims under top edge of spacers. If VFO is jumpy, clean all wipers on drive and capacitors. In addition, clean gear teeth with some cleaner. I use a TV combination cleaner/lube called "Blue Stuff" by Tech Spray. 

6. ADDENDUM to 2A. 

In 2A it is recommended that the aluminum foil be covered with a piece of card material cemented in place. The use of Glass tape (3M) can also be used to insulate the file. 

The engineer closes his letter with these comments, "These little rigs can really do a fine job when set up properly. Very few ever got out of the factory quite right." 

I hope that some of you folks with FPM-300s find this list of interest. As with any modifications, use these at your own risk. 

Jim Zellmer, KA0VSL

This page last updated 07 Apr 2001.