Wednesday, December 14, 2016

FAA Amplfier AM-6154

A few years ago I bought some FAA amplifiers to repair and put on the air with the intention of placing one in service at a repeater site and using one on my TS-700A.  This is the first amplifier of the project.  There are many sites and pages posted on the web for modifying these amplifiers.  Most of that information will not be restated here.
This was the first amp that was put on the air.  It required only a few repairs to use it on 2 meter FM.  The first thing I do when working on an amplifier is disconnect it from the power line and short the plate supply to ground.  The metering resistors (R1 &R2) in the H.V. supply were changed to 3 watt non inductive 2%.  See the 2nd and 3rd photos.  Then the bias was modified to allow full cut-off at idle and correct drive when keyed.  The filter and directional coupler were removed since they will not take the power created on the ham bands.  Nothing else was changed in the cavity.  Not making any mods in the grid circuit of the cavity will make input tuning very sharp which is what I wanted for repeater duty.  If garbage cannot make it through the input it can't be on the output. 
The next change was to add the keying relay.

The wiring was done through the rear connector and D connector (J4) for the grid supply. I chose this method since the FAA had a zero idle current mod. for the amp that used the same wiring in the D connector and key input on the exciter connector.  I did not use the ECG-123A and photocoupler circuit.  I added a vacuum relay mounted to the back panel that holds bias at cut-off when not keyed and allows full plate current when keyed.  Reverse polarity is protected by a diode and the relay contacts protected by a 10k resistor mounted on the grid supply.  Thermal protection for the cavity originally is provided by power from the exciter.  I mounted a separate DC supply to power the circuits for cooling.


 This view is the high voltage supply showing the 2, 10 ohm resistors (R1 & R2) that get replaced.  They are the 2, 1/2 watt resistors in the lower left corner of the circuit board.  Be careful to safety the capacitor before doing any work.  Also note the strapping of the TB.  It should be set to the line voltage you intend to use to power the amp.  The TB on the grid supply also needs to be strapped to the correct line voltage. The close-up shows the new resistors installed.
 The cover of the H.V. supply must be removed and the board removed from the stand-offs to change the resistors.  The power supply does not need removed.  I jumper the capacitor to ground as soon as I lift the cover and before starting any work.  
This is a close-up view of the new resistors.  I used ambient light for this so it is a bit dim.
The original Eimac 8930 tubes are getting quite scarce and expensive I made a reducing ring to use 4CX250B tubes.
Here is the cavity converted to use a 4CX250B.