-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Predator 400 amplifier
- Thread starter DR456
- Start date
Shadetree Mechanic
Delaware Base Station 808
Billy's ghost appears again.
As in Billy D. Ward. Pretty sure this was his idea in the old Gray 300 amplifiers.
Textbook name for this is a "gimmick" capacitor. Wrapping an insulated wire around another one gets you a small-value capacitor.
The intention is to neutralize the amplifier, and prevent it from oscillating.
Barely works.
Better solution would have been to put a negative feedback network across each transistor. A 100 ohm 2 Watt resistor in series with a 0.1uf ceramic disc capacitor is placed with one end of the series pair on the collector of each transistor, the other end on the base lead.
Works for Texas Star.
Never did find out why Billy didn't apply such a simple solution.
Good chance the transistors are MRF455. They have a lot of gain, and this makes them likely to oscillate.
Once the NFB network is in place, one on each transistor, you can unwrap the blue wire and put a short piece of wire directly from the center pin of the coax socket to the circuit board.
You should place a SWR meter and coax jumper between the radio and the radio socket on the amplifier after doing this. Key the radio with the amplifier operating, and read the SWR between the radio and the amplifier.
If it's much over 2 to 1, this could give the radio a headache. Won't try to cover how you would remedy this problem for now. And if it reads below 2 to 1, it's not worth improving.
73
As in Billy D. Ward. Pretty sure this was his idea in the old Gray 300 amplifiers.
Textbook name for this is a "gimmick" capacitor. Wrapping an insulated wire around another one gets you a small-value capacitor.
The intention is to neutralize the amplifier, and prevent it from oscillating.
Barely works.
Better solution would have been to put a negative feedback network across each transistor. A 100 ohm 2 Watt resistor in series with a 0.1uf ceramic disc capacitor is placed with one end of the series pair on the collector of each transistor, the other end on the base lead.
Works for Texas Star.
Never did find out why Billy didn't apply such a simple solution.
Good chance the transistors are MRF455. They have a lot of gain, and this makes them likely to oscillate.
Once the NFB network is in place, one on each transistor, you can unwrap the blue wire and put a short piece of wire directly from the center pin of the coax socket to the circuit board.
You should place a SWR meter and coax jumper between the radio and the radio socket on the amplifier after doing this. Key the radio with the amplifier operating, and read the SWR between the radio and the amplifier.
If it's much over 2 to 1, this could give the radio a headache. Won't try to cover how you would remedy this problem for now. And if it reads below 2 to 1, it's not worth improving.
73
Last edited:
