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I wanted a little amp, with a bias circuit already in it,
and to be driven by my Cobra 148.
I checked my pile of broken amps that were given to me.
(I got that RM Italy amp working, but no bias, sucks in SSB!)
Took a look at that DYNAMO amp, and recognized the board
as the same as a PALOMAR ELITE 250. I know the circuit and
layout well, since I just worked on some of them.
Did a few quick tests on this amp, and it kind of works,
with some problems, so thought I would bring this
DYNAMO back to life.
1. - Forced it to key up, and the bias is about 5 amps!
So changed the big bias resistor to 100 ohm 2 watts.
Now the bias is about 1 amp, both MRF455 transistors get a little
warm after a while, so I assume they are both drawing current.
.
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2. - Receive signal cuts out sometimes, after messing with receive
pre-amp switch and banging on the pre-amp relay. There is no need
for a receive pre-amp. Get rid of this! Cut the wires at the circuit
board to the pre-amp. Then with a piece of wire, bottom side of board,
bypass the receive signal path through that relay.
.
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3. - and next, the bad input circuit, common in all these PALOMAR
type amps. the LOW-HI-MED switching of resistors, and on some
amps they have a variable resistor, which is usually burned out.
then some guys put in a 1k pot, and it burns out quickly.
but when switching at this input circuit, it changes the input impedance
of the amp, causing the driver radios' output circuit to change impedance,
possibly causing damage. (I use an old tube Teaberry T when testing and
driving these amps at first. Don't want to blow out any 2SC1969's!)
Get rid of this also! Cut the wires at the board from the input level switch.
Cut the resistors off the board. From the past, I know putting around 27 ohms
in series with the input will give an OK match. So the 27 ohm resistor
that was cut out, was soldered in series with this capacitor that was lifted, that
goes to the RF input transformer.
.
.
.
Did some more tests, and the amp is alive!
So moved it over to the station bench, and connected the
Cobra 148 "Dee" (China made) radio to it. Keyed up in AM mode,
and got 45 watts of carrier out. (but bad downward modulation)
The internal SWR meter on the Cobra shows no reflected power,
and of course, 3.8 Watts forward power. The input impedance of the
amp is looking good. Cobra radio is a happy camper!
Then went to SSB mode, whistled into the mic, and hit just over
125 watts. I have 120 watts PEP output or so. will do
a proper power tests later on. The SSB auto key up circuit works.
so went on the air into an antenna. LSB ch 16, 36, 37, 40
talked all over, (from Louisiana) to California, up into New York,
New Hamshire, Canada. They all said I sounded clean.
If you have any of these, make them work!
.
now that it works, got some "fine tuning" to do.
since it is already there, will wire up the Receive pre-amp switch
to be manual key-up. (to check and adjust the bias.)
and the pre-amp LED will be a keyed-up LED indicator.
And possibly make a regulated and adjustable bias circuit.
For some "bling" the RF output LED, in the middle, replaced
with a super bright white LED with a circuit so that is varies
some with modulation.
The amp input needs more attenuation. so that any 3.8 watt
radio can drive it. With SSB you just keep the mic gain down lower.
But I then connected a RCI 2970 type radio to this amp.
in AM mode, adjusted the carrier for about 25 watts out of this amp,
and the AM was great on the scope. A little upswing on the meter.
hopefully all this helps somebody.
now, can I have a beer! anybody got a beer!
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