Hi Raoul,
Nice to see you are still at it. You will be better off with the version 2 predriver. 95 Watts on 15m is a good sign. If you have not seen this yet, here is a discussion of the V2 board from the Drakemod paper that may be of interest.
The problem with the later model board is the bias network on the
MRF476. Its bias level is such that the transistor will go into
thermal runaway or may latch up by itself. The 270 ohm resistor from
base to ground is not enough to prevent this. The 300 ohm resistor
and 1N4005 diode is an acceptable method of providing bias, but with
the grounded emitter, there is no way to guarantee thermal stability
around the transistor. You'll notice this if all of a sudden the
transmitter output drops or, on the lower bands, the ALC is gone and
more mic gain is required. You let up on the mic for a few minutes
and all is well. If you were to feel the heatsink on the MRF476, it
will be very, very hot. It may also be possible that the predriver
board 'eats' MRF476s. You find its bad - usually leaky and low gain
- replace it, and soon the new one dies an inglorious death also.
The cure is to lift the emitter off ground with a resistor. Make a
tight bundle of 3 - 1.8 ohm 1/8 watt resistors in parallel. Cut the
emitter lead of the MRF476 about where the lead changes width. Re-
move the stub from the circuit board and put this resistor network
between the emitter and where the the stub went into the circuit
board. Removing the stub can be interesting for its soldered on both
sides of the board.
Yes, raising the emitter will decrease the gain. The degenerative
feedback also makes the MRF476 easier to drive, so the net result is
a wash. This one change for this specific board type is highly re-
commended, especially if you're having problems with MRF476 longev-
ity.
73
-Jim
NU0C
On Wed, 20 Mar 2024 08:40:30 -0700
"Raoul Meunier" <rmeunier@...> wrote:
Good morning Jim,
It's been a long time since your last message... My apologies for taking so long to answer you; lots of home distractions here...
Trying to obtain more power in 10 and 15 meters, I modified the Pre-Driver Version 1 according to the information in DL7MAJ paper ("Full output on 10m").
The result was good but not enough: 25 Watts on 10 and 95 Watts on 15 meters.?I ended up pulling the Pre-Driver Version 1 and installing a Version 2 one.
I'll keep on trying to find what's wrong with the Version 1 unit later on...
Best 73 de Raoul CE3RY
--
73
-Jim
NU0C
Jim Shorney