Low Output (was Sunday)

ashhar_farhan <farhan@...>

--- In BITX20@..., "david.g3ryp" <david.g3ryp@n...>
I wonder how much output everyone's getting from the IRF510?

I can get 2.5 watts rf maximum but this drops to well under a watt
with the lpf in circuit. I've checked the lpf cut-off and I would
expect some attenuation but not that much. I tried an irf530 which
promptly overheated and destroyed itself as I increased the bias!

I'm hopeless when it comes to getting rf out of solid state stuff!

hehe, david worry not. there are a couple of things that you ought
to check.
1) cut the supply to the IRF510. then measure the RF output of the
driver at 50 ohms resistors across the RIF510's gate. it should show
5 volts peak (that is 10 volts between the +ve and -ve swing). If
you not are getting that much, then you have probably wired the
transformer incorrectly. they are probably in parallel to each
other, or worse, the
drain and the power supply are connected to the separate ends of the
winding. check it carefully.

2) if you are not getting 5 volts at the 50 ohms resistor across the
IRF510, then measure the output from the predriver. it should be a
more than one volt peak (2 volts peak to peak on the 'scope). if
either of these is a problem. check the following:

a)that the collectors are showing somewhere north of 10 volts and
the emitters are showing about 2 volts on both the driver and the

b)if the DC voltages are alright, suspect that the AC connections
are a problem. Most probably it is a mis-wired transformer. take
them out of the circuit and check them carefully. i frequently make
a mistake with shorting the opposite ends of the SAME wire and
leaving the other wire in the pair isolated.d

c) the 6.3 ohms, in my case is approximated by paralleling two 10
ohms. check that the 10 ohms IS 10 ohms and not 100 ohms. it is easy
to confuse brown-black-brown with brown-black-black.

above all, trace! measure the outputs at the RF fitler, pre-driver
and the PA. tell us how much you get at each point. we will 'divide
and conquer'.

i dont know if i mentioned in the article that you should set the
drain current to between 50 and 80mAs. I set it at 80mA, a bit on
the higher side, but the linearity is better.

- farhan