[AMRadio] RE: Another receiver update...

[Brett Gazdzinski]

Hello all AM'ers.
I have been working some more bugs out of the home brew receiver.

I put the bigger dial onto a shorter front panel, because
the other dial was just too small.
I marked the frequencies on it, only to find them off
after a while.

I did some checks and the thing was drifting about 10kc
or more from a cold start.

So I did some tests....I removed the power from the built in power supply
with its hot dropping resistors and VR tube, and powered
it up with the heathkit adjustable voltage power supply remotely.
I still had the drift.

I figured the lo tube (6j5) might have something to do with it,
so I replaced it with a spare.
Interesting results!
The warm receiver with the cold 6j5 went back to
its original frequency, then, after the tube warmed up
a bit, the frequency went up quickly, almost a jump.
The original tube did this also....

I have a glass 6j5 and tried that....no jump!
Anyone got an idea why a metal tube would act this way?


So now the warm up drift is about 2kc, and I have ordered
some NPO caps from mouser to replace the silver mica caps
in the LO.
Perhaps that will cut the drift even more.


I also played around with muting.
I changed things so the receiver still works when I transmit,
just the audio output is grounded, and the mixer input is shorted
to ground through a reed relay.
This way, I get an IF output for the O scope on both RX and TX.

Besides the remaining drift, it works very well.

Brett
N2DTS