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October 2nd 03, 02:29 AM

In article , (John
Ely) writes:

I have a BC-342 receiver that has (mostly) been converted using an old
Surplus Conversion Manual. I am a little worried about this method
after tracing out the wiring (workmanship is pretty bad) and making
some voltage measurements.

I've got a Farnsworth (!) BC-342-N that has had those mods. Works pretty good
for being 60 years old.

The conversion raised the screen voltage on both 6K7 RF amplifiers by
lowering the high side of the screen grid voltage divider from 40K to
20K ohms and lowering the cathode resistors. This results in screen
to cathode voltages of about 160 VDC. The tube manual lists maximum
screen voltage at 125 VDC.

Don't worry about it.

Using 33K in place of the recommended 20K lowers the screen to a
little over 130 VDC.

Am I crazy or isn't the original screen configuration better than the
conversion?

The original design sacrificed gain and noise figure for strong signal
performance. Remember that any military receiver had a high probability of
being in close proximity to several operating transmitters.

Also, any opinions on trying to regulate the screen
voltage (to ground) with a 125 volt zener diode? I know zeners are
sometimes used in special circuits to produce broadband noise and
don't want to introduce noise on the sensitive screen grids of the RF
amplifiers.

Not needed!

Finally, the conversion removes the first RF amplifier from the manual
gain control. Is there a better method (cheap way to delay gain
reduction). Perhaps diode with the anode at the high end of the gain
control and the cathode to the cathode of the first RF amp?

I'd put it back on the RF gain control.

Any thoughts?

First thing to do is download a manual - the kg7bz site has 'em for free
download, I think.

Next, decide how "original" you want the rx to be. The 342 is a pretty
conventional design, and you can do lots of mods, or just restore it to
original condition. They are good receivers in some ways (solid, geared dial,
etc.) but there's only so much that can be done without doing radical things
like tearing out the IF cans and replacing the 6K7s with more modern types.

Some folks like to use separate RF and AF gain pots. Some like to install an
external S meter or tuning eye, often using the front panel connector (which
can be replaced by an octal socket fairly easily). Some like to redo the xtal
filter, which wasn;t too hot in the first place.

I find mine is best as a general-purpose general-coverage receiver with minimal
mods.

73 de Jim, N2EY

check out the stages one at a time, starting at the last audio and working
forward.

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