[HBR] help on local oscillator

[Walt Hutchens]

Bill said:
> I seem to be getting a really raspy l.o. signal.

Ripple on your B+ is unlikely to produce what you'd call a 'raspy'
signal.  You may get hum (60 or 120 cps, depending on half or
full-wave rectification), you might also get FM, most detectable when
listening to a CW signal or (better yet) by listening to an LO
harmonic using the BFO of very stable receiver.

This 'listen to the harmonic' test is always a good idea for a new
receiver, and ten times more important if you want undistorted SSB
reception.

In circuits that have a 'hot' cathode, you can get serious hum or FM
via heater-cathode leakage but not 'raspy' -- and this circuit has a
grounded cathode.

A VHF parasitic oscillation (as suggested by Ian) is a definite
possibility. With a modern tube I'd put money there; the 6J5 isn't so
likely because the gain is so much lower, especially at VHF. They were
used in early WW-II VHF equipment at frequencies of over 100 Mcs,
however, so it can happen, particularly if the tank circuit leads are
long in your design.

Another possible culprit is a fixed capacitor across the tank circuit
for padding purposes. While this circuit doesn't show one, almost all
ham applications will have such a cap. This LOOKS like a low voltage
application, but it's not: Even a receiver LO can develop in the
ballpark of 100 volts p-p across the tank and the usual modern 50 wv
ceramic cap will break down in a manner that will cause raspiness.

This cap should be rated 250 WV-up.  Yes, lower ratings will
sometimes work.  And sometimes, not.

Also, this cap must be able to carry substantial current, even for a
6J5. For best stability, use the physically largest silver mica you
have available. Still better, use two or three (roughly equal) caps in
parallel.

C1 (the grid coupling cap) can also have a voltage breakdown problem
but is less likely to do so, because it handles much less power, hence
less heating to reduce ability to handle high voltage.

Check the grid resistor, R1. If the product of C1 (grid coupling cap)
and R1 (grid resistor) is too high (100 mmf/47k specified) you have
the possibility of a blocking oscillation as in the simplest types of
superregenerative detector circuit. That'll DEFINITELY sound 'raspy.'

A bad R1 (carbon composition, with poor contact between the lead and
the resistance element) could be arcing a bit and this would certainly
cause raspiness.

Substitute another 6J5.  A slightly gassy tube might possibly cause
something like this.

That's all I can think of.

Walt Hutchens
KJ4KV