[Central-Electronics] 10B harmonic problem

[Jim candela]

	Art,

   I don't have any 10B's, but there is a schematic and write up of the 10a
exciter in the W6SAI Radio Handbook, 1970 edition, pages 387-388-389. It
talks about two traps on the plate of the Mixer tube (6BA7) to "reduce
unwanted responses of the mixer which are apparent when the unit is
operating in the 14 Mhz band". There is a series L-C (C23 and L7) on the
mixer screen grid, and a parallel L-C (C25 and L6) lightly coupled through
C24 (3pf) to the mixer plate.

You mentioned trying L6 to little effect. Did you also try L6? Is the VFO
1625 plate tank tuned to resonance?

FWIW on one 20A of mine I did away with these traps, and raised the 'Q' of
the Mixer tank to not only raise the mixer gain, but to reject those spurs
without the need for traps. On the 20A there is a parallel resistor with the
Mixer tank that is 10K on some and 22k on others. The rigs with 22K also
have a ceramic bypass capacitor at the B+ side of the mixer tank whereas the
20a's with 10K have a Mica bypass capacitor. If you combine the higher 22K
with the Mica bypass capacitor, the Mixer plate tank 'Q' goes way up along
with the gain. The Mixer tuning gets pretty touchy however. It looks like
the 10a in the schematic has the parallel resistor as R32 (no value given)
and the bypass capacitor is C28 (.006uf).

Hope this helps,
Jim
WD5JKO

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of Arthur
Delibert
Sent: Thursday, October 12, 2006 9:30 PM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] 10B harmonic problem


Hoping somebody can help me with this problem, which I didn't see anywhere
in the archives.

I am a new owner of a 10B -- actually 2 of them, and they both have the same
symptom.  When I use the 458 VFO on 20 meters, there is a very strong
harmonic at 3 times the VFO freqency, or about 15,500 MHz.  The manual says
to expect this, and to tune L6 for a minimum output on the harmonic.  It
says the harmonic should be only a few percent of the 20M signal.

I have a harmonic that's as strong as the 20M signal, and tuning L6 through
its whole range does almost nothing to eliminate it.

Using a grid dip meter on L6, I can see some dip at about the right
frequency.

I'm using the same VFO and the same 20M coil on both of my 10Bs.

Is my 458 VFO hyperactive, and generating a much bigger harmonic than it
should?  Have the components in the 15 MHz trap drifted in a way that the
nulling action is in the right frequency range but isn't very prominent?
Something else?

Any help or thoughts appreciated.

Art  KB3 FJO


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