[Central-Electronics] 10B harmonic problem

[George Maier]

Hello Art: 

Glad to hear more people running 10B's.  There were a few factory changes in
the mixer circuit that you may or may not have in your unit.

1) There should be a 180 Ohm resistor from the cathode of the mixer to
ground, and it should have a .005 uF cap (5000 pf) across it. If the
resistor is there, but not the cap, add a .005 ceramic disc.  
2) The 13 MHz trap, consisting of C27A (3 pf) coupling cap, and C27B (25 pf)
which is across L7 (yellow coil), was moved from screen grid circuit of the
mixer to the plate circuit.  

The factory notice for these mods talks about improved linearity, but I
suspect that it has some effect on the way the 13 & 15 MHz traps tune.  
I've done this to my 10B, but I can't remember what the ultimate rejection
is at either frequency, only that the traps seemed to tune as they should.
In addition to the items that Jim (WD5JKO) mentioned, you might consider
replacing C25, C25A, C27A, and C27B with brand new silver mica's; I also did
this as just as a precaution.  If the old caps are a bit leaky, the Q of the
trap could be down.

Thanks to Kenn Grimes, K4XL, the details of this and all other factory
changes are available on BAMA (http://bama.sbc.edu/); just download the 10B
djvu file.  

73 & Good luck!

George - W1LSB





-----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 10:30 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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