[TenTec] 1220 "birdies" revisited

[Bob & Linda McGraw K4TAX]

Guys:

A couple of things to check.  Are the birdies stable in frequency?  or
do they "move around".  Are they there with a dummy load and move or go
away when the load is removed.  If so, the RF amp is most likely
oscillating. If the birdies are stable (crystal like) look at the math. 
The IF freq plus or minus the received freq. equals injection freq.  Run
a few derrivities of the number and see if they produce one or more of
the birdies.  That might be a clue as to an idler circuit not correctly
adjusted.

I'd find it hard to believe that a single conversion RX converter would
have so many whistles unless something is being overdriven such as the
oscllator or the osc is mis aligned and is parasitic or the RF amp is
mis aligned and is acting like a oscillator.  Maybe a ferrite bead was
omitted from a lead on a transistor in the assembly process.

TIP: I have a small bead in which I carefully cut in half lengthwise and
glued each pice to the tips of a pair of tweezers.  Now I can reach in,
clamp the bead around a transistor lead with closed tweezers and observe
the result.  You'd be surprised how many times the noise floor goes down
as a result of an oscillation not heard but affecting noise performance
of a low noise receivng system.

I wish I had a 1220 on the bench, I'd find the problem for you guys.

73
Bob K4TAX

wb5jnc@juno.com wrote:
> 
> Greetings,
> 
> After seeing Don WA4IML's post ("This is interesting. My 1220 has the
> following birdies:") I went back through and listed everything that broke
> the squelch with a dummy load connected. Here's my list with approximate
> strengths (squelch set to approximately 11:00; just quiets on blank
> frequency):
> 
> Frequency       Approximate strength
> 143.240         weak (i.e., barely breaks squelch)
> 143.350         strong (squelch at 3:00 to quiet)
> 143.600         weak, choppy
> 144.000         strong -- probably 4.00 crystal harmonic
> 144.050         strong
> 145.350         strong
> 145.395 or .400?        strong
> 146.050         weak
> 147.350         moderate
> 148.000         strong -- probably 4.00 crystal harmonic
> 148.050         strong
> 
> Other notes: frequency coverage 142.995 - 149.000; ROM rev. unknown
> 
> Any thoughts, guys?
> 
> 73, Al
> 
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