Help! NC-46 Tuning Question
 

I've got a National NC-46 that I'm resurrecting. The alignment is waaay out
of wack, with the top ends of the tuning ranges as much as 25% higher in
frequency than they should be. Furthermore, when I increase the trimmer
caps on the RF coils the resonant frequency appears to go _up_ instead of
down (I even tacked on a fixed capacitance to verify my sanity). I've owned
and maintained newer tube radios, and built transistor rigs, so in theory I
should know what I'm doing, but this problem has me stumped.

Does any of this sound familiar? Are there any factory manuals available
for the NC-46 that detail the factory's opinion of the correct alignment
procedure? Where can I get one?

Thanks in advance.

-------------------------------------------
Tim Wescott, KG7LI

Tim Wescott wrote:

Does any of this sound familiar? Are there any factory manuals available
for the NC-46 that detail the factory's opinion of the correct alignment
procedure? Where can I get one?

Thanks in advance.

-------------------------------------------
Tim Wescott, KG7LI

Download at
http://bama.sbc.edu/national.htm

-Bill

Tim Wescott wrote:

I've got a National NC-46 that I'm resurrecting. The alignment is waaay out
of wack, with the top ends of the tuning ranges as much as 25% higher in
frequency than they should be.


You probably know this already, but it bears repeating:
Alignment is never waaay out of whack - unless someone with
a "golden screwdriver" was in there before you. Look for
a defective component in the circuit - open/shorted/changed
value caps come to mind.

You probably know this already, but it bears repeating:
Alignment is never waaay out of whack - unless someone with
a "golden screwdriver" was in there before you. Look for
a defective component in the circuit - open/shorted/changed
value caps come to mind.

The rig has obviously been worked on -- it had a silicon diode on the
rectifier, a new power supply cap of unknown vintage, and no glass in the
window. I originally suspected a golden screwdriver type because of that,
but the problem now is that it just _won't_ go where it's supposed to.

I was hoping that the symptom (the coils tune the _wrong_ direction with
capacity change) would ring a bell for someone; it looks like I'll actually
have to dig in and engage my brain on this problem.

Thanks

Tim Wescott wrote:
You probably know this already, but it bears repeating:
Alignment is never waaay out of whack - unless someone with
a "golden screwdriver" was in there before you. Look for
a defective component in the circuit - open/shorted/changed
value caps come to mind.


The rig has obviously been worked on -- it had a silicon diode on the
rectifier, a new power supply cap of unknown vintage, and no glass in the
window. I originally suspected a golden screwdriver type because of that,
but the problem now is that it just _won't_ go where it's supposed to.

I was hoping that the symptom (the coils tune the _wrong_ direction with
capacity change) would ring a bell for someone; it looks like I'll actually
have to dig in and engage my brain on this problem.

Thanks


Could this be a case of low-side versus high-side LO injection?

-Bill

"- - Bill - -" exray@coquidotnet wrote in message
...
Tim Wescott wrote:
-- snip --
I was hoping that the symptom (the coils tune the _wrong_ direction with
capacity change) would ring a bell for someone; it looks like I'll
actually
have to dig in and engage my brain on this problem.

Thanks


Could this be a case of low-side versus high-side LO injection?

-Bill


I wish it were, but its in the input circuit -- when the dial (in band 'C')
reads 3MHz the coil tunes at 4.5MHz, as verified both with a grid-dip
oscillator and with a signal generator/scope. At the low end of the range
it's not so bad.

Tim Wescott wrote:


I wish it were, but its in the input circuit -- when the dial (in band 'C')
reads 3MHz the coil tunes at 4.5MHz, as verified both with a grid-dip
oscillator and with a signal generator/scope. At the low end of the range
it's not so bad.


I just looked at the schematic...I gather that there is a "T1" for each
band although only one is shown?
Is it just that one band or do all of them misbehave?

-Bill

"- - Bill - -" exray@coquidotnet wrote in message
...
Tim Wescott wrote:


I wish it were, but its in the input circuit -- when the dial (in band
'C')
reads 3MHz the coil tunes at 4.5MHz, as verified both with a grid-dip
oscillator and with a signal generator/scope. At the low end of the
range
it's not so bad.


I just looked at the schematic...I gather that there is a "T1" for each
band although only one is shown?
Is it just that one band or do all of them misbehave?

-Bill


Yes, that's the arrangement -- I've checked the two lower bands ("D" and
"C") and they both have the problem. The other two don't, which is odd --
you'd expect the problems to be worse at the high-frequency end.

Tim Wescott wrote:


Yes, that's the arrangement -- I've checked the two lower bands ("D" and
"C") and they both have the problem. The other two don't, which is odd --
you'd expect the problems to be worse at the high-frequency end.


Hmmm...shorted turns comes to mind since those coils would have the most
turns. Whithout doing any major surgery could you try to add a few
turns as an experiment to see what the result might be?
Another guess might be moisture having impregnated the coil forms.
Often a good DVM can read a leakage resistance right across the cardboard.
Do you have the correct 6K8 in place (glass vs metal)

-BM