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Old July 17th 03, 05:29 PM
Chris
 
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Default National NCX-5 transmit/receive offset problem

With Norm's posting from last January, I know I am not
the only one experiencing a strange transmit/receive
frequency offset problem with the old National NCX-5
transceiver.

http://groups.google.com/groups?hl=e...cgocable.n et

He contacted me and we have together worked on the issue
from many angles. The first concern was the receiver
offset tuning circuit. That checks out ok. And the
VFO looks ok too. Now he has found a document which
seems to imply this is something in the 1st or 2nd
transmit mixer.

I'm wondering if this is a problem with the old mark I
rigs when they were converted to a mark II. It seems
odd that the factory would ship a rig with an offset
problem. Maybe some key component has gone bad with
age.

The instruction manual has a single paragraph about checking
for this condition, but it has no instruction about
what to do if the condition is found!

So far, I am working around the problem by using the
RIT control to syncronize the VFO between T/R
while listening to an external shortwave receiver
lightly coupled to the VFO.

Chris Howard
Estes Park, CO

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Old July 19th 03, 02:57 AM
Chris
 
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Here are the things I have tried:

1) examined the RIT circuit to make sure it was not
pulling the VFO

2) replaced the zener diode in the VFO power input with
a three-legged voltage regulator. (my schematic says
something like +6.8 but the measured voltage was about
double of that. I believe I ended up putting a 12 volt
regulator in there.)

3) replaced the 10pf capacitor between pin 1 of V-3 and
ground. (this had the most positive effect)

Chris
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Old July 19th 03, 02:57 AM
Chris
 
Posts: n/a
Default

Here are the things I have tried:

1) examined the RIT circuit to make sure it was not
pulling the VFO

2) replaced the zener diode in the VFO power input with
a three-legged voltage regulator. (my schematic says
something like +6.8 but the measured voltage was about
double of that. I believe I ended up putting a 12 volt
regulator in there.)

3) replaced the 10pf capacitor between pin 1 of V-3 and
ground. (this had the most positive effect)

Chris


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