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Well, not sure if this is the right group or not, but we will find out.

Working on restoring an NCX-500 (very similar to the National 200, or NCX-200) transceiver. I am located very close to a 50kw AM broadcast station on 1030 Khz and have noticed that these rigs are subject to front end overload in the receiver. I found it necessary to put a high-pass filter inline with the antenna to eliminate an abundance of birdies and other stuff.

Something to be aware of...

Bob,? K7DYB

Robert,

It's the right group, just hardly anyone here. Not a lot of people who
have National radios have found this group yet.

I started it just before the rules changed for basic groups here.
Otherwise, any new group after that change would incur a fee after
membership reached 100. I created some for the more popular vintage
radio brands and this is one of them.

Whether it grows to become useful in time we will have to wait and
see. The Icom group grew pretty fast, and the antenna tuners group is
growing at an even faster rate. But the vintage groups aren't building
up steam yet.

Donald -- AD8DY
Formerly KJ3I

On Mon, 26 Apr 2021 15:49:38 -0700, "Robert Kembel"
<rwkembel@...> wrote:

Well, not sure if this is the right group or not, but we will find out.

Working on restoring an NCX-500 (very similar to the National 200, or NCX-200) transceiver. I am located very close to a 50kw AM broadcast station on 1030 Khz and have noticed that these rigs are subject to front end overload in the receiver. I found it necessary to put a high-pass filter inline with the antenna to eliminate an abundance of birdies and other stuff.

Something to be aware of...

Bob,? K7DYB

----------------------------------------------------
Some ham radio groups you may be interested in:
/g/ICOM /g/Ham-Antennas
/g/HamRadioHelp /g/Baofeng
/g/CHIRP

Hi All,

Here is another note on the above transceivers (applies to both models).

I noticed that the received signal seem "gravelly" or "raspy". This seemed to be worse on strong signals and initially I thought that it may be an overload issue. Upon further investigation, I noted that the BFO frequency was being pulled by the product detector. The plate of the BFO tube is connected to the cathode of the product detector through a 120pf capacitor. As the signal at the product detector varied, the impedance of the cathode circuit varied and changed the loading on the plate of the BFO. This was very noticeable when monitoring the plate circuit of the BFO oscillator.

I added a 100pf cap in series with the existing 120pf cap that is between the BFO plate and product detector cathode to reduce the interaction. This appeared to clear up the problem with the BFO frequency changing as a result of the received signal. I did not yet try a different BFO oscillator tube to see if it might be a tube issue.

I know these are not common rigs, but I wanted to document my experiences with others. I will post some additional notes as time permits...

Bob,? K7DYB