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Author Topic: NC-300 repair info wanted  (Read 929 times)
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N1KK
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« on: January 08, 2012, 09:22:28 AM »

I just picked up a non working NC-300 receiver. Would there be any sites
that might off some repair tips or Elmer type help repairing it.

As received, all the tubes light up with a very low hum at speaker.
The S-Meter is all the way to the right and does not move.

I have not done any repair work in a long time and could use a little
help or tips from someone who might have some repair experience
with this receiver.  I have always wanted a NC-300/303  receiver
and this came across at the right price.  I just downloaded the operating
manual.

thanks
Ken
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KM1H
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« Reply #1 on: January 08, 2012, 09:34:58 PM »

Thats a pretty easy receiver to work on; I have 2 in the shop now.

Besides replacing electrolytics, paper caps and checking for out of tolerance resistors, its usually just cleaning tube pins & sockets by reseating several times, cleaning switches and pots if noisy and check alignment/performance. May need a few tubes.

Carl
KM1H
National Radio 1963-69



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N1KK
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« Reply #2 on: January 11, 2012, 08:41:23 AM »

Thank you for replying.

I have had some success.  I found 2 bad tubes using a simple
EMISSIONS tube tester.  The HF Osc. tube and one of the I.F. amp
tubes.
I had a 6AH6 to replace the HF Osc.  But I did not have a 6JB6 for
one of the I.F. amps.  One of the two 6JB6 tubes used in the 2 I.F.
amps checked out GOOD, no Grid Leakage or Shorts.  The other
6JB6 shows GOOD Emissions but BAD on Grid Leakage and SHORTS.
If I tap the tube it would show GOOD on Grid Leakage and no Shorts.
So I thought the tube might have dirty contacts and I cleaned them.
No change. 

The things I found so far,   The Accessory socket near the back of the Chassis
uses rivets to mount it to the chassis. I was probing around with a insulated
tool and when it hit the area I got a low level hum.  The grounding  of the socket
which has a couple wires connected was bad.  So now I have low level hum
all the time. Continuing my probe I tapped around the 12AT7 and I noticed
a cap connected to pin 1 with the other side not connected but it look
like it was touching the chassis.  I moved it and the audio can on really
loud. I moved the tube and the audio would come and go so I cleaned
the socket and now I have good audio all the time with no hum.

The receiver looks like it has a couple of coupling caps that have been
changed. 

Where I am right now.  I thought the receiver was working. In fact
on 160 80 and 40m it appears to be as sensitive as my IC-735.
Now for the strange part.  After it warms up, if I change from 80m
to 160 or 80 to 40 then back to 80m the receiver is some what deaf.
I can hear stuff very low in the receiver and tuning the dial has
no effect on what I hear.  If back on 80m, power the receiver off
for 2 or 3 seconds, power back up it works again on 80m
This I discovered yesterday and have not had a chance to do anything with
it.
If the receiver has been off for a while, turned on it works fines. Its only
after it warms up do I loose 80m and I think 20m too. If I leave it
on 80m it continues to work fine.  Its only if I change bands
and back to 80m does it go deaf.  I am wondering if the HF Osc
circuit has a out of tolerance resistor and when I turn power
off and on it shocks it circuit into working???

Any thoughts?  Would you like to take this off line for discussion?
Thanks
Ken
N1KK

Thats a pretty easy receiver to work on; I have 2 in the shop now.

Besides replacing electrolytics, paper caps and checking for out of tolerance resistors, its usually just cleaning tube pins & sockets by reseating several times, cleaning switches and pots if noisy and check alignment/performance. May need a few tubes.

Carl
KM1H
National Radio 1963-69




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KM1H
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« Reply #3 on: January 11, 2012, 12:28:13 PM »

Reply to your PM on the way.
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N8ETQ
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« Reply #4 on: January 11, 2012, 02:46:15 PM »

Hey Ken,

     Just thought I would add a comment here about that "Damn" switch
on the AF gain pot. It's explained in the manual but nothing on the
front panel about it, I always forget about it and sometimes think
my 300 has crapped out. It removes the  RF gain pot from the
RF amp. Nice idea but they could have put something on the Front
panel about it for those of us who can't remember what day it is!
Carl will get it straightened out. it's a Great RX.

GL es 73

/Dan
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N1KK
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« Reply #5 on: February 20, 2012, 08:05:11 AM »

I got a second NC-300.  Real clean.  Was dead but after cleaning
all switches its working.   Like my other NC-300 the wheel drum
for the frequency string is broke.  The other one had only one side broke,
the left when looking at the picture in the manual. I just replaced
the left side with the exact measurements as in the diagram and I was
back in business.

This new NC-300 has the strings on both sides of the chain broke.
I have the chain, spring, and triangle hook.  What confuses me
is the chain measurements.  The chain measures 6 1/2 inches
where the manual states 6 inches (just tiny balls)  Where the manual (just below diagram)
states 6 1/2 inches (guessing end to end ?) this chain measures 7 3/8 inches end to end.

The manual states CORD LENGTH = 20 inches.  With a total of 39 19/32
how do you get (I assume left and right side of chain) to be 20 inches?
If I do the math and assume half of the stated 6 inches of chain on each
side I get 16 7/32 plus (half of 6 inches =3) = 19 7/32 inches for the left
side.   39 19/32 (total per diagram) - 19 7/32 = 20 3/8 for the right side.


Are both strings suppose to be equal or not?  Just using the numbers
in the manual I get 16 7/32 + 6 = 22 7/32,   39 19/32 - 22 7/32 = 17 3/8
Left side = 16 7/32 Right side 17 3/8

Anyone know the scoop on what the measurements should be?

Thanks
Ken
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