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Author Topic: NC300 IF Selector Switch Question  (Read 2782 times)
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w9bea
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« on: January 05, 2020, 07:01:13 PM »

Hi All:

I have an NC300 that I have replaced all the wax paper caps in it along with three weak tubes and some other repairs.  I needed to drop the front panel off slightly to get at a couple of them.  The knob is in the lower left corner of the chassis, and the knob is offset with a cog-like wheel.

I am getting no audio out of the radio and think that something got messed up in the IF stage.  I buzzed the input of the AF stage with my audio generator and that is fine.  I measured the voltages on the 6BE6 (V8) and found them all out of whack.  Pin 1 was calling for -8 volts and I saw 165 volts..... Yikes.  Then the voltage went away and the pin displayed -10 volts.  and I do not know why.

I loosened up the knob  and hand rotated the shaft that drives the three rotary wafers.  Then I replaced the outer knob/shaft assembly.  The numbers on the knob do not match up with the positions of the switches, and I am guessing that this is the major factor in no audio to the AF stage.

Anyone ever had the front panel of the NC300 off the chassis, and know what the reassembly procedure is?

I assume that the wiper is supposed to make contact with one contact at a time and not strattle two contacts?

I would love to talk to someone who really knows the NC300 radios well.

73--Wally W9BEA
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Wally Klinger W9BEA
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« Reply #1 on: January 14, 2020, 10:07:06 AM »

Haven't seen an update on your troubles on the -300. 

I am using one daily that I resurrected a couple of years ago. 

No, the switch does not straddle two terminals from my recollection. 

If I or someone took a photo from underneath after sliding the set partly out of the cabinet, would that help?

Any progress by you so far?

Tom
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W2JBL
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« Reply #2 on: January 17, 2020, 09:57:58 AM »

The wafers on the selectivity switch only make contact with one terminal at a time. You have the switch out of sync with the gears\front panel. I had a customer bring me a dead NC-300 years ago he tried to repair himself. The guy forgot to mention to me that he messed with the selectivity switch and it gave me fits for hours beforeI figured it out. In the end the real problem was a shorted screen bypass in one IF stage.
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KK4YY
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Your best isn't as good as you can be.


« Reply #3 on: January 17, 2020, 12:06:57 PM »

A new component is not guaranteed to be a good one. I like to check them before soldering them in. You might have a shorted coupling cap... it wouldn't be the first time a new part was a bad one.

Other than that, give the radio a close look over. Something might have been moved when you worked on it, and shorted-out somewhere. Point-to-point wiring is susceptible to having leaded components short to each other when disturbed. This is what spaghetti tubing is made to prevent — but it's not always used.


Don
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All your worries won't add a day to your life, or make the ones you have any happier.
w9bea
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« Reply #4 on: January 20, 2020, 12:47:06 PM »

Hi All:

Well I found two problems.   First, a wire came off of the cathode of V3.  I found this while I was probing around.  Some of the outer insulation was spiked to the pin on the tube socket, giving it an illusion that the wire was connected.  I cleaned the pin and re-soldered the wire.

Also I loosened the IF switch knob and reached in with my fingers to rotate the inner toothed cog or wheel.   Currently I have the three IF positions working.  It took me about 30 minutes of loosen/move/tighten to get the alignment right.  The polarized shaft that joins the three wafers together in sync seems  to be fine.

I let the receiver cook on the bench for about 2 hours.  Then I followed the IF and RF alignment procedures.  I was a tad scared to wiggle the wires inside of the coil forms so I left those alone.  All I did was to set my generator as per the manual and peak the S-Meter on each band on the two caps (one small bread slicer type and one compression cap with the screw adjustment).  I picked up some signal, not a lot.  Except on 11 and 10 meters I picked up a few S-units.

The S-Meter adjustment was the last thing I did.  There is a high(er) wattage variable resistor in the bottom of the chassis that you zero the S-meter with.  It is a tad noisy, but I set the S-Meter zero as best as I could and let it cook for a bit.

I am a bit disappointed in the receivers audio quality on loud signals.  I wonder if I jacked up the IF alignment, perhaps narrowing the bandwidth of the IF?   I understand that it is better to align an IF stage with a sweep generator rather than a regular RF source like my URM25D?  I do not own a sweep generator (yet) nor do I know how a sweep gen is used to properly align the IFs.

Also, when you switch to CW mode, the S-Meter pins hard to the right and the audio goes into distortion.  I have to use the RF gain control to make the audio palatable.

Your thoughts?

73--Wally W9BEA
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Wally Klinger W9BEA
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« Reply #5 on: January 20, 2020, 10:50:45 PM »

Sorry wrong post.



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W2JBL
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« Reply #6 on: January 27, 2020, 10:25:39 AM »

      On CW the NC-300 disables the AGC. You have to ride the RF gain control. If you want AGC on CW use the SSB position. There are mods out there to increase BFO injection and clean up the audio on SSB. Also one for the AGC time constant (slow realese for SSB).  I have done them and they help a lot.

     Both the NC-300's I had in the 70's and the one I have today overload and distort quite easily on AM and suffer cross modulation form nearby signals. I detune the antenna trimmer, which helps a lot, and also run the RF gain back. make sure you have the switch on the volume control in the right position. The "wrong" one runs the RF stage wide open with no gain control. 

    You can easily add inverse feedback to the audio chain. I did that and fattened up the coupling caps, removed the cap accross the output transformer primary and the radio sounds really good. Also- jump out R-47 (grid of first audio amp, and remove C-57 (connects to R-47) and you get better highs. 
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