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Author Topic: NC-300's again  (Read 3934 times)
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k7mdo
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« on: February 27, 2017, 09:05:51 PM »

I  am finally back on the hunt for the real reason two of the NC-300 IF crystals are "off" frequency by up to 15%.  

However, in the meantime the third receiver unit that I thought was resurrected has failed and the owner has dropped it by for a little "look see".

I was in his shack listening to some 75 meter AM when all of a sudden some CW overlaid the phone portion of the band.  I have not seen this before but thought someone on the forum might have a good guess before I start looking through the sack full of capacitors and such that might cause something like this.

It is as if there has been a band "fold back" of the CW area right on top of the AM region.

Anyone seen such a thing?

73, Tom

All of the tubes have been checked and most replaced with another tube with no improvement.

 

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MikeKE0ZUinkcmo
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« Reply #1 on: February 27, 2017, 10:39:09 PM »

Have you hooked it up to a generator yet to see if the dial calibration is OK?

As for the crystals, have you put them in an oscillator to see what frequency they oscillate at?   If they oscillate at or very near their marked frequency, the problem is elsewhere.

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Mike KE0ZU

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k7mdo
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« Reply #2 on: February 27, 2017, 11:31:28 PM »

Mike, yes, I took one of the crystals out and tested it. It came up nearly exactly as marked.  I, too, suspect one of the associated capacitors or resistors in the circuit....  that is the next project.

The third set though had no operational issues until about a month after I recapped and aligned it.  Tomorrow I will go through the alignment again and see which IF is the problem with the band compression or fold back or.....?

I've been on vacation for some time and am just getting back to these three sets.  I thought I had one all fixed until this problem.  When it was working it was a pretty decent receiver. I'll figure it out.

Tom, 73
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MikeKE0ZUinkcmo
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« Reply #3 on: February 28, 2017, 07:04:43 AM »

Don't talk yourself into thinking the problem is this or that, I've chased my tail more than once with that mind set.   

I bought a known DOA 300 about fifteen years ago, and the problem turned out to be a corroded wire in an IF can.   Easily found problem but took an act of congress to get the can out to fix it.

Good luck!!

Mike
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Mike KE0ZU

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k7mdo
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« Reply #4 on: February 28, 2017, 03:38:32 PM »

The "good" NC-300 ended up on my bench this morning and I immediately went through a complete calibration routine to see if I could find a problem.... none found... now wondering if for some reason it is getting "shorted" somehow when in the case.  I have never had it in it's case, only outside on the bench.  My friend always has kept the case at his house... just wondering.  I'll run it on the bench for a few days and go from there.

The NC-300 is sure a great receiver, I have an NC-303 integrated into my AM station and like it a lot but I like the phasing control on the -300 better than the "Q multiplier" used on the -303.... but that isn't a deal breaker.

Now I am going to re-tackle the crystal filter issue from an earlier post on the two sets I have on the shelf that aren't anywhere close to 2215 KC in the crystal filter circuit.... the hold up has just been the difficulty of getting into the circuit components inside the small box where the crystal is hidden.... but, no other issues are pressing and it is winter here in Oregon.

73, Tom
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MikeKE0ZUinkcmo
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« Reply #5 on: February 28, 2017, 04:57:12 PM »

Interesting, I have a 303 as well.   For AM fone I much prefer the 303, but for CW I like the 300 better.   Its surprising how much of an impact, subtle design differences make.
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Mike KE0ZU

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k7mdo
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« Reply #6 on: March 02, 2017, 06:56:15 PM »

Well I am stuck again on the recently re-capped -300.  Initially it aligned and operated very well, then all of a sudden the set started exhibiting issues....  worst one is that now it has garbled (slightly) audio in both AM and SSB and it seems to get worse the longer it is on, as if heat may be an issue.

I have traded out all tubes, one at a time with no success.  I spent today with the scope looking for anything I could identify as a cause.  I put in a nice clean RF signal from the URM-25 and can't see the problem in the waveform though I do see at maximum gain and volume, a flat topping of the sine wave.  but just slightly backing off and I am back to clean signal. BUT, the audio seems to slowly get more garbled as the set warms up.

I do notice that the audio output transformer windings are quite loose from the laminations (jiggly loose) but wiggling the windings does not seem to affect the garble on the audio.  I suppose it could be the problem but?

Anyone with an idea?  One thought I had as I was typing this rant would be to chill spray the capacitors one at a time, etc. throughout the set while listening to it, will try that tomorrow.

73, Tom
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N8ETQ
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Mort


« Reply #7 on: March 02, 2017, 07:18:25 PM »




   Hey Tom,


         Just a thought, there is a switch on the
back of the AF GAIN knob that will disconnect the
AGC from the 1st RF stage.  It could be stuck, I
always forget about that thing.. It only switches
in either the extreme CW or extreme CCW rotation,
unlike the AC power switch on the rear of the RF
GAIN pot...

GL

/Dan


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MikeKE0ZUinkcmo
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« Reply #8 on: March 02, 2017, 07:49:59 PM »

Since you say it happens on both AM and sideband, my first inclination would be to look at the audio amplifier by applying a 1kc audio tone to the top of the volume control and with a distortion meter, measure the distortion at the speaker terminals.   This would be the reference for your other distortion measurements.   Remember to measure the distortion at the output of your audio signal source FIRST, because you'll subtract this from the rest of your measurements, to get the net distortion.

Next I'd do the same basic measurement with a 100 micro-volt or so 95% modulated signal injected into the grid of the first stage of the low freq I.F. amplifier.   This puts the variable band pass circuits of the low IF, as well as both the detector and noise limiter circuitry in the picture so you can use the distortion meter to measure what their individual contributions to distortion are.   As a side note, noise limiters often add distortion to receive audio.

 
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Mike KE0ZU

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k7mdo
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« Reply #9 on: March 02, 2017, 10:49:15 PM »

I'm going to concentrate on the audio section but my distortion analyzer happens to be my ears....  Will report any success.

Thanks for the thoughts, Tom
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k7mdo
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« Reply #10 on: March 05, 2017, 08:04:46 PM »

Eureka!  Found the problem with two of the -300's.  Everyone probably knows this but it turned into news to me.

It turned out, after tracing every circuit in the set numerous times, that it was simply a single tube.  The V-8 CWO 6BE6.  I have so many of these tubes that I had just gabbed a couple and tried them as I substituted tubes trying to see if it was just a tube.

All of my 6BE6's tested healthy and strong on the checker but in service, as it turned out only two of about 10 actually worked properly in the radio!

The first indication was healthy audio volume levels on AM but lower audio and garbled audio on SSB.

When, finally, a good tube was inserted, the audio came up in volume and the clarity immediately improved.  In fact the tubes that worked in that location were 6BE6W's or I believe the mil spec was 5750. 

Now I can focus on the third set which has a badly out of tolerance IF crystal filter that I have mentioned before on a previous thread.  Still working on that one.

73, Tom   
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MikeKE0ZUinkcmo
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« Reply #11 on: March 05, 2017, 10:50:27 PM »

Good news!!!   Glad to hear of victory.   Now #3 should seem less intimidating, and, with the exception of the crystal, I'd suggest wholesale parts replacement within the assembly.   Fortunately, with only 3 connections you can test it outside of the receiver.

Mike
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Mike KE0ZU

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