NC-300 crystal frequency drift

[k7mdo]

One of the more recent finds by a friend of mine was three (3) National NC-300's that were stored in a non-climate controlled climate for an unknown time but long enough!

All three were complete but one of them with the highest serial number stood out as the best one to tackle for immediate restoration.  I cleaned the chassis and found the set unmolested.  Ordered and installed complete capacitor kit and aligned it per the manual with great success.  It works like new and my friend is quite happy with it. but he wanted to have me test and align the other two so he could re-coup some of his money (hoped to end up with one for free!)

I cleaned them and then carefully brought them up with the variac method only to find both worked OK but with the usual variety of alignment issues.

Then I tried to align the sets per usual with my URM-25, O'scope and frequency counter.  This is where I ran into an issue with both as the IF of the 2215 KC crystal seemed to be much higher, in one at 2245 and the other 2265 KC.  In reading about the sets I found several people conjecture that the crystals drift up with age....  So having no other way to tackle the sets we removed the notch filter that contained the crystal of the "worst" offender.  Clipped one lead of the crystal and measured its frequency..... incredibly it was 2214.99 KC.  I can only guess that it is not all of the story.... I was going to replace it but not now.  I wonder if I am missing the point of the purported drift? OR, was the process of clipping out the crystal enough to "joggle" the crystal a bit and cause it to make better connection inside its case?   Or, is the "drift" a function of other issues related to the resonance of the circuit?

We are in the process of re-inserting the notch filter housing into the set for a second try at alignment but thought I would query the masses who may have experienced this before to see what the consensus is.

Thanks for any thoughts 73, Tom

[W6MQI]

Tom,
 When I aligned my NC-303 I got real lucky and found the crystal frequency was basically spot at 2294.5Khz (2295) kind of unbelievable. These crystals will drift in frequency so you need to adjust the I.F. frequency accordingly. I found my crystal freq. by taking a 1X scope prob connected it to my frequency counter, then I made a small multi turn coil connected to the scope prob which was used to sniff the crystal freq without any danger of loading it. Make sure your 2nd I.F. is spot on 80kc, then adjust your 1st I.F. accordingly. Say you crystal was 2298 then you would set the 1st I.F. to 2218, if your crystal was 2288 then set 1st I.F. to 2208. This how I was told to do it and it worked great my NC-303 sounds real good.

73, Dave

[k7mdo]

Yes Dave I see what you are doing with that method.

That will probably work and I will experiment with that thought.

Maybe my big question is this, when I temporarily removed the Crystal, it measured exactly per the book at 2215. Whereas before, in circuit, it exhibited 2265.

I don't have the set back together enough yet to retry the alignment but I will report the results.

I think the inactivity of the Crystal in its holder may have been cured by the rough removal when I "snipped" one lead for testing purposes.

Seems like once in a while I had to take apart ft-243 crystals and clean them with alcohol to get them to operate, particularly the war surplus ones.

Tom

[W6MQI]

I was going to say take apart the crystal and clean it, but if its like the crystal in the 303 that's not an option. Had to do this in a SX-28 which I understand is normal procedure when restoring those old girls.

Good luck 73

[w3jn]

You aren't the only one to have noticed this.  http://www.antiqueradios.com/forums/viewtopic.php?f=5&t=162708

I suspect that doing this in-circuit is causing a spurious response somewhere.

[k7mdo]

Well the other fellows description follows my experience except for the alignment. Aligning at the "new" frequency of 2265 resulted in the inability to achieve linear bandspread.  I could set the band's "high" side but the spread was off significantly.

In a week or so I will get back to the receiver for the next try after having now found it not to be the crystal's fault. I do agree that it is possible that corrosion of some internal contact could be a culprit.

Wife has me "vacationing" this week.... hmmmm.

[WA4WAX]

That brings something to mind:

Own an HRO?

Do not align the IF at 455 KHz!

Instead, find the resonance of the crystal filter.  Seldom will it be 455 KHz.  Mine is 456.2 KHz.

Align the IF to the crsytal filter reosnance frequency.

Some people miss this trick........

[k7mdo]

Well, here is some more information that someone might find helpful.  I removed the "actual" crystal from the NC-300 and measured it directly and found it to be 2214.99 kc, close enough to the published "2215", not bad and certainly not "drifted" up or down.  So, replaced it back in the circuit only to find exactly the same problem. The peak is back up where it started, 2240.

Somewhere above I mentioned I am working on two of these sets simultaneously and one measures 2240, the other 2265 or so....

I read the link above and agree with that poster's assessment except that further alignment does not give me satisfactory band spread. 

I have some time to further investigate this during the week but at this time none of it makes sense in trying to salvage these two radios and it seems others have had similar problems but with better alignment results.

73 and thanks, Tom

[k7mdo]

Well, this is starting to get to me!

In summary a friend acquired 3 NC-300's which were all complete but a long time in storage.  I went through them and they were all untouched so I selected the one with the highest serial number to work on first.

I re-capped it and aligned it per the zeroxed manual with great success.  In addition the IF crystal frequency, when swept with the URM-25, came in exactly at 2215 KC (per manual).  All went well and my friend is happy to have a fully functional radio out of the three units he purchased.

Now, to the next set which had an early (1955 era) manufacturing date but the "best" chassis from a corrosion standpoint.  

This is when the trouble started for me.  I didn't re-cap it as it came to life to some extent when brought up on a variac. But did of course check the tubes, etc.  Then I attempted to align the set and immediately found the prescribed 2215 KC IF to sweep at 2240 KC.  Ie drifted up.  Then I checked to the 3rd set and its IF turned in 2265 KC.  I researched it a bit on the internet and found the references to "crystal drift" and at least one poster suggested to just use the new frequency and align the set.... that is what I tried but there was a hitch.  There appeared to not be enough "band spread" to align the RF section... I could set the RF generator at the prescribed 4.000 MC and adjust the C-71 variable for maximum output, then adjust the URM-25 to 3.800 MC and try to adjust the "padder" cap to achieve the peak "s" meter reading at that end of the 80 meter band.... can't do it.  The peak when I looked for it by tuning around is 35 KC higher than the prescribed 3.800 MC.. no adjustment will get that section to achieve the 0.200 MC spread.  

At this point I decided to "look into" the box containing the 2215 crystal to see if the crystal itself had changed frequency.... what I found was NO. I clipped one leg of the crystal loose and it checked at 2214.99 KC... couldn't ask for more. soldered the leg back into the simple circuit and replaced it in the radio.... tried again to sweep it and it came back in at 2240 KC with the same problem, not enough spread for alignment.

I don't see how the circuit is dragging that crystal frequency up, there is no obvious way to align the set with that IF that far off that I can find.

Any ideas would be appreciated.

73, Tom

 

[KC2TAU]

I would recommend re-capping both sets before you proceed with the alignment. Considering how the re-capped first set aligned properly and the other two have not been re-capped I would eliminate that first as a variable.

Be sure that the holder has been cleaned with high percentage isopropyl alcohol.

A good suggestion mentioned by w3jn in the link provided to the Antique Radio Forums:

"If you hooked the sig gen and counter directly across the crystal, you're loading it down to 25 ohms. A more valid measurement would be lightly coupling the generator to the crystal thru a 5 pF or so cap, and looking for the peak on a scope with a 10 meg probe."

If the crystal measures correctly out of circuit but changes when in circuit then something is loading it down. If, after replacement of all capacitors and all out of tolerance resistors, the crystal peak still does not rest at 2215khz I would suspect perhaps a failing capacitor in one of the previous stage IF transformers.

[k7mdo]

OK on the ideas.... and thanks. 

One thing that still glares to me is that in all the previous threads on the subject no one "solved" the issue, they simply tuned at the new frequency.... I just can't get to that stage and would really like to find the culprit as this is apparently a pretty common problem in the NC-xx receivers.

Maybe the best answer would be to the question of "... does a crystal 'really' drift and then stay at a new frequency?...."  30-60 KC is quite a drift from a physics standpoint of the crystal design from my experience.

The capping thing would be nice but until I am sure the sets will align, I am hesitant to put more money into them.

Thanks again, Tom, 73

[KC2TAU]

But if a leaky capacitor is loading down the circuit and preventing the crystal filter from being aligned properly...?

[WD5JKO]

A possibility is a fixed frequency DDS crystal replacement. The Progrock from QRP-Labs is a tiny module that has three outputs. One could be the 1st conversion oscillator replacement, and the other two could be for LSB/USB select in the BFO.

Interfacing to a tube rig would require a power source, and a LPF to make a sine wave from a square wave (they sell those too for about 5 bucks). Then the oscillator tube would need to be converted into a class A amplifier.
    

   http://qrp-labs.com/progrock.html
ProgRock - triple GPS-disciplined programmable crystal  $18.00
This kit is a simple minimalist controller for the Si5351A Synth kit (included with the ProgRock kit). It is intended as a programmable crystal replacement. It has three independent outputs with frequency range 3.5kHz to approx 300MHz, and can be optionally GPS disciplined.

Jim
Wd5JKO

[Jim, W5JO]

Saw this once before.  The little output coupling cap in the crystal filter was shorted.

[w3jn]

Jim, the crystal in question is the filter, not for the LO.  The second LO in the NC-300 is a L/C circuit.

[Jim, W5JO]

Jim, the crystal in question is the filter, not for the LO.  The second LO in the NC-300 is a L/C circuit.

So you are saying that the crystal in the crystal filter is directly connected to the 1st. IF after the filter?  Last I saw, there is a small, about 3 pf cap between the grid of the first IF and the innards of the filter.  Am I wrong about that?

[Jim, W5JO]

Jim, the crystal in question is the filter, not for the LO.  The second LO in the NC-300 is a L/C circuit.

Downloaded the diagram, the 3 pf cap labeled C 20 
but I would take a long look at L4, there is a 39 pf cap in that thing, maybe the coil??? 

[w3jn]

Sorry JO, I was referring to WD5JKO's post proposing a DDS.