[Boatanchors] National HRO-7 IF/Xtal Filter Alignment

Thu Jan 4 11:17:46 EST 2018

Bill,

It is possible that someone installed a crystal element that was well away from the intended frequency.  Pull the crystal from the crystal filter and hook the hot lead from your signal generator to one pin and the input from your scope (or a good RF voltmeter) to the other pin and clip the ground leads from the generator and scope together.  Set the signal generator for unmodulated output at a high level and then slowly sweep from around 438 up and you will get a sharp response at the crystal’s resonant frequency.

If it turns out that someone did use a 438 Khz. crystal to substitute you can continue to use it.  The difference from the intended IF is not great enough to severely impact calibration or more importantly tracking (how well the HFO tracks with the tuned circuits for the input, RF interstage, and mixer input circuits).  But I would still try to find a proper replacement at 455 Khz. If the crystal in your set is not correct.

One sometimes overlooked source for the now harder to find 455 khz. Crystals is the FT-241 series marked in channels instead of frequency.  Channel 46 in the 2 digit series and channel 327 in the 3 digit series are right on 455 Khz. and one channel marking either side of this is still far closer than what you have now.  The commonly available 455 khz. ceramic resonators don’t provide the high Q of the crystal element and are not a good substitute but better than nothing for those completely missing a crystal.  I would rather have an off frequency element like you may have rather than a ceramic element.

It is also likely the crystal element itself needs cleaning regardless of what frequency it is on.  The procedure is in the earlier HRO manuals but I can’t remember if it is in the 7 manual.  Basically you carefully disassemble the crystal holder, clean the element itself and the contact pressure points with the highest purity isopropyl (i.e. “rubbing”) alcohol your store sells, then wipe them dry with another clean paper towel and reassemble being careful not to touch these parts with your fingers or the finger oil will undo much of what you just did. I like to do a before after test with the signal generator and scope across the element to assess the effectiveness of the cleaning.  Depending upon the holder it likely has a little ceramic or porcelain “gasket” between the two sides and this is easily cracked.  If cracked you can repair it with generic “super glue” however as you can see from the need for the cleaning there isn’t an airtight seal anyway and a crack is of little consequence.

Rodger WQ9E

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________________________________
From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of Bill Stewart <cwopr at embarqmail.com>
Sent: Thursday, January 4, 2018 9:25:51 AM
To: boatanchors
Subject: [Boatanchors] National HRO-7 IF/Xtal Filter Alignment

GM All and a Happy New Year,
I have been working on and playing with a HRO-7. It works well, but the selectivity seems to be off a tad.
Little or no attenuation on opposite side of zero-beat. The IF is spec'ed to be 456+/-2kc. I hooked up
the sig. gen., as specified in the manual, and at first did not get a signal from the spkr. I tuned the SG
until I found a sig., from the rcvr, at a SG freq around 438kc. Before I start turning screws, is this off-freq
condx a sign of miss-alignment and hopefully can be adjusted back to 456kc or does this indicate the xtal is
way off its intended freq. If that's the case, can the IF be re-tuned to 438kc and still work properly, including
proper xtal filter action, etc.

Tnx for any hints, suggestions etc.....73 de Bill K4JYS

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