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Crystal off Frequency

The tip of your probe has internal capacitance to its ground clip. This will tend to pull the frequency of a crystal downwards, as if you had soldered a 10pf (or so) disc cap to the same spot in the circuit. Textbook name for this is 'circuit loading'.

That's why moving the ground connection changes your reading. The ground clip of the probe is in series with the capacitance of the tip. Since capacitors in series reduce the total capacitance, leaving the ground clip loose reduces the "loading" capacitance of the probe tip on the crystal. Not all counters are sensitive enough to give you a stable reading this way.

The "floating" reading you got will be the most accurate. Looks as if you're chasing a problem that was caused by your measurement method, not by a fault in the radio.

This is what caused us to stop using frequency counters for setting crystal frequencies. A communications service monitor has both an accurate frequency reference and a receiver input. The probe tip becomes an antenna and only has to "sniff" the signal from alongside a crystal without touching any part of the circuit. The trimmer gets set to zero beat with the monitor's setting.

But a counter is more accessible than a service monitor. Cheaper, too. They work best when you can probe to a point in the circuit that's "downstream" from the crystal itself. Some PLL chips like the MC145106 found in many RCI-made radios have a pin that puts out half the 10.24 crystal's frequency. This pin is not connected to the crystal itself, and the probe won't disturb the crystal if you use that pin to make this check. No trick to double the frequency error you see on the 5.12 MHz reading you'll get.

And if your counter is sensitive enough to show a stable reading with the ground clip loose, that's the simplest solution to the loading problem.

73
 
Greetings!

You are splitting hairs with those readings...keep it floating from ground if you can - it may radiate but it radiates something they can tune out versus grounds that may generate more problems due to multiple points sourcing of noises (at lest it can be a single source of noise)

Ok, to spare the reader a long winded reason listed below, you may need to "ground" ; test and tune; release from ground and check RF frequency results - if not - repeat the previous step - because to get the Freq counter to MEASURE right - review the test jig setup in the service manual - and realize that you have to leave the Xtal mess floating for reasons beyond your control...but tune and check to get the counter to register it - then release ground to float - test? - repeat until you have success...this process let's you obtain a known frequency that is not on spec, but when all is removed and radio brought to normal operation to show the device is in tune - I've done this more often than not, due to loading and unforeseen events the radio getting tested places on your test bench equipment...This is a SINGLE POINT GROUND thru FL3

The solder wire is just a means of tying all the metal cases of the Xtals together physically to keep a ringing effect from occurring as you use, operate or transport on a rough road - vibration - at least you don't want the salts to lose their tune and resonate at another frequency if it gets bounced around a bit.

Tying off is just a method to keep (insert working class metaphor here) them tied together and working together as a physical unit.

A Xtal case is not always SHIELD - it's simply the metal case - what you do with it makes a difference in radios - soldering the case to other xtals helps keep them rigid and stable - also can provide physical support also electrical as well as thermal support and predictable characteristics.

If they weren't tied together you may hear a ringing effect from the freely vibrating Xtal slightly out of tune from the others. Hence the metal case does not mean it's grounded - you make it so when you tie off the metal by soldering it to either foil ground or another part that provides electrical, physical and mechanical support, like tank cans or other xtals.

A secondary effect of not tying the crystals to ground versus shielding (using ground) is a eddy current and noise injection - multiple point grounding may provide a low resistive path for DC values, but serves as a means of noises from the external to interfere and even resonate - as in cause a separate image signal based upon an errant xtal or multi xtals generating a BFO only much lower in frequency affecting and using the boards' foil ground as an antenna radiator...due to all the xtals are not exactly on the same resonate frequency adding a performance hit to the radio let alone the ability to be shielded from it.

There is an interconnection both capacitance and inductive in traces an effect making the section REQURE it to be small compact and simple to keep reactive effects to a minimum.

You already know the radios IF of 10.7 and 10.240 leaves an image of 450/455/460 kHz - all three of those signals propagate inside the radio and its chassis case grounds - and you have the SSB / AM and FM issues of frequency tuning - you don't want too many mixing artifacts floating around in that radio as you try to listen in...

So, if you have an RF probe you can use as a sniffer, test to see a radiative effect the unshielded has, versus grounded to chassis - there is a noticeable difference in performance and operation if IF leakage becomes the issue.


Welcome to the world of "single reference timing" limited by the "Multiple point attachments to real world devices" and "Initiatives used by Quality control of substrates in a multi gang component series of systems operated by humans" - gives your results with "Unlimited possibilities"

Makes for a seminar if not an ongoing blog...

The Xtal seems low in Hz, but when replacing it - keep some NPO caps (as suggested earlier) around to help you make the Xtal resonate by tweaking. You may not have a choice unless you live next to an Xtal factory - or make your own. Each xtal interacts with what is there so what is stamped on the cans' case may not mean you get those results in the box you're putting it in. Xtals that resonate spot on frequency - and don't need tweaking are ideal, I do understand the issue of adding capacitance or even inductance is asking for more drift and you'd rather spend your time taking on the radio versus fixing it or trying to keep it in tune..

... but again real-world events add color to your day...

Welcome to the world of RCI/Galaxy - a Universal Enigma...

Regards!
:+> Andy <+:
 
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For anybody having problems finding reference crystals, you can get 10.240 crystals out of Hong Kong for just $1.19 for 10 of them (10.240 in Ebay’s search). The batch I got was all active and so far they all tweaked on frequency without any problems. Some people call them “half height” crystals or the newer HC-49S size, but don’t be concerned about their size. It doesn’t matter.
 
For anybody having problems finding reference crystals, you can get 10.240 crystals out of Hong Kong for just $1.19 for 10 of them (10.240 in Ebay’s search). The batch I got was all active and so far they all tweaked on frequency without any problems. Some people call them “half height” crystals or the newer HC-49S size, but don’t be concerned about their size. It doesn’t matter.
Just ordered 10 xtals for ~$1. Can always use more. For a buck, it is worth the risk. You say they work? Thnx for the tip!
 
Just ordered 10 xtals for ~$1. Can always use more. For a buck, it is worth the risk. You say they work? Thnx for the tip!
Removing these out of scrap radios was getting old, and like you I was curious about the performance of these. I doubt you will experience any problem with them. The batch I ordered from that source is several years old now. If I remember correctly it took about 2 weeks for them to arrive. After they arrived I checked them immediately to see that they were all active. No dead ones or too far off frequency ones were found. I’ve used 3 or 4 of them so far with no stability problems. In some ways it doesn’t surprise me that you can get these at such a low price. The 10.240 reference is used in so many other applications today besides CB PLL’s. Mass quantities are being produced today in China to cover that need. It’s unfortunate that the U.S. isn’t doing the same. I’d gladly pay a higher price from a U.S. manufacture, but paying $20- $35 for NOS or a custom made one is ridiculous.
 
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I sure learned a lot!! Always fun fixing something that's not broke:rolleyes:

I saw those crystals on ebay and wondered if the could handle the current?? since the factory crystals are rated at 1mw and most of the other crystals were rated at 500uw or less?? any info on the specs or is it irrelevant??

Since the FL3 wire is not technically a ground were is the best place to pickup a ground since the chassis is not always a true ground??

Thanks for all the help
Plumber-G
 
Greetings!

I sure learned a lot!! Always fun fixing something that's not broke:rolleyes:

I saw those crystals on ebay and wondered if the could handle the current?? since the factory crystals are rated at 1mw and most of the other crystals were rated at 500uw or less?? any info on the specs or is it irrelevant??

Only if your nose has developed a nasty case of Tendonitis and the doctor has recommended that you not longer use a Nellie Pot to prevent forest fires...

...Continuing to do so may require them to amputate, starting from the neck up...

Refer to ACA and Label On Bottle that Reads: Failure to follow the label directions is a Felony Offense and is punishable by law - up to and including the Death Penalty - SEE: ACA

You raise a good point:

How much can a Crystal - vibrate - before the circuit it's used in - loads the crystal and dampens, Changes, Alters, the RESONATE frequency...
(The IF the Woodchuck could chuck wood...paradox...)

Now you know why FL3 is the only point of ground for it.

Too much of one reactance is not good for the other...for to cancel out one the other must be used; each one is a hit on performance.

So, the wire "thingy" they do, can help in offsetting the capacitance thru an inductance...thereby reactance becomes more resistive...but a lot of things have to line-up before you begin the alignment process...

Due to the nature of Xtals, they have gone thru several design changes including their size of chip to make the frequency. Older units had the larger salts with larger retainers - today the "salts" are tiny grains and are getting tinier as each revision shows it works - and are even more fragile than the previous ones. And this reduction in size affect their ability to be powered and resonate let alone even produce a signal that can be amplified for oscillation.

So that's why the "loading" issue comes up - some are directly coupled to the uP chip they are the timing reference for - but the uP drives heavy loads like automotive ignition and lighting - so they can't tolerate a large load on their leads - they simply stop working as crystals and act more like a small capacitor - which - in the application - accomplishes nothing...
the FL3 wire is not technically a ground were is the best place to pickup a ground since the chassis is not always a true ground??

Thanks for all the help
Plumber-G

I've always kept the ground wire AWAY from the FL3 mess, I clipped to a Can by it or used the CASE itself but you can also refer to many points of (grounded) interest along the way.
It's best to maintain ORIGINAL Alignment done by Factory as much as possible - leaving all their work as points to focus on later if problems ever arose - at least the areas they (Factory) fixed first are to be treated as potential suspects.
A good way to remember is the Galaxy DX models are UNIVERSE-al - as in they use Chassis Grounds to the case that ties off to Foil board ground and Tank Cans can provide board grounds TOGETHER - Negative Ground Only - so if it works in the FORD but not the Subaru Brat (Aussie: Positive Ground Ignition) - Sue the Brat and remember that the other is often Found On Road Dead - (Take that MR. Ford CEO - you NAFTA dodger that simply will stop making cars in countries calling it "saving money" by un-employing those very ones that make your crap you say it takes away from their payouts and your own pocket...)

Regards!
:+> Andy <+:
 
I sure learned a lot!! Always fun fixing something that's not broke:rolleyes:

I saw those crystals on ebay and wondered if the could handle the current?? since the factory crystals are rated at 1mw and most of the other crystals were rated at 500uw or less?? any info on the specs or is it irrelevant??

Thanks for all the help
Plumber-G
To help answer your first question, In this case I’m not overly concerned about the maximum drive specification. Besides the crystal, other factors in the design of the oscillator determine the actual amount of drive used. The main objective of most oscillator designs is to apply the least amount of drive necessary to create a stable oscillation under varying conditions, such as temperature and voltage fluctuations. Designing anything higher past this ideal level will increase the crystals aging effect (drop in frequency over a shorter period of time) and possibly add unwanted spurs or noise. Considering the engineers design objective, the ideal amount of drive is usually far below the maximum the crystal will allow. Although it has been done with some success, I’m not talking about using a small crystal designed for a solid state oscillator and placing it in a much higher drive tube oscillator. Back in the early Johnson CB days, adapters were available so you could use walkie talkie crystals instead of the radios normal HC-6/U size. The huge FT-243 crystals (40’s, 50’s and early 60’s) haven’t been made for years. Finding them on the surplus market is not a problem, but finding the frequency you want is extremely difficult. Some nostalgic or restorative Hams are getting around this issue by simply replacing the wafer that’s inside an FT-243 case with a HC-18/U. I’ve made a few for my Heathkit DX-20 CW transmitter, so far so good, but I’m a little concerned about their longevity. I expect they will age fast or eventually destroy themselves under such stress. Buying a custom made HC-6/U would be best, except for their high cost, plus I wanted to keep the jack original. Not all modified FT-243 are acceptable. Reports about chirp, frequency drift etc abound with many others that are trying them on their particular rig. It only proves to me that these little guys will handle a lot, but you really cant expect them to act perfectly or last under those extreme conditions.
 
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The ongoing saga of my 2547 alignment, Having changed probes and grounding I have been able to bring the PLL alignment to spec except the USB frequency jumps around at least 1khz I can center on a frequency but it seems to go up and down, the IFT (L24) coil seems to make large random changes. LSB is rock steady to spec I am suspecting the IFT coil ??
Any thoughts or checks i can make

Thanks
Plumber-G
 
plumber-g, switch the USB and AM cans and see if the problem follows the can.
you can switch the VCO cans no problem as they are the same, and you can switch the USB and AM IF cans as they are the same, but the LSB IF can is a different number.

If the problem does follow the can, you can be pretty well sure the can is bad and replace it.

I have personally found bad cans in this area a few times in galaxy radios.
LC
 
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