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Crystal Problem




 
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Author Topic: Crystal Problem  (Read 3121 times)
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WD8KDG
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« on: December 14, 2013, 10:12:38 PM »

The little jewel arrived this afternoon and appears to be almost a dud. I ordered a crystal from ICM for my Johnson 500. The holder style is a HC17U and has the same pin spacing/diameter as a FT243.

So, the frequency desired is 7.294MHz. From what is in the manual, on 40 meters the Junkston operates on the fundamental, so I think a crystal cut for 7.294 fundamental should transmit on 7.294 with the 500?

Placing the 500 in the tune position, CW, and starting from scratch loading; with the key down the crystal starts to work, but around 7.2934MHz. If I let the key up and then key down,  the crystal quits & no RF outpoot.

What's a poor man to do in a big town? Got any ideas?HuhHuh??

No problems on 75 meters with Brian's  (AF4K) crystals. VFO works OK on 40.

Tnx,
Craig

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flintstone mop
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« Reply #1 on: December 15, 2013, 09:15:17 AM »

Usually the manufacturer will have the specs for the particular osc. circuit and design the crystal accordingly. Capacitance, loading, etc etc.
Do you know for sure that the crystal is designed to work in the Johnson 500? The crystal may be in the wrong mode and is giving the off-frequency operation.
The unit you have is either defective or not designed properly to work in the Johnson oscillator circuit.
Fred
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Fred KC4MOP
WD8KDG
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« Reply #2 on: December 15, 2013, 10:10:41 AM »

When I ordered the crystal from International Crystal; the first question they had was capacitance loading? The only thing I know, the TX used FT243 crystals and the TX is an EF Johnson Viking 500.

The sales person put me on hold and did some searching. Then came back on the line and said a HC17U has the same spacing/pin diameter, then put me on hold a second time. At last the sales person said they could make the crystal.

Invoice description of their crystal is EFJVIKING500  FC 7.294000    FX 7.294000

At this time, I'll call ICM Monday morning.

Tnx,
Craig

PS: So, what is this capacitance loading thingy? Sure ain't in the manual.
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Tom WA3KLR
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« Reply #3 on: December 15, 2013, 10:50:48 AM »

I looked at the Viking 500 schematic and see that the crystal oscillator screen grid is bypassed with a 0.005 uf cap. which is good.  The Viking II and some other transmitters had no cap there to ground ("electron-coupled oscillator") causing very little/unpredictable feedback and trouble with some crystals here that worked in my transistor test oscillator.  I added 10 pf to ground and that fixed my problem.  This is not your problem but I mention it for the benefit of others.

Have you tried other 7 MHz crystals?

Have you tried the crystal in another oscillator?

Have you tried the crystal in the other position?

Have you tried another 6CL6 or have a tube tester?

Check the screen grid resistor value and the low B+.


Crystal oscillator configurations are either series resonant or parallel resonant.  Most crystal oscillator circuits operate in the parallel resonant mode, as the 500 circuit does.   ICM presumed parallel resonant correctly and need a loading value, which is usually in the range of 10 to 32 pf.  The crystal is operating slightly off of its series resonant frequency in the inductive side of resonance (High side of series resonance).

The loading value is for frequency correlation between their test jig and your application circuit.  If your crystal turns out to be off frequency (after you solve the present problem) and you order from them again, lend it back for them to figure out your loading capacitance along with your very accurate measurement of the frequency.

GL OM.
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
WD8KDG
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« Reply #4 on: December 15, 2013, 11:05:44 AM »

Sad to say, don't have another 7 MHz crystal, Brian's 75 meter crystals work just fine!

No other oscillator sort of, could disconnect the Ranger from the Desk KW..........more pain

Yes, tried the other crystal slot, no joy

Could try another 6CL6 and I have a tube tester

Going to disconnect the RF deck this morning and get it belly side up n' check things.

I seem to remember some silver mica caps around the octal socket/crystal holder. They are not on my skizmatic? Have to get it belly side up for that look see. Having a grey beard and past the social security age is my excuse.

Tnx,
Craig

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W3RSW
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Rick & "Roosevelt"


« Reply #5 on: December 15, 2013, 02:09:04 PM »

A very definitive 6CL6 page. Has been referenced on AM fone before but still really neat to read again.
http://faculty.frostburg.edu/phys/latta/ee/6cl6xmtr/6cl6schematic.html#ecoscillator

Crystals, fundamental or overtone and needing Capacitors in series or in parallel references can be found in Wici or here:
http://www.bliley.net/XTAL/

Meant to add this," the company song."
Quote
Songs a La Departmentalle
Tune -"Home on the Range"
 Volume - QSA 5 - R9
 +++++++++++++++++++++++++++++++++++++++++ 
Calibrating

Oh give me an ohm, where the amperes roam,
 Where the volts and the decibels play.
 Where the rocks that you see,
 Are all called BC3,
 With their frequencies put there to stay.

Chorus:

Ohm, ohm in my cage,
 It was there .that I first learned to pray,
 When each LD2 was blown neatly through,
 And the quartz was all cloudy all day.

Slicing

Oh give me a room, way back in the gloom,
 Where the dust is knee deep on the floor,
 Where the quartz is in chunks,
 And it's cut by bohunks,
 And the time clock clicks on by the door.

Chorus:

More, more every-day,
 While Wimpy stands by keeping score,
 And the number we slice-still turns up no dice,
 And the damn bonus Jackpot won't pay.

Lapping

Oh give me some air that's scrubbed 'till it's bare,
 Put six extra locks on the door,
 With sugar and spice,
 And Charlie's advice,
 We'll hit ninety millionths or more.

Chorus:

Dust, dust everywhere,
 We do better work every day,
 In spite of all this, we really can't miss,
 Our boss says our work is O.K.

Holders

Oh give me some steel, an old emery wheel,
 Where the solder can never grow cold.
 With hot carbon-tet,
 Our britches are wet,
 And we make our mistakes as of old.

Chorus:

Tools tools everywhere,
 For every man must be supplied,
 But with all of this stuff, it's still not enough,
 To people who take 'em, NO FAIR!


++++++++++++ The End! +++++++++++++++
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RICK  *W3RSW*
Tom WA3KLR
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« Reply #6 on: December 15, 2013, 04:25:44 PM »

Craig,

If a previous owner did add "padding capacitance" across the crystal socket in an attempt to lower the crystal frequency, this will lower the net activity of the circuit, because as the load capacitance gets larger, trying to push the parallel resonant frequency closer to the series resonant frequency of the crystal results in a situation where the circuit response just gags and can't run.  So if you make the discovery of extra capacitance installed, removing it should be a great help.
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WD8KDG
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« Reply #7 on: December 15, 2013, 07:40:05 PM »

Success!!

No problems with wiring, switches, and all that stuff. The silver micas around the crystal socket were for other circuits and the extra connections on the socket were open for their use.

While the beast was belly-up a new 6CL6 was plugged in, no harm in trying. The old 6CL6 tested great on the TV7DU, but.....................that was the only change. Screen grid resistor measured 68.5K on my meter, with no sticker traceable to NIST. Parts list shows a value of 68K for that resistor.

The rock oscillates around 7.2935 or there abouts according to two freak counters. The big difference between the Johnson drifting VFO/DFO, Brian's (AF4K) crystals, vs the ICM crystal; when first loading the 500, the drive has to stay at 10 mils or a little higher. If the PA drive drops to around 8 mils the rock quits with light loading on the PA. Near recommended loading (300 mils AM) drive isn't a problem.

Many thanks for the suggestions!

Craig,

PS: Going to keep an eye on TX frequency and loading for a while. 


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Tom WA3KLR
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« Reply #8 on: December 15, 2013, 07:51:48 PM »

Well, I'm glad to hear that it is working, but I would think that the crystal oscillator operation should be independent of the later drive level.?  Keyer adjustment?  It's time for me to stop thinking and get ready for bed.

73
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
WD8KDG
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« Reply #9 on: December 15, 2013, 08:04:37 PM »

Keyer adjustment? Could be.........I'll sleep on it & look at the skizmatic. Getting to the adjustment shouldn't be too much of an issue. A piece of small diameter piano wire thru the top, can do. But why no issues with the VFO or other crystals? One of the PNW AM group is sending a couple 40 meter crystal to play with.

More questions to ponder.

73's,
Craig
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WD8KDG
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« Reply #10 on: December 16, 2013, 06:45:02 PM »

Keyer adjustment is fine. At this point in time, I'll leave well enough alone. A couple FT243 rocks will be heading towards Springtucky soon. Wonder how they will function? Grin

Tnx for the help,
Craig
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Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
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