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Post by BBB on Sept 7, 2013 10:45:42 GMT -5
I have had some limited success "pulling" transmit crystals using small pico farad capacitors in a couple of radios. Usually by just putting a cap in parallel with the crystal and wrapping the legs of the cap around the crystal pins then re-inserting the crystal back into the socket will bring them up a little. Was wondering if anyone has documented or knows how to pull crystals on way or the other with capacitors properly. We sometimes touch on the subject on CRR but I usually forget what the procedures are one way or the other. I'm assuming you can only "pull" one way or the other so far before the crystal will not oscillate properly. I'm currently trying to get a Regency CR-142 up from 27.112 to 27.115 on transmit. The culprit seems to be the 10.635 Xmit and/ or the 37.750 Mixer crystals since the receive is off a bit farther than the Delta tune knob will allow also. Adding caps in parallel with either of these is having no effect at this time. Maybe there is a tuning slug I'm missing or I have to wire the cap in series with the offending crystal. I do not have a crystal tester like Sencore has in their CB testers but this crystal tester looks interesting for only $32: www.ebay.com/itm/New-Frequency-Meter-49S-Crystal-Oscillator-Tester-Support-Test-IN-Circuit-/251255436496?pt=LH_DefaultDomain_0&hash=item3a7ffdb4d0Just didn't want to have to spend $30 for a new crystal to get the radio back on the air. Thanks. |
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Sandbagger
Administrator/The Boss 
Posts: 6,249 
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Post by Sandbagger on Sept 7, 2013 16:32:30 GMT -5
I have had some limited success "pulling" transmit crystals using small pico farad capacitors in a couple of radios. Usually by just putting a cap in parallel with the crystal and wrapping the legs of the cap around the crystal pins then re-inserting the crystal back into the socket will bring them up a little. Was wondering if anyone has documented or knows how to pull crystals on way or the other with capacitors properly. We sometimes touch on the subject on CRR but I usually forget what the procedures are one way or the other. I'm assuming you can only "pull" one way or the other so far before the crystal will not oscillate properly. I'm currently trying to get a Regency CR-142 up from 27.112 to 27.115 on transmit. The culprit seems to be the 10.635 Xmit and/ or the 37.750 Mixer crystals since the receive is off a bit farther than the Delta tune knob will allow also. Adding caps in parallel with either of these is having no effect at this time. Maybe there is a tuning slug I'm missing or I have to wire the cap in series with the offending crystal. I do not have a crystal tester like Sencore has in their CB testers but this crystal tester looks interesting for only $32: www.ebay.com/itm/New-Frequency-Meter-49S-Crystal-Oscillator-Tester-Support-Test-IN-Circuit-/251255436496?pt=LH_DefaultDomain_0&hash=item3a7ffdb4d0Just didn't want to have to spend $30 for a new crystal to get the radio back on the air. Thanks. Usually the rule of thumb that I go by is caps in parallel will drop the frequency. Caps in series will raise it. Depending on the type of circuit (tube, transistor) and the frequency of the oscillator, you should be able to pick a small pf cap value that can "pull" the frequency up 2 or 3 khz before the oscillator stops. If all else fails, the 37 and 10 Mhz crystal scheme is VERY common among 23 channel AM radios. I probably have a dozen or so of those in the junk box, I'm sure we could find a suitable replacement.... |
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Post by 2600 on Sept 12, 2013 16:22:46 GMT -5
In general, putting a capacitor in series with the crystal will raise its frequency.
Putting an inductor (choke) in series with it will lower the frequency.
A capacitor in parallel with the crystal lowers it, too. Never have tried putting a coil in parallel with a crystal, only in series.
There is always a limit to how far you can go with this. Just how far that will be often comes down to how old and sluggish the crystal itself may be.
Not every oscillator circuit behaves the same with this trick. Push it too far and the crystal just won't oscillate.
If you have a way to observe the output level of the oscillator, you can see the output level fall off if a series capacitor gets too small, or a series inductor gets too large, or a parallel cap's value gets too large.
The familiar "Delta Tune", "Clarifier" or "Voice Lock" control on a radio is using this same trick, but with a variable element connected to the knob. Older 23-channel radios would have an air-variable capacitor behind that knob, wired in line with the crystal. Newer radios do this with a special diode called a 'varactor' that acts like a capacitor, changing its capacitance as you apply a variable DC voltage to it. This lets them use a simple potentiometer on the radio's front panel to pull the frequency of a crystal up or down.
One drawback of this trick is that it tends to cause the circuit's temperature to affect the crystal's frequency, making it drift more as the radio warms up inside. General rule is that the farther you "pull" the crystal's frequency the more temperature drift you will add to the circuit.
A capacitor or choke coil is always cheaper than a new crystal, but only if it gets you back on frequency. If the crystal has drifted too far or has gotten too weak, you may not get it all the way back on channel this way.
73
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Post by BBB on Sept 14, 2013 10:14:25 GMT -5
Thank you for all the information. I was only able to get the Regency up to 27.113 using the cap trick at this time. Audio reports were decent and no one noticed it was off yet.
Since Sandbagger mentioned that was a common crystal configuration, I will open up a couple of old radios to see if I have those crystals floating around.
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Sandbagger
Administrator/The Boss
Posts: 6,249
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Post by Sandbagger on Sept 15, 2013 9:16:38 GMT -5
Thank you for all the information. I was only able to get the Regency up to 27.113 using the cap trick at this time. Audio reports were decent and no one noticed it was off yet. Since Sandbagger mentioned that was a common crystal configuration, I will open up a couple of old radios to see if I have those crystals floating around. First you need to figure out which crystal is the problem one. If channels 13, 14, 15, and 16 are all equally off, then it will be the 37.750 Mhz crystal. If it's channel 1, 5, 9, 13, 17,and 21 that are off, then it will be the 10.635 Mhz crystal. You could also have a situation where both crystals are partially off, and it compounds the problem. If ALL of your channels are a bit off, there may be a cap value in the oscillator(s) itself that you can change to shift the whole range. |
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