Trimmer Caps for old Quartz Crystals?

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kruser

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Hey Gang,

I've been restoring several of my old scanners and others that I purchased for very cheap.
I'm pretty much done with that part and have now moved on to testing all my old crystals. I had many that were unknown as the marking had faded away but I was able to determine the frequencies with freq counters, signal generators, or whatever was easiest at the time. Some were bad and tossed but most still oscillated close to the original frequency.
So now I have all these old radios running (mostly old Electra's but some Regency's) and they are working well. The exception is that some of the crystals have drifted off frequency maybe from rough handling or simply age or poor construction.
And as luck would have it, the ones that are slightly off are the ones for the agencies I like to monitor the most. I can still hear the audio but at times it is annoying due to being slightly off frequency.
I recall some old radios with crystals also had a trimmer cap for each crystal socket so you could "tweak" the crystal's oscillating frequency slightly. If memory is correct, you could really only tweak one way or the other (higher or lower). I forget which it was. I think if the crystal was high in freq, you could use a trimmer to lower it slightly but it may have been the reverse of that and the trimmer cap would raise the freq slightly.
Mine are close enough that I think a cap would help on many. It's worth a try anyways figuring I can get small trimmer caps for pennies and new crystals cost a ton.
The thing I don't recall is the range in pF or value that a trimmer cap used for this purpose would have had.
I also do not recall if the cap simply inserted in parallel with the xtal socket or if it would require a trace be cut and then inserted in series with one leg of the xtal socket. I dug through all my old service manuals and found none that had the individual trimmers for each socket so I'm hoping someone here can help me remember what value trimmer to try and also how they were wired electrically in the circuit.

I also kicked the idea around of just purchasing new crystals but at today's price, that is not a good option. I then found the used offers on eBay and at least one individual that will sell individual xtal's on the freq you want if he has them. None of them can say if the crystal is still on freq and with my luck, they would all be bad.
I'm figuring I can fudge maybe 50% of them with trimmers back to a good enough setting and then the others I may purchase new or just be happy with what works.
Many of the agencies I monitor have swapped to the narrow bandwidth that will be required but no new systems have shown up on the new "splinter" channels that will be free. Once they start filling the new narrowband slots then I guess the old crystal units will become doorstops unless one wished to go in and tighten up the bandwidth which I'd imagine is not as easy as it sounds.
I actually did perform a narrowband mod on one of the old Regency units. I purchased some narrow 10.7 (may have been 10.245) IF crystal filters and simply replaced the wider stock filters and it worked great. I was trying to minimize adjacent channel bleedover from a very strong transmitter 15 kHz away. It ended up eliminating it!

If anyone recalls how this was done, it would be appreciated if you can pass it along.

Thanks All.
 

kruser

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Location
St. Louis County, MO
Its been awhile but IIRC the average trimmer cap range used to be between 1 - 10PF or 1 - 15PF
and they were always in parallel with the crystal.

Oh - Here is a company that still sells narrow band kits.

Narrow Band Filters
Thanks for the link regarding the narrow filters. I've heard of this company before but had forgotten all about them. The encoder/decoders are something I may toy with in the older Icom's I have.

I did purchase some small 2-20pf trimmers and I was amazed at how many off freq crystals I'm able to "trim" back into life! I was not going to invest much cash on new crystals so this allowed me to get what I can from those I already had.
My county has a new license app on the FCC site and it looks like they will be going with a P25 800 MHz system so one of my old crystal scanners may see an earlier shelving then I had anticipated. I do have a bunch of stuff still on VHF lo and hi in the area so I guess it may be worth keeping one of the old units running for a few more years yet. Our highway patrol changed frequencies some time back so I may end up buying those crystals new. They are still on low band vhf and I always liked the audio of the old bearcats so I may as well keep one of them running for the highway patrol alone.
They had a statewide system planned but I think the governor has halted the project for a second time just recently. If that is true then I don't see them switching anytime soon.
I really think they would be fools to kill off the low band system due to our hilly terrain in much of the state. The northern half is fairly flat so most anything would work there I guess.
 
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