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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 7th Nov 2020, 5:33 am   #21
falcon123
Triode
 
Join Date: Jul 2020
Location: Angeles City, Philippines.
Posts: 19
Default Re: Hallicrafters SX-43 FM drift.

yes i do as it parallels the tuning cap. given the style of construction of the original its any ones guess as to its tolerance. the alignment if it was found to work would easily take care of that. by the way,at what temperature do you suppose these are baselined at an interesting question. and at what temperature was the radio designed to operate? its greatest amount of drift cold was in the office that is airconned. and was that way even when first removed to the workshop. and now after being out there for a month is considerably better. if it was easy to access it id even remove it and try a mica which are very stable. this part of the world has little temperature variation its warm all the time. oh well i guess thats why this is a HOBBY!
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Old 19th Nov 2020, 10:17 pm   #22
falcon123
Triode
 
Join Date: Jul 2020
Location: Angeles City, Philippines.
Posts: 19
Default Re: Hallicrafters SX-43 FM drift.

well ive finally figured it out, it was c26. c26 is on the top of the band spread cap which also serves as fm tuning. the c26 trimmer sets the 106 mc range. i discovered it by heating various portions of the set with a hair dryer when it was cold. it was really quite amazing how warming that cap brought the tuning back to where its supposed to be. its a different style trimmer than the other two on the band spread cap.its not two metal plates with mica in between. rather its two contacts one bottom one top on either side of the ceramic. by rotating the upper (top) adjuster it seem it varies the effective thickness of the ceramic between it and the fixed lower contact and thus varying the capacitance. un fortunately i didnt think to just clean the thing before i removed it. i replaced it with a conventional mica padder that immediately corrected the outlandish drift. it still drifts a bit but nothing like before and is pretty good for a set with no afc. does anyone (if they can understand the different trimmers i write of) know why the type of trimmer i replaced was used in the first place? and is there any advantage in replacing it with a like item? as thing stand im going to leave it unless shown otherwise
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