53181a trimmer capacitor replacement - Page 1
Forum section: electronics / repair

author: hp3310a
author_rank: Regular Contributor
timestamp: April 11, 2025, 04:08:02 pm
content: Hi everyone,I recently got a nice Agilent 53181a frequency counter, the regular one without one of the high-precision OXCO options. This version has a trimming capacitor to "calibrate" it. It has a range from 5.5pF-18pF and is single turn. The adjustable frequency range is easily +-2kHz (given 10MHz input). This makes adjusting to a 5Hz interval really difficult, if you're lucky and have a lot of patience you get to about 1Hz accuracy, there is no way to make fine adjustments in the sub-1Hz range.Well, I would like to be able to do that (no matter how nonsensical it may be given the rather bad temperature stability). I thought it may be possible using a multiturn trimmer capacitor. There actually are some available, not quite the range but close enough (1pF-20pF for example). They are rather expensive (>$30), but available.I was wondering if someone already did this before?

author: TimFox
author_rank: Super Contributor
timestamp: April 11, 2025, 04:13:25 pm
content: Multi-turn trimmers are normally "piston" construction, and are very good.Do you have space for such a device?A good source for surplus piston trimmers:https://www.surplussales.com/categories/1679-piston-trimmer-capacitors-index/

author: hp3310a
author_rank: Regular Contributor
timestamp: April 11, 2025, 04:18:39 pm
content: I think those are too long, the available space is rather limited.

author: TimFox
author_rank: Super Contributor
timestamp: April 11, 2025, 04:28:31 pm
content: They range in physical size: did you look at the Surplus Sales of Nebraska listings?I don't know of any other construction for multi-turn trimmer capacitors.

author: hp3310a
author_rank: Regular Contributor
timestamp: April 11, 2025, 04:56:08 pm
content: Actually I may have misinterpreted my previous research, those that I found (on Digikey) all seem to be single or half turn.I found one that may be suitable size-wise (https://www.surplussales.com/items/107051/air-trimmer-capacitor/) but that one has a mirrored pin arrangement compared what the Murata T5.5-18A which is on the PCB has (two legs in front one in back, instead of two in back one in front).

author: TERRA Operative
author_rank: Super Contributor
timestamp: April 11, 2025, 05:00:20 pm
content: That internal timebase in these units is really crap and pretty apparent that it is there simply to allow the counter to work after a fashion, with the intention that an optional OCXO be installed or an external timebase (GPS, Rubidium, etc) be used for real measurements.Even if you were to replace the trimmer capacitor, the stability of the internal timebase is bad enough that I think you'll end up constantly chasing your tail. Getting more precise adjustment is no good if the target is moving around all the time......

author: hp3310a
author_rank: Regular Contributor
timestamp: April 11, 2025, 07:28:36 pm
content: I agree, it's kind of beside the point. But if something works that badly (the trimming process due to the one turn cap), it just bothers me.For a "ballpark" measurements the standard OCXO seems to be more than fine, you just need to understand its stability. I'm doing a long term test, recording readings for a 10MHz GPSDO square wave input and the chassis temperature. I already know it has a drift of about 4Hz from 24.2°C to 26.2°C (at 10MHz input). If that turns out to be repeatable (which I don't know yet) it's really not that bad.For precise measurements I can always use the GPSDO as reference input.