Any trick to repair sealed 10 turn trimpot? - Page 1
Forum section: electronics / repair

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: March 31, 2015, 03:14:31 am
content: This little guy doesn't change value and I can feel the end detent turning in either direction.Can I whack it a certain way to get the screw to engage again? Or what?Ya think I can get a normal trimmer and drill holes myself?These are quite expensive. It's a Helitrim 78PR5K.

author: acbern
author_rank: Frequent Contributor
timestamp: March 31, 2015, 07:51:39 am
content: I would not try to repair. Its a precision part, and you need precision in its unit I would think, and its not sure whats going on inside.Use either a direct replacement with holes, I would think you get it reasonably priced at mouser or digikey, or buy a standard one, epoxi-glue it on an FR4 strip with mounting holes drilled in and thats it.

author: electr_peter
author_rank: Unknown
timestamp: March 31, 2015, 08:13:22 am
content: Probably contacts have broken off or glued to resistive surface. Practically impossible to repair - structure is sealed, no easy access, it won't work right afterwards. Get a proper replacement part or precise resistive divider (if some fixed ratio is needed).

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 01, 2015, 01:02:48 am
content: Ha ha! Behold stubbornness, a propane-powered heat gun, and a dental pick!As I suspected, the little thingy/carriage fell off the thread, after a bit of prodding it engaged again. I didn't need to burn off the epoxy after all, just jam a box cutter in there to rip open the side.

author: pickle9000
author_rank: Super Contributor
timestamp: April 01, 2015, 01:13:54 am
content: Quote from: Alex Eisenhut on April 01, 2015, 01:02:48 amBeing a dickhead an not listening to people can be a big advantage in life, nice job.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 01, 2015, 01:20:02 am
content: Quote from: pickle9000 on April 01, 2015, 01:13:54 amUm, thanks, I guess? Should I put that on my CV?

author: pickle9000
author_rank: Super Contributor
timestamp: April 01, 2015, 02:29:54 am
content: Quote from: Alex Eisenhut on April 01, 2015, 01:20:02 amI'm the type that says "Sure I can do that". Luckily I have never been in a situation where I wasn't able to pull it off. So yes put it on your CV. It was definitely a compliment.The beauty of this field is that there is so much to learn and such variety.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: April 01, 2015, 02:47:32 am
content: Don't be surprised if Vishay suddenly adds a new line of Audio Resistors to go with their VAR Z-Foil's. See-through pots will be the new rage. Patent it while you can.

author: SeanB
author_rank: Super Contributor
timestamp: April 01, 2015, 04:18:32 am
content: You could buy see through multiturn pots for a long time, I remember Bourns did have some in their range.

author: Joule Thief
author_rank: Regular Contributor
timestamp: April 01, 2015, 04:33:46 am
content: Quote from: pickle9000 on April 01, 2015, 01:13:54 amLove it.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 01, 2015, 04:10:01 pm
content: Now I'm wondering if simply jamming some thin feeler gauges under the screw head to pull on the screw would have helped instead of the destructive approach?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: April 02, 2015, 05:01:21 am
content: Maybe.But you have to let it know who's boss. Now it's properly afeard of you, and will not cross you again.

author: Simon
author_rank: Unknown
timestamp: April 02, 2015, 07:12:58 am
content: A precision part with picture hooks jammed in it for terminals ? well i never

author: pickle9000
author_rank: Super Contributor
timestamp: April 02, 2015, 07:44:33 am
content: Quote from: Alex Eisenhut on April 01, 2015, 04:10:01 pmWith the side open you are able to see the mechanism, clean the tracks, ensure there is no debris. I'd say you did it perfectly. Too many unknowns the other way.So is the hole getting covered with tape? I have used adhesive foil (aluminum) to seal things like this. It's used for furnace duct sealing. Lasts forever.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 02, 2015, 12:34:13 pm
content: I don't really consider this a precision part. I took a picture of a precision pot.I got a bit flustered by the multiturn pot because it was sealed, and I am used to peeling apart single turn pots.The picture hooks are crimped to the normal leads before the epoxy pour.FWIW, the trimmer works, it's just setting a DC balance current. I just put the plastic panel back (it cracked off in one piece) and put scotch tape over it. It's screwed together with 3 other pots in a cluster on the side of the instrument.

author: electr_peter
author_rank: Unknown
timestamp: April 02, 2015, 12:59:27 pm
content: Fix make it work again, but would you consider selling/installing such part in commercial application? I would not.Anyway, I have a question regarding multi-turn pots. If it is not supposed to be very accurate/stable in the first place, why one would even bother installing multi-turn pot? My guess would be that either more suited form factor or that circuit is so poorly designed that it requires adjustment from 0 to 100% in a specific place. Otherwise single turn pot with some resistors will do nicely.

author: Rerouter
author_rank: Super Contributor
timestamp: April 02, 2015, 01:10:14 pm
content: who needs accuracy in a pot, you adjust it until it is set correctly, only thing is stability, and for that i have found it to be enough for most projects i have done, where you want to dial in on a value, over a large range, e.g. low voltage threshold on a pump timer, the pot i used was a multi-turn because the circuit works from 2V to 50V, and its much easier to set a multiturn than a single turn 270 degree pot if your aiming for a specific voltage, and know the temperature will not shift much (18-33C)

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: April 02, 2015, 01:13:53 pm
content: Quote from: electr_peter on April 02, 2015, 12:59:27 pmWell, I'm sure the calculation was different in 1969, when transistors cost 20$ each, and one trimmer cost less than one pot plus two resistors and the space on the board for the resistors.As for commercial applications, some customers would be quite happy if they can get 6 more months of use out of a 10000$ instrument and make money with it. I know I did repairs on NDT equipment for that reason. We're talking about mirror galvanometers that were museum pieces even in 1995. The customer approved the kludges.This multi-turn pot is to get the resolution to tweak balance setting, it's clearly not more accurate or stable than any other resistor of the same size and material.