Question/opinion... To solder or braze this broken knob...  - Page 1
Forum section: electronics / repair

author: pez.diSpencer
author_rank: Contributor
timestamp: September 28, 2017, 09:12:51 pm
content: Restoring a B&K 1570 analog scope (actually a kenwood cs2070 in a B&K box) that was badly damaged in shipping. The seller shipped it in a used LL Bean box with about 4" open space all around with no packing materialNeedless to say, it was rolled into a ball, but I got a full refund and got to keep the scope. Amazingly, the CRT wasn't damaged, and I've finally set about making repairs. I've been able to repair it electrically, but I'm hung up at this point:The outer shaft on the a/b intensity control, a concentric 1k pot, has sheared off right at the standoff. I could mount another pot somewhere else, but it's the B sweep intensity, which I honestly won't use much if ever anyway. So, in the interest of honorable restoration, I'm trying to save the functionality of the original component.So far, I've cut off about 1/8 inch of the standoff to expose more of the brass shaft. Then, I drilled out a little of the brass end so it fits snuggly on the exposed shaft. Looking at the pictures, would you attempt to braze this? I'm afraid solder won't hold. (Disregard the center shaft in the picture, I would obviously have that out before attempting a weld).There is more of a gap between the brass end and the standoff than visible in this photo, allowing access to the joint.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: September 28, 2017, 09:52:28 pm
content: i'd assume that the pot internals contain plastic elements. so, any treatment that involves heat may wreak havoc on the potentiometer (unless you manage to fully dismantle it to get hold of the shafts alone).i've been quite successful at those repair adventures using a two-component Methylmethacrylate glue, like this:https://www.adgrind.co.uk/accessories/consumables/pattex-pse6n-stabilit-express-80g-adhesivenot to be confused with standard 5-min epoxy, which is absolutely unsuitable for such a job.hth!

author: pez.diSpencer
author_rank: Contributor
timestamp: September 29, 2017, 05:08:47 am
content: Quote from: Le_Bassiste on September 28, 2017, 09:52:28 pmYes, this is my concern... I had/am considering just using a two part like you mentioned or good ole JB weld. The outer portion fits snug enough on the exposed brass that it will stay on and turn the pot by itself, so I really just need something that will hold it on when the knob it attached... The problem is that there isn't much material to work with, and the center must stay clear for the concentric shaft. I'll probably try JB before trying an application requiring heat.

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2017, 05:16:30 am
content: A two part epoxy adhesive is mostly likely the best approach. Research the available choices before automatically picking JB weld as a name brand. To give the adhesive the best chance of holding, make sure to slightly roughen and scrupulously clean/degrease the mating parts before gluing.I am sure ordinary electrical solder would be strong enough, but there is the risk of heat damage to adjacent parts as mentioned. Glue is probably better for that reason.

author: Ian.M
author_rank: Super Contributor
timestamp: September 29, 2017, 05:19:56 am
content: Epoxy doesn't bond that well to brasses and other copper alloys, but it *may* be good enough.

author: CJay
author_rank: Super Contributor
timestamp: September 29, 2017, 06:10:22 am
content: That looks very similar to some of the dual concentric pots used in CB and some Ham radios, it might pay you to spend a little time browsing for CB spares, even if you can't get the exact values you may find one physically the same so you can swap the 'wafers' into it.

author: Armadillo
author_rank: Super Contributor
timestamp: September 29, 2017, 06:59:42 am
content: You should do neither and don't let this be a pain come haunting you forever. Change a new one or a used one.

author: Assafl
author_rank: Frequent Contributor
timestamp: September 29, 2017, 07:20:36 am
content: Quote from: Armadillo on September 29, 2017, 06:59:42 amHe can always search (and maybe find) a replacement. Why not try?Great model builders can use adhesive to fix almost anything. I wonder if a machinist can work the surfaces to make a friction fit?BTW - if you do use any acrylic based adhesive (like CA) remember humidity is what polymerizes it. Since the brass surfaces are 100% non porous, Humidity will have to permeate through the crack to cause the acrylic to polymerize, you will have to wait for many hours (I recommend 24h) before you touch the parts. I usually cure any acrylic adhesive parts in a combi-steamer set at 60-80% humidity at 40-60C.Remember to mark the correct orientation, and if it fits really well (which would mean the metal did not break - just a previous cold weld was broken) I'd avoid roughing it up but would clean with a bit of IPA (I usually prefer Acetone but it may ruin conductive plastic or seals/orings on the spindle). I would try to avoid getting it into the spindle so it wouldn't dissolve the lube and (worse) have the lube contaminate the spindle.

author: Armadillo
author_rank: Super Contributor
timestamp: September 29, 2017, 08:47:35 am
content: Quote from: Assafl on September 29, 2017, 07:20:36 amUnfortunately, the models ended up only for display purpose and not for handling and general usage.I would say dreaming and day dreaming is alike so a more real to life analogy would be needed here.The question centred on 2 things, and the 3rd must be own efforts to satisfy oneself if he so desired.

author: pez.diSpencer
author_rank: Contributor
timestamp: September 29, 2017, 11:16:08 pm
content: Quote from: Armadillo on September 29, 2017, 06:59:42 amLol... Find me a concentric 1k/1k pot with the same dimensions and I will gladly pay you for it. I have searched quite exhaustively for a directly replacement, and this scope isn't really worth investing in a parts scope.However, I like the idea of finding one of same dimension and swapping wafers-- might epoxy in the meantime until I can find a replacement:Quote from: CJay on September 29, 2017, 06:10:22 am

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: September 30, 2017, 01:53:49 am
content: I'm kinda thinking why not make your own PCB with more modern and available selection of pots to suit. Even if your pot need a spacer or shaft diameter change?Anyway this isn't going to be that difficult? Well...... here's me not knowing the availability of suitable replacements!fwiwhttp://www.ebay.com/itm/like/263204273206?lgeo=1&vectorid=229466&item=263204273206&rmvSB=trueHeres a 1K5K 9mm dual adj pots. Being 9mm easy to convert to 1K each?

author: amyk
author_rank: Super Contributor
timestamp: September 30, 2017, 02:40:48 am
content: Carefully disassemble the pot so that you can get the broken part alone.It could be of such a simple shape that you could, or get someone to, turn a suitable replacement in a lathe.

author: Armadillo
author_rank: Super Contributor
timestamp: September 30, 2017, 06:12:00 am
content: Quote from: pez.diSpencer on September 29, 2017, 11:16:08 pmDouble adjust potentiometer are not cheap even for compatibles and since you deemed it's not worth investing then forget about getting same dimensions part because it will come from donor "parts scope" and it won't be cheap.Custom turns part seems like a good idea if you turn it yourself.In the marketplace, I can't find cheap old parts, nowadays.I guess you are left with the epoxy, craftsmen style. Hope it works for you.

author: Ian.M
author_rank: Super Contributor
timestamp: September 30, 2017, 07:49:41 am
content: If you can get the metal part on its own without breaking anything, soldering it is still a better choice than gluing it.Low temperature hard silver solder (e.g SSF-6) would be one option, but I expect you can get away with SnCu or even SAC Pb-free soft solder at a much lower temperature - use the absolute minimum possible to fill the joint completely, as the thinner the solder layer, the more it will alloy with the brass and the stronger it will be. If using soft solder, it will help to tin both surfaces then wick off all excess solder before assembly. Appropriate flux for the solder is essential