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Having issues finding this simple switch

  1. Apr 16, 2016 #1
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    This is a on off switch for a McIntosh 2120 stereo power amp. It is a snap switch of sorts built in a potentiometer case with the switch attached. The shaft is 25mm. Due to the fact that it closes a relay I don't think it has much current running through it.

    I just can not seem to find anything to replace it. Both Digi-Key and Mouser say they don't have anything like it.

    Any ideas??

    Cheers,

    Billy
     
  2. jcsd
  3. Apr 16, 2016 #2

    dlgoff

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    I've got this one I would give you but after checking the resistance, it seems the pot is open. :oldcry:

    However you may be able to remove and use the switch. If you want it PM me.

    maybe.jpg
     
  4. Apr 16, 2016 #3
    Hi Don.

    Thanks for the kind offer. The actuall switch part is working fine. It is the plastic arm in the pot case that failed. I have seen some pots with a switch for sale and even some that had more or less the correct shaft length but not the half shaft D shape ending. All these seem to be in the mili amp range and low voltage I want to use the original knob from the unit if that is possible. The good news is that the original knob can be modified to except a round shaft or split shaft.There is no way to mount a short shaft control on the amp as the face plate is some distance from the mounting surface. What one gets into messing around with old electronics...lol

    I even took the old switch apart and super glued the plastic part that failed back together but I don't think it will last very long. If push comes to shove I can machine a new part and repair the old switch. I have a small four axis CNC mill. That would require a good bit of work. I don't have a 3D printer, which would be ideal to make a new part with.

    I guess I expected there would be a simple rotary on off switch in common shaft lengths to be had. There are many but not any I have found that will fit.

    Thanks,

    Billy
     
  5. Apr 16, 2016 #4

    jim hardy

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  6. Apr 16, 2016 #5

    davenn

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  7. Apr 16, 2016 #6
    Thanks Dave, that is what I have done. I have not found one with the correct shaft length or D shape on the end but I will get around that issue. I may need to make a connector sleeve but that is less work than making the part to fix the old switch.

    Jim, this is just a on off switch built into a pot case. It's only function is to turn on a power relay.

    Thanks guys,

    Billy
     
  8. Apr 16, 2016 #7
    ALPS (the switch mfg. shown in your photo) is still in business and their catalog can be found at: http://www.alps.com/products/e/pdfcatalog_list.html

    BTW that shaft style is called 'half flat' and it looks to be 0.250 in. diameter, which would make it '1/4 in., half flat, 1-3/8 long'. Shaft length is measured from the the device mounting surface, i.e. the shoulder on the threaded bushing that is against the inside of the chassis when mounted.

    Clarostat brand used to make a bunch of 'mix-n-match' pots and switches for the electronic repair industry. A Google search for 'Clarostat pots' returns >50 000 hits. Clarostat was bought out by Honeywell and their parts aren't widely distributed these days. The shafts an these are aluminum these days and the very old originals had brass shafts. Some imports have plastic shafts. So a file and hacksaw can readily reshape them if the shaft is long enough. There used to be, and maybe still are, 'shaft extensions' available. They slip onto the switch shaft, are held on with a set screw and have a standard shaft on the distal end.

    Suggest you use epoxy. Superglue (cyanoacrylate) is not very strong and quite brittle.

    Let us know what you find.
     
  9. Apr 16, 2016 #8
    Hi Tom,
    Clarostat was the second place I looked after ALPS. I buy Clarostat pots from time to time. They make good parts and are well suited to some of the pre 70's Fender amps. I had searched the ALPS site without results.
    I also remember there being such a thing as a shaft extensions I have seen in the distant past. I may dig a little before I make one. I am sure I can make one on my small machine lathe from aluminium or better brass in short order. Brass is easy to work with and aluminum is a bit sticky and harder to work on a lathe.

    Perhaps something like the original part exist somewhere but I have not found it. In any event, I can produce a solution.

    Also, thanks for the "half flat" reminder. I had forgot what they we called.

    Thanks,

    Billy
     
  10. Apr 16, 2016 #9

    jim hardy

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    Now i'm confused. Are you looking for a switch or for a potentiometer with a switch on the back ?

    I've been known to file a flat on a shaft...
    Mouser claims to have 26 of this in stock
    it has a 1.5 inch shaft with a flat on it, dimension on datasheet
    http://www.mouser.com/ProductDetail/CK-Components/A10215RNZQ/?qs=sGAEpiMZZMvNbjZ2WlReYkMRQ%252b%252bBkxIQBjHdF6Jb1WM%3d [Broken]




    I notice this outfit lists several switches specific to guitar amps; maybe worth a perusal.
    https://www.tubesandmore.com/products/switches
     
    Last edited by a moderator: May 7, 2017
  11. Apr 16, 2016 #10
    Good one Jim.
    @Planobilly That one has coin Silver contacts, fine if you're switching line voltage. You mentioned it drives a relay coil but didn't state the voltage. Silver contacts need some minimum voltage to punch thru the tarnish on their surface. I don't recall the exact number but 25V comes to mind. I don't know about coin Silver. That switch also comes with Gold or Gold over Silver contacts if you are switching low voltage. The problem with Gold though is it vaporizes at over a few tens of milliamps of current, especially if there is an arc. Arrgghhh, tradeoffs. :devil:
     
  12. Apr 17, 2016 #11
    upload_2016-4-17_10-24-9.png

    Here is the schematic.

    Bill
     

    Attached Files:

  13. Apr 17, 2016 #12

    Nidum

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    Ebay ?

    That was just a random shot . Probably lots more to be found on there .
     
  14. Apr 17, 2016 #13

    jim hardy

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    Well, that's sure enough just a switch. Thermistors are there to prevent destructive inrush current but i cant read their cold resistance value..
    I'd think that switch would be okay - datasheet says it's rated 5 amps continuous carry.
    Silver is good contact material for substantial current
    and the filter caps that i assume are downstream of bridge will snub inductive kick from the transformer so you don't need tungsten arc-proof contacts.
     
  15. Apr 17, 2016 #14
    Sort of what I thought Jim. There is nothing special about the switch. Cold resistance is 4 ohms.

    I have another question but will put it in a new post.
     
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