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 Post subject: Threaded Knob Repair
PostPosted: Feb Wed 21, 2018 1:29 am 
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Joined: Sep Mon 18, 2017 2:23 am
Posts: 1652
Location: Plymouth, MI
I recently acquired a 1922 Magnavox amplifier that has four threaded bakelite knobs for the binding posts that have worn out threads, they do not tighten down and are very loose. Is their a way to repair the threads?


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Dan

I know just enough to be dangerous.....
(Member - Michigan Antique Radio Club)


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 Post subject: Re: Threaded Knob Repair
PostPosted: Feb Wed 21, 2018 1:56 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12602
Location: S. Dartmouth MA 02748-1225 USA
Dan,

It appears the insulated knob has a brass insert. It would take a LOT of manipulations to wear down the insert of the threaded stud.

IMHO the knob thread is 10-32 over a 8-32 stud or an 8-32 knob thread over a 6-32 stud...

Use a micrometer and a thread size chart to determine what is going on.

FWIR McMaster-Carr may have insulated domed knobs with threaded brass inserts. These may make an acceptable substitute.

I have not seen "odd" threads on electrical connectors used in the U.S. in the 20's. radios. Such "odd" threads usually appeared on fine instruments, textile machinery, pianos, industrial process instruments, etc.

FYI On 20's radios, knobs, dials and binding posts can be made with at least three different materials, Bakelite, hard rubber and a shellac/coal composition know as "mud". Mud can be damaged by almost all solvents and temperature greater than 150F, it is thermoforming. Mud also chips very easily. If mud is over cleaned the coal filler gets exposed and it tends to "sparkle" only hope is to paint flat black lacquer... There is risk of melting mud components if a solder repair is made to a stud or other electrical post that has a mud part physically attached.

YMMV

Chas


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 Post subject: Re: Threaded Knob Repair
PostPosted: Feb Wed 21, 2018 2:31 am 
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Joined: Sep Mon 18, 2017 2:23 am
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Location: Plymouth, MI
I'll look at the McMaster-Carr website and see what they have. I may also look into remaking the posts so I can use the same knobs. Thanks for all the info Chas.

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Dan

I know just enough to be dangerous.....
(Member - Michigan Antique Radio Club)


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 Post subject: Re: Threaded Knob Repair
PostPosted: Mar Wed 07, 2018 7:46 pm 
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Joined: Sep Wed 06, 2017 6:06 pm
Posts: 135
Location: New Castle, IN
This might sound stupid, but is there enough material for a heli-coil-type repair?


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