[Test-Equipment] Tektronix T935A Scope - seized knobs

[Fuqua, Bill L]

The worst case of this that I have ever seen was with a number of Stanford Applied Research  HR-8 Lock-in amplifiers. They used some sort of white grease on the shafts of their switches that hardens after a number of years and it requires removing the switches, removing the wafers from them, then taking off the shaft retaining ring. We had to drive the shaft out with a hammer and sand it down to remove the stuff and clean out the bushing, reassemble them with proper lubricant. This was difficult because these switches had many wafers with 1% resistors on them.  I still have one or two in my garage that are in the same condition. 
   These are great devices for weak signal detection.  Now days if you don’t need precision measurements you can use a sound card and FFT. 
73
Bill wa4lav
 
________________________________________
From: test-equipment-bounces at mailman.qth.net [test-equipment-bounces at mailman.qth.net] On Behalf Of Barry [n4buq at knology.net]
Sent: Thursday, July 22, 2010 11:46 PM
To: test-equipment at mailman.qth.net
Subject: Re: [Test-Equipment] Tektronix T935A Scope - seized knobs

> Found no binding on the knobs. I had to open the cabinet and look
> internally.
> Followed VAR knob shafts to 1k pots on board - Found seizure to be in 1k
> pot bushings.
> Sprayed bushings lightly with Chemtronics "Trol-Aid" from 1970's.
> Worked in lube and they slowly loosened up enough to turn from front red
> VAR knobs with just fingertips.
> Will re-assemble tomorrow - how much should I charge myself ?
>
> Back to tube radios,
>
> bob...w2ami x wn2ami 1962

I had a similar thing occur on a Tek 465B I recently acquired (okay, "won" on eBay).  The "Focus" pot was extremely difficult to twist.  I worked in some DeOxit (yes, I know, it's not exactly the correct chemical for the job but it worked).  The shaft eventually was very free to turn; however, about a week or two later, it is beginning to show signs of hardening lube again.  Looks like a more permanent solution is to disassemble the pot and clean it correctly.

73,
Barry - N4BUQ

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