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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 10th Oct 2016, 7:11 pm   #1
stevehertz
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Default Hifi multi pushbutton assemblies - stiff action cured

A couple of the miniature pushbuttons in an interactive row of the same on a hifi receiver were not returning properly to the 'off' position after being pressed to release them. This resulted in two pushbuttons being operated simultaneously that should never be on together. I applied Servisol to the switches' contacts and also to their mechanical linkages which work in conjunction with a pressed metal plate that slides sideways to nudge one switch 'off' when another is pressed 'on'. Liberally applying Servisol made little if any difference.

I then turned my attention to the actual pushbuttons and the cutouts in the front panel through which they fit, thinking that some stiction may have been occurring there. After a thorough clean and polish with a dab of silicon polish, that too made no difference.

I decided to take the top off one of the switches and take a closer look to see how the switch is actually operated by the sliding metal plate. The tops of the switches can be seen as the slotted blue parts in the photos below. To do this you have to bend back four tiny metal tabs that hold each switch in place as one assembly on a length of U section pressed steel. I carefully pried up the tabs, but not enough to allow the blue plate to - as I feared it may do - spring off in a shower of small parts and springs. At that point I again checked the operation of the switch and to my amazement it was now operating smoothly! Those tiny metal tabs were obviously pressed down so hard as to be physically pushing the top blue part down onto the inner parts and restricting them from moving back and forth freely. I'm guessing that when new, the amount of compression by the tabs - although tight - was insufficient to create a problem, but over time the outer plastic part had succumbed to the pressure and was now pressing down on the internal moving parts. So I just bent the tabs gently back down, but just enough to retain the blue cap this time. I did the same to another switch that was acting in the same way, and that too now worked perfectly.

An odd and difficult to locate problem, but one that I won't fall for again. It was a Tandberg receiver but these interactive rows of switches are used on many different makes of receivers, amps and equipments in the 70s.
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Old 10th Oct 2016, 8:32 pm   #2
SiriusHardware
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Default Re: Hifi multi pushbutton assemblies - stiff action cured

Quote:
Originally Posted by stevehertz View Post
I carefully pried up the tabs, but not enough to allow the blue plate to - as I feared it may do - spring off in a shower of small parts and springs.
Just a thought - when I think that something I'm about to dismantle may turn into an impromptu exploded diagram of itself, I put the job, and my arms, inside a large clear plastic bag and do the final separation with everything inside the bag. There have been a few times when I probably would have lost a spring or other parts propelled by the spring if I had not done this.

Of course, catching all the bits is one thing... working out where they all were just before the whole thing flew apart is quite another...
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Old 11th Oct 2016, 5:29 pm   #3
Herald1360
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Default Re: Hifi multi pushbutton assemblies - stiff action cured

It's possible that the plastic material has simply swollen with time, moisture absorption or whatever. The fix is the same, though......
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Old 11th Oct 2016, 6:33 pm   #4
stevehertz
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Default Re: Hifi multi pushbutton assemblies - stiff action cured

That's right, swelling of materials over time, all manner of things could have happened. But yes, the cure is to release the pressure and allow the inner components to move freely again.
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