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Radio Shack Sound Level Meter: Switch Repair

My trusty Radio Shack Sound Level Meter recently began misbehaving: switching to the most sensitive two ranges (-60 and -70 dB) caused it to turn off. Finessing the switch got it back in operation, so I completed the mission (a string quartet in Vassar’s Skinner Recital Hall topped out around 90 dB) and laid it out for repair:

Radio Shack Sound Level Meter - PCB solder side

Radio Shack Sound Level Meter – PCB solder side

After cleaning the already pristine gold-plated (!) contact pads and putting it back together, the switch failed the same way.

A bit more poking & prodding revealed that slightly loosening the upper case screw (in the boss just left of the switch pads) made it work perfectly.

Ah-ha!

Come to find out that the rear case presses on the PCB to hold it in place, which moves it slightly toward the front of the case. The switch rotor, being firmly attached to the stem in the middle of the pads, doesn’t move, which suggested that the bifurcated spring contacts on the rotor had take a bit of a set.

Un-bending them very, very gently to add a millimeter of springiness solved the problem.

A piano solo topped out in the high 80s…

Update: Another meter owner shows how to cure the problem, rather than treat the symptom:

I found your older note about the switch problem on the digital R.S. SLM to be helpful, in that mine had a similar problem, but only on the 60 dB scale, not both the 60 and 70 dB scales. Your diagnosis about the back putting pressure on the board seems to be right on. However, for me, re-bending the switch contacts didn’t help.

What did fix it was filing ~2mm off the back case boss around the upper screw hole. That was the source of the pressure on the board. 1 mm didn’t quite fix it, but 2mm off did.

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  1. #1 by Jason Doege on 2014-02-24 - 09:46

    Evidence that people enjoy art in vastly different ways…

    • #2 by Ed on 2014-02-24 - 13:09

      I’ve always wondered just how loud those concerts were; that’s my story and I’m sticking with it.

      Even though I don’t hear very well any more, I found myself using foam ear plugs at the concerts to reduce acute ear pain during the louder passages. Now I know that my ears demand foam plugs above 85 dB, which isn’t very loud by objective standards. It seems they’re both less sensitive and less tolerant of pure tones. [sigh]