Fixing an intermittent issue in a CEL-231 Sound level meter

[DaJMasta]

The Cel-231 is an old, fairly cheap sound meter that can be found on ebay fairly easily, but while it doesn't have multiple weighting modes, peak storage, dosimeter storage, etc., it does offer a +-1dB base accuracy which is decently higher spec than many of the cheaper meters available.

I picked one up with a Cel-282 calibrator for cheap and after swapping the batteries in both, I was able to get it running and taking reasonable readings.  There was one odd issue though, it seemed that squeezing on the middle area of the unit (and flexing the relatively thin plastic housing a bit) could trigger the reading to jump momentarily.  If you left it alone, it was just fine, but handling it was an issue.  I opened it up to see what was going on, and it's neat to see inside this older handheld instrument.  The cal sticker I broke was dated September 19, 1992.



Taking out the board an having a look, there wasn't a real obvious leaky cap, burned part, or big break, but since it seemed like a potentially physical problem, I resoldered a few joints that looked a little sketchy and tinned the ground around the mic wire entering the PCB that was frayed and had a couple of small breaks.  Partly reassembling it, I found the problem still present.



This little ceramic based "hybrid" (I guess it is a hybrid with both caps and printed resistors) looked like it had some unknown residue on it, and though it maybe wasn't the safest idea, I swabbed it just a bit with alcohol to get rid of the residue to see if it made a difference, and it didn't.



I wanted to go and probe the board to see what was going on, but because of the batteries loading in both sides.... it was a pain.  I didn't know if it was tapping off the center of the 4 battery compartment for a low voltage split rail or what, so I didn't want to just stick it on my single output bench PSU, so i loaded the batteries in one side, loaded them in a spare AAA holder, then jumped them onto the contacts with alligator clips.  The hack let me power up the board with it fully exposed, so I started poking.  Flexing the PCB and some larger parts was worth checking since the issue showed up with mechanical stress, but no luck... until I bumped the range switch.  After a bit of wiggling around, I found that in the high range, the reading was stable, but when on the low range, even slight bumps could make it jump 10-20dB momentarily,

I pulled off the plastic switch top, busted out the contact cleaner and sprayed a bit in both sides of the switch, then exercised it a bit with a few dozen switches.  That little shot of spray completely fixed the problem, and the unit calibrates just fine to the calibrator module (adjusted with the graph for current pressure level, of course).  Really interesting to see the ceramic hybrid in there, and in the little bit of probing I did, it's got at least one boost converter and some decent 14-15V opamp rails from these AAA batteries.



A cheap little addition and a quick fix, I'll take it.

[gbrettell]

could it be that when squeezing the unit the mic is picking up the noise induced by mechanically stressing the enclosure?

[DaJMasta]

The case certainly does flex, so while I hadn't seen the issue since the switch cleaning fix, I just gave the unit a twist.  When it got loud enough for me to hear while messing with it (mostly twisting the upper plastic part against the blue plastic sheath on the bottom) I could only get about 10-12dB of increase on the level.  The flex that had been causing an issue before (squeezing down on the center of the unit, just below the screen section with the switches) wasn't audible to me when it was happening and could get to 15-20dB on the reading.

There's definitely some conducted vibration since the sound I was hearing of the case creak while flexing it was quite quiet and it was being generated halfway down the meter... it shouldn't have been a 12dB increase in level with an ambient in the low to mid 30s, but this is still quite a bit under what I was seeing initially with much less flexing.