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RS SPL METER (42-3019) MODIFICATIONS

Yet another meter mod!

At my request, Sean Collins sent me the schematic to this Radio Shack meter so that I might suggest some modifications to extend its frequency response (thanks Sean!). The schematic is almost exactly the same as the current anaolg meter that RS carries, the 33-2050, with the main difference being it employs a three terminal microphone element, rather than the two terminal type found in the 33-2050 and 33-2055. This makes any external mic mod difficult unless you use a stereo jack or something for the interface. I'll leave any such mods to the ingenuity of the reader.

The frequency response mod described here should makes the electronics flat over the entire audible range for the "C" setting of the A/C switch. After perfoming the mod, direct meter readings should be accurate for bass SPL measurements down to 20Hz or so using the internal mic without using any correction factors. The treble response of the meter will be limited only by the frequency response of the internal microphone (probably -3dB @ 15kHz at best for the stock cartridge).

FREQUENCY RESPONSE MOD

Instructions are as follows:

- To extend the LF response: -

CHANGE (either replace or solder in parallel to the old value, your choice, use electrolytic caps):

  • C1, C2 from 1uF to 10uF
  • C3, C4 from 1uF to 47uF
  • C7 from 10uF to 220uF
  • C8 from 22uF to 220uF
  • C9 from 22uF to 220uF

- To extend the HF response: -

CHANGE (replace, use ceramic or NPO cap):

  • C12 from 33pF to 12pF (controls HF response)
REMOVE (see possible calibration note below!):
  • C13 100pF
The easiest way to change these caps (except for C12, which must be replaced of course) is to leave the originals on the PC board, and solder the new caps (the values given above) to the back of the board, where there is plenty of room.

Case Disassembly:

  1. Remove the two phillips head screws on the back, one is behind the battery door.
  2. Pull the back away about 1cm from the mic section. Watch out not to crack the plastic hooks on the bottom!
  3. Holding the unit so that the microphone is pointing up, pull down on the back so that the plastic hooks on the bottom let go. Remove the back.
  4. Unscrew the two brass posts holding the pc board in position.
  5. Unscrew the philips head screw on the rotary switch, remove the phenolic wafer.
  6. Remove the pc board along with the mic (with housing) and meter. A gooey substance may fight you when you remove the mic housing from the case.
Possible Calibration Note: This mod should not affect the calibration of the meter, since only the poles in the circuits have changed, not the ac gain in the passband. The possible exception here is the removal of C13, which has a (CAL) marking next to it on the schematic. These old opamps, like the Lord, work in mysterious ways, and I'm not completely certain what this cap is doing. All I can say is that this cap is not in the 33-2050 meter, though all of the surrounding circuitry is the same. If I were modifying this meter, I would hold the meter about an inch away from the dust cap of a small woofer producing a 1kHz tone, and switch this cap in and out of circuit. If no steady-state level change results then it is safe to just remove the cap. If a steady-state change occurs, you should re-calibrate the meter with the capacitor removed based on the level experienced with the cap in-circuit. Use the same setup as just described to accomplish this: note level on meter with cap in-circuit, remove cap, adjust "cal" pot until you get the same level as before.

If your modified meter displays instability (needle jumps around, stays pegged on the lower ranges, is influenced by the position of your hand over the needle area) increasing C12 is the way to fix this. Be aware that the larger this cap is, the lower the hf rolloff will be, though of course the meter is useless if it is not stable. If you are not interested in extending the high frequency response, or if you use the "A" weighting setting a lot, leave C12 and C13 alone and you should experience no instability problems.

FAQ SECTION

(This section reserved for any FAQ-type stuff in the future.)
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