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Hi folks, many many years ago I was a happy owner of an analog Radio Shack SPL meter. I'm getting back in the hobby, but my old meter has since been discontinued, and even the used market dried up. The new RS stuff does not seem to get as much praise as the older stuff, so I've decided to get a Chinese clone of what appears to be the RS analog meter that I've once owned.

Would someone with the real RS meter be so kind as to open it up and take a picture? Or verify that the circuit board is indeed identical, which would seem to imply that it is a true clone, and the SPL correction tables available online could be applied to this model also.

If this is proven to be a good clone, this might be a good source for a cheap and decent meter that can be had for only about $25-$30 shipped!
 

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This is a page from the manual that came with this "No Label" Chinese meter, all the specs, and the wording appear to be identical to Radio Shack 330-2050. :D

Now if only the circuit board layout can be confirmed.
 

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I have recently purchased one of these, mine was labelled "Silverline" but they are sold under a variety of different labels. Mine was faulty when recieved, as it was very cheap and a mail order buy then I decided to repair it rather than return it. The problem it had was that on all ranges the needle would not drop below the -4 position although it was working as far as detecting sound was concerned. After re-soldering the badly made joints of the link wires on the PCB and cleaning the gold PCB switch contacts then it all appeared to be working well.

At this stage I decided to compare the circuit with the Radio shack version and while most of it is very similar the RS use a single BA301 op-amp and this new meter uses a C4558 dual op-amp and quite a few different component values, so NO it is not the same meter. It appears to be updated from the original circuit by having larger value coupling capacitors so it should be better at low frequencies than the old one.

I remove the microphone insert (9mm dia marked EPE) and checked it's frequency response to find that while it was reasonably flat it showed quite a drastic loss above about 7kHz, so that wasn't good.

I next checked the frequency response of the meter without a mic attached with a spectrum analyzer (with the meter set to C weighting) to find a very large dip in the response around 8kHz, again not good.

I cured the dip by putting a 47pf capacitor across resistor 2R10 (the 750k near the A-C switch) and although this caused a very slight loss in the response at low frequencies it was now virtually flat from 70Hz to 20kHz.

I have since changed the mic insert to a Panasonic WM61A which has a much flatter response than the original and all tests so far show that it is all now greatly improved.

Both of these modifications will affect the calibration so if you are planning to do the same then you either need a calibrator or some sort of very stable source of sound where you can make before and after comparisons that enable you to reset the meter to the correct level.

Be warned that if you you decide to start playing about with the component values on the board then some of the components originally fitted do not correspond with the board markings so the circuit has already been modified from the original design.

Otherwise, Yes it is a good meter and well worth the very low price. If you are trying to use it unmodified to
measure sound levels at different frequencies then it isn't so good but if you are just using it to compare different sound levels at the same frequency then you would not notice it's deficiencies.
 

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clicky, thanks for the detailed information! I'll try the cap and microphone replacement mods. As is, using REW, there is indeed a significant and unexplained drop after 7-8khz.
 

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Glad to be of assistance Eskim. If you do replace the mic insert then you will need to find a small rubber grommet to secure the WM61A into the microphone housing because it is only 6mm diameter as opposed to the original which is 9mm. Make sure that the back of the mic insert is flush with the back of the housing because if it any further forward then it seems to cut the high frequencies.

I would interested to hear what results you get from yours after modification.

I have drawn out the complete circuit diagram, if you or anyone else wants it then let me know.

At the moment I have done some very extensive modifications with my meter to give it an extremely flat response and although I have suceeded it does tend to go unstable as soon as I turn my back on it, so it looks as though the quick fix that I outlined above may be the best compromise.

Incidentally with all SPL meters, the specification of the C weighting curve does show a gradual decline after 8kHz so in theory it shouldn't match the results that you get with REW.
 

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I am returning this unit and have purchased a Galaxy Audio CM140 CheckMate SPL Decibel Meter.
It should arive next Thursday or Friday. I hope to have better results with the new meter.
 
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