Radio Shack 33-2050 Sound Level Meter - Service

[MartinQ]

I have a Radio Shack (Realistic) 33-2050 sound level meter that is on the blink and it looks like something is wrong with the analog meter itself. The schematic on Eric's site is a bit hard to read (http://mysite.verizon.net/tammie_eri...50/rs_spla.gif), but it looks like the meter is rated for 135 uA and 2k ohm. This one measures in the megaohms and putting a 9V battery across it (9.22V) only provides about 15% movement on the needle.

The only markings on the meter are TEC M-700B. The card inside is printed 700B-91 (assuming that means 1991).

Any suggestions (except 'get a new one') would be great. I would hate to put down a good piece of gear especially after it's been cleaned and all the contacts coated with DeoxIT. I was just getting ready to swap out the capacitors as described here:

RS SPL METER (33-2050) MODIFICATIONS

[MartinQ]

I found a slightly more readable schematic that was uploaded by Salas in an old thread.
SPL meter corrections?

So, it looks like the 2k ohm meter is in a voltage divider with R19 (82k) for the battery check circuit. That means I should be getting full-scale deflection with less than 300 mV.

[Conrad Hoffman]

Well, if the meter was good before, putting a battery across it is guaranteed to burn it out instantly. My guess is it will be near impossible to find a new meter for the thing. The only distant hope is that the meter developed a bad connection somewhere that you might be able to find and fix. It's a very distant hope. Use a magnifier and inspect any wires and coils you can see for breaks or oxidation. One *never* checks a uA or even a mA meter without putting a large resistor in series with it.

CH

[MartinQ]

Hi Conrad, yes I understand the meter should blow by having 9V put across it. I was trying to find out why I could not get any significant deflection on the meter, even for the battery check (2k/82k voltage divider with a 3k load across the battery). Like I said, the meter measures in the megaohm range so putting 9V across it has little effect.

I'll try to post some pics, but so far everything tests out as I stated. I can get the meter probes right down to where the tabs connect with the coil, but it's still reading 1-3 megs. I can't see the wire connections with the bare eye, so I'll see if I can get some macro photos to post.

[MartinQ]

A few pictures of the TEC M-700B meter.

[Conrad Hoffman]

Nice photos & you have as good access to the coil as you're going to get. About all I can suggest is reheating the joints where the wire attaches, otherwise the open must be in the coil somewhere. Try to get it under a low power stereo microscope- when working with hi-fi always try to use a *stereo* microscope. If that doesn't work, I'd look for a meter movement from whatever you can find, and try to adapt it to the case. If it's the same sensitivity or more, you can just scale it with a resistor. If less, more fiddling is involved. Probably sacrilege, but you could also make an adapter plate and put in one of the little inexpensive digital meters various places sell.

CH

[MartinQ]

I pulled the coil out to have a better look at things. Even with it out, it's impossible to see the coil connections with the naked eye so I did some snooping with a cheap USB microcam. I managed to get some up-close visuals on both ends of the coil, but I can't detect any problems or see the actual connection. I think a proper microscope would definitely help.