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[ajmogis]

I'm building a remote control for an EMT140 plate from the schematics at danalexanderaudio.com.  It seems pretty straight forward, except the value on the decay time meter.  All it says is 377.5uA.  Is this indicating a maximum current tolerance?  Current for full deflection?

Any insightful input is appreciated.
-AJ

[David Kulka]

This spec normally refers to the amount of current that will produce a full scale reading.  Typically, small meters require 1 milliamp for a full scale reading, so you need a meter that's a little more sensitive than the average model.  But it should be fairly easy to find, I'd try Nebraska Surplus Sales, Digikey, Mouser, Ebay.

Of course, the meter will not have a scale that reads in seconds, like an original EMT meter.  Full scale was about 5 seconds, I believe.  You might be able to print your own custom meter scale and glue it inside the meter.  Or maybe you can find a meter that reads "5" full scale, and that would be pretty close.

If you can't find a 377 uA meter you could use one that is more sensitive, and shunt it with a resistor.

[ssltech]

Or just change the resistances around the meter...

Don't go too crazy looking for a meter scaled to '5' though... the EMT scale was highly non-linear, after all, and still won't match the typical (evenly-spaced) moving coil meter scale... you'll have to print your own, either way.

When I first came to town, the local NASA surplus place had some WONDERFUL reversible DC gear-headed motors in stock, and I schlepped one back to the UK to the studios that I worked at over there. We built our OWN EMT remote using a nice big (4" face) meter and a spring-centered toggle switch. One of our consoles already HAD a remote, the other was manual. We just coupled a pot onto the shaft of the plate control, wired the motor from a reversing circuit triggered by the switch, then  used a variable resistors to series/shunt the meter until it read FSD at maximum setting, and rest position at minimum... then wrote our own faceplate with reference to the scale engraved on the plate box.

WE also added a couple of microswitches, and placed them so that the "screw-head" on the coupling shaft would open the microswitches, one at each end of it's travel, then we wired the microswitches as 'Endstop' limits, which basically broke the two wires coming from the up/down switch.

Easy, cheap. I think the whole thing cost about $15 ($10 of which was the motor!). You can do it the EMT way, but it's  basically just a geared reversible (usually DC) motor, a pot a meter and some resistors... you don't have to do it EXACTLY like EMT did, and there's no difference in the sound after all!

You want a reversible gear motor which does approx. 1RPM... that way the full 90

[David Kulka]

Keith, good points.  I thought of suggesting that the OP adjust the resistor and trimmer values in the circuit, but I thought it might get too confusing.  AJ, if you are comfortable with electronics and are into spending some time experimenting, this would give you more options.

All those resistors adjust the calibration and tracking of the meter but its accuracy isn't critical to most people.  Maybe it's a little like a fuel gauge in a car - knowing at a glance whether you're 1/4, 1/2, or 3/4 full is enough, you don't need to know how many gallons remain.

Attached is a jpg of the original EMT meter, which may be helpful.  You can see that the scale is kind of spread out at the top and bottom.

[ajmogis]

The saga continues...

It's been a while since I've had time to look at this.  The damper has been "stuck" at a usable decay time so this project got put to the back burner.

First of all, thanks for the helpful input.  You're right David, I'm not concerned with incredibly accurate metering.  Just something to be able to tell vaguely where it's at.

The whole reason I asked about the meter is because my remote was not working sans meter.  I thought perhaps this circuit simply isn't happy without the meter in there.

It turns out that the motor seems frozen.  Has this ever happened to anyone? Should I be able to move the motor by hand with the power off?  The relays are clicking but the gears are not turning.  The whole thing feels stuck.

Any thoughts appreciated.

-AJ

[John Monforte]

Just pop the motor off and be sure the damper mechanism moves freely. The basic unit just had a big knob there.

If thats good you trace it down between the motor and gears

[ajmogis]

The damper itself moves freely.  The motor and gear assembly isn't turning.  Looks like I'm going to have to take it apart and see what's happening.

[ajmogis]

It's definitely the motor.  I've got the whole thing disassembled and the motor is frozen solid.  The gears that run the limit switches and meter pot move just fine.

The motor assembly seems to consist of the motor itself (silver) on top of a gear box (black).  The whole thing won't budge.  I was unable to remove the gear box to see if the problem is in there.

Input from anyone with ideas is greatly appreciated.

-AJ