Hi guys. I'm new to the forum and I could use your help. I have a test box that is used to measure volts and ohms of some of the components in my car's airconditioning system. The A/C system has a number of servo motors that uses potentiometers to transmit their rotational position to the box via the meter. Furthermore, the voltage to the servo motors are also displayed on the meter. An object fell on the meter damaging it beyond repair. The box does a lot more than what a multimeter can do. I purchased the test box three years ago, unfortunately, the company that made the test box has gone out of business. Here's where I need the help of the forum. In addition to replacing the now inop meter, I would like to use a digital volt/ohm panel meter instead. Could someone please tell me how to go about sourcing one of these digital panel meter and if there's any wiring modification that needs to be done? The analog meter has two wires going to it. A flip of one of the panel switches allows me to read servo voltage or variable ohms from the potentiometers. The volt meter scale has 0-10. The pot scale is 0-100, this number is unique to the car's A/C system. By the way, the tag on the back of the analog meter has: SPD 50VA. I apologise for writing such a long post. I'd rather give the forum more information than too little.

Regards

Patrick

 

Can you take a jpeg of the damaged test box and the meter and post it here?

hgmjr

 

What is the brand name, company name and model number of the box?

 

Hi guys. Thanks for the reply. I will respond to this post and the one above about taking a picture of the test box. First off, the test box is a one-off. It was made by a company in England. It's not an item readily available here in the US. Nevertheless, the components are "off-the shelf" items that can be bought at any decent electronics store. Having said that, I have since removed the panel meter and sent it off to a friend to have him take a look at it. I took a picture of the meter before the damage occurred and have attached it for everyone to see. I previewed the image but it seems a bit small in spite of image size. Perhaps someone could enlighten me on how to upload an image within the constraint of the rules.

 

I couldn't see where you're from, but there should be lots of guys can help you with a digital indicator.

 

Hi,

There's probably only one pair of leads running to the meter movement. The big mystery is the movement sensitivity and wheather it's a voltmeter or ammeter.

Grab the alalog meter's wires with a digital meter's leads and switch to volts. Hook the inputs to the test box to a known voltage - like 12 VDC. See what the digital meter reads. Try different ranges. If it's always low and poorly scaled, try switching to a current setting. If the reading seems to be more meaningful, then the old movement might have been something like a milliameter.

The fun part is that the old movement might have had some off full-scale response, so calibration will become an issue (so you can read the digital meter and tell what the old meter movement would have indicated). You'll have to check several known voltages and resistances to establish that.

 

Thanks. Ok, here's a bit more information. The meter, as you have suspected, has two lugs for a pair of leads. One of the lug has a "+" below it. There's no mystery about the meter's purpose, it's a voltmeter. The voltmeter scale tops out at 10vdc. Furthermore, the upper scale, as you have again confirmed, is used for, "off full-scale," readings from two potentiometers in the circuit. To be more specific, the upper scale reads output voltage from the pots in the circuit. The pots are mechanically connected to a geared servo motor. When, for example, the servo motor is energized, an arm connected to it moves a flap in the airconditioning system. The movement of motor, via the pot, is indicated on the upper scale. The lower voltmeter scale is used to measure various voltages going to the servo motor. A switch on the panel allows me to read servo motor position via the upper scale and voltage to the servo via the lower scale. In other words, the meter is a dual purpose meter. By the way, the pots are also used to calibrate the meter. A selector switch allows one pot (250 ohms) to "zero" the meter on the upper scale. Another pot (10kohms) allows me to set the upper scale to the "100" mark. With this additional information, it seems like a typical panel meter could do the job, since there are provisions already built in to the test box to, (1) limit the input voltage to a maximum of 10vdc. (2) the pots are used to calibrate (set) the upper scale to read correct servo motor position. (0-100) Now then, here's the big question: if a digital meter were to take the place of the analog meter, would it be safe to say that the digital meter would respond the same as the analog meter to the inputs and adjustments for accurate readings? If I had a working analog meter, here's another idea: Install the meter and connect the test box and select volts, for example. Let's say the analog meter reads 8vdc. Power down and exchange the analog meter with the digital meter. Power up the test box again and see if the digital meter reads 8vdc also. This is only a theory because of the demise of the original meter.

It seems you have a very good grasp of what I'm trying to accomplish and what obstacles I need to overcome to do the conversion. I hope the above information sheds a bit more light on the problem for both you and the rest of the forum. I've been reading about meters and electronic circuits in general on the web site. I learn something every day from you folks. You seem to know a lot about something you can't see that affect our daily lives in more ways that we can count.

I've updated my profile for everyone to see.

Regards

Patrick

 

Howdy, if you can't sort this out any other way, check out Ram Meter, Inc. Instrumentation and metering are their specialty.