View Full Version : Panel Meter Repair

K8BAR

05-22-2004, 02:34 PM

Please Help; Does anyone know of someone or of a company that can repair a grid meter for a Drake L-4B amplifier? Or maybe a NEW meter or a USED meter http://www.qrz.com/iB_html/non-cgi/emoticons/confused.gif

Any hepl would be appreciated.

Thanks, Bill

K9STH

05-22-2004, 03:16 PM

Just what is wrong with the meter?

There are some things that you can do yourself whereas other things require a specialist (if they can even repair the meter!).

Glen, K9STH

K8BAR

05-22-2004, 04:02 PM

The Grid (Multimeter) on my Drake L-4B will not stay zeroed. It work after a fashion, but going below zero really bugs me. I have tried to zero the meter as instruction in the manual, but it just doesn't work. The only solution is repair it or find another meter.

Thanks, Bill, K8BAR

K9STH

05-22-2004, 05:51 PM

When you zero the meter is this when the rig is turned off or with it on? Believe it or not, that does make a difference. When you answer that, I can better tell you which direction to go.

Glen, K9STH

K8ERV

05-23-2004, 01:01 PM

I do MINOR meter repair. Neither a job nor a hobby, and don't particularly like to do it. A sticky needle means there is some mechanical interference. If you are lucky, it is the needle touching the faceplate or glass. If you are not lucky, it may be junk in the magnetic gap, which can sometimes be cleaned out. If you get really desperate, send the meter to me and I will try. Tom K8ERV

K9STH

05-23-2004, 02:35 PM

I have been in contact with him on the Drake reflector. It seems that the meter is working but he doesn't believe that it is reading correctly. As such, I sent him the following:

I doubt very much that the meter itself has changed calibration. That almost never happens! The problem is almost certainly to be in the meter shunt (a resistor that is put in parallel with the meter when it reads current and another resistor is put in series when the meter reads voltage).

Meter shunt resistors can change value over the years for any number of reasons. Or, the contacts on the meter switch can become "dirty", no longer make connection, etc.

I would check the meter shunts as well as the switch and associated wiring. If you replace the meter movement I am fairly certain that you will still have the same problem.

Various shunts have to be used with a meter to allow it to read different currents. Basically, the meter movement will be something like 50 uA, 1 mA, or something like that. The shunts then allow most of the current to go through the resistor with only a small portion going through the meter movement itself when reading current. When reading voltage, the series resistor drops the voltage (per Ohm's law) and the resultant current is read. This current is then calibrated as a voltage reading even though you are actually reading current. When the resistance of the shunt changes then the meter will not read correctly.

If the switch is not making proper contact, then the appropriate shunt is not in place and the meter will not read correctly then as well.

Glen, K9STH

K7KBN

05-23-2004, 10:14 PM

Little picky note: #it's a "shunt" when it's connected in parallel with the meter movement (to produce an instrument that can be calibrated in terms of DC current). #

If it's inserted in series with the movement to limit the current that flows through the combination, it's called a "multiplier" (to produce an instrument that can be calibrated in terms of voltage, whether DC or AC).

It's an odd bit of irony, I guess, but a DC ammeter responds to VOLTAGE developed across a shunt resistor, and a voltmeter responds to CURRENT flowing through the resistive combination of the movement resistance and the selected multiplier resistor. What a world!

K9STH

05-24-2004, 03:57 AM

KBN:

A shunted meter does not read the voltage across the shunt. A meter can only read current. If you use the formulas for finding the value of a meter shunt you will see that the shunt carries most of the current while the meter movement carries the remainder. Adding a resistor in series only causes a direct reading on the meter scale showing the relationship between the current and the voltage.

For example, when a meter is "shunted" to allow reading a higher current, if the meter has a resistance of 100 ohms and the shunt is 100 ohms, then 1/2 of the current will go through the meter. If the meter movement has a full scale of 100 mA then the total current running through the circuit (comprised of the shunt and the meter) will result in the actual current of twice what the meter actually reads up to a maximum of 200 mA at which the meter will be "pinned".

If a shunt of 11 ohms is placed across the meter then the total current of the circuit as read by the meter will be 10 times the reading (within 1 percent).

This can be calculated by the resistance formula of (1)/((1/R1)+(1/R2)). 1/R1 where R1=11 ohms = 0.090 1/R2 where R2 = 100 ohms = 0.010 ohms. Therefore, (1/R1)+(1/R2) = (0.090)+(0.010) = 0.100

(1)/(0.10)=10 giving the multiplication factor of the "shunt".

Therefore, the ratio of current between the two resistors is 9:1. That is, if 900 mA is going through the shunt, then 100 mA is going through the meter. Since the maximum meter reading is 100 mA, then the current through the total circuit is 1000 mA or 1 amp. Thus, the "multiplication" factor is 10 with the meter reading 100 mA when the total current through the circuit is 1 amp.

Now, if a resistor is placed in series with this same meter then you can get a correlation between the current reading and the voltage. However, especially at lower voltages, you have to consider the effective resistance of the meter as well as the value of the resistor in series. Thus, if you place a 900 ohm resistor in series with the 100 ohm meter movement, the total resistance across the circuit is 1000 ohms. As such, using Ohm's law, the maximum voltage that can be read by this combination is (100 mA) x (1000 ohms) = (0.1)(1000) = 100 volts.

In the "real world", the meter movement often has a maximum current reading of 1 mA (or even less, like 50 uA). With a meter resistance of 100 ohms (not that unusual) and a series resistance of 99,900 ohms, the effective resistance of the circuit is (99,900)+(100)=100,000 ohms.

Again using Ohm's law, the maximum voltage which can be read with this arrangement is (100,000) X (0.01)=1000 volts. For "average" use, once the series resistance gets to be between 20 and 25 times the meter resistance, you can usually eliminate the resistance of the meter from the equation and be well within 1 percent. Now, for lower voltage readings you definitely do have to enter the resistance of the meter into the equation (and for more accuracy do this even at higher series resistor values).

To keep from loading down the voltage, a meter with the lowest possible maximum current reading, which allows the use of a larger series resistor, shoud be used. That is why many VOMs, etc., use meter movements like 50 uA. For a meter calibrated for 1000 volts maximum reading, again using Ohm's law, the total resistance through the circuit would be (1000)/(0.00005)=20,000,000 ohms. At this resistance, a meter effective resistance of 100 ohms is right at 0.0005 percent and, for all practical purposes can be ignored. Also, the loading on the voltage source is going to be minimal at this resistance and the voltage being read is going to be very close to the actual voltage.

This would represent a loading factor of "20,000 ohms per volt" which is what many of the old "analog" VOMs are rated. The calculations are just the total value of the series resistor divided by the maximum voltage to be read. Thus, (20,000,000 ohms)/(1000 volts)=(20,000 ohms per volt).

Glen, K9STH

K7KBN

05-24-2004, 06:47 PM

Glen -

If you take a look at a switchboard meter that's calibrated in terms of, say, zero to 500 DC amperes, you'll see somewhere on the meter face or on a label on the back of the meter, a notation reading "FS=50 mv" or "FS=100 mv". #This means that, if you were to put 50 mv DC (or 100 mv DC) across the meter terminals, the meter would deflect to full scale. #Now, this meter is calibrated in terms of AMPERES; it's an AMMETER; if I understood your last post, you're saying that it WON'T "read" voltage. #This is wrong.

The meter will be connected to a large resistor. This resistor is the "shunt", and it is itself an "instrument" that requires periodic calibration along with its associated meter. #The calibration process passes a known current through the shunt, and the voltage developed across the meter terminals on the shunt is measured. #A "50 mv" shunt will develop 50 millivolts when its rated current (in this example, 500 amperes) flows through it. #The "standard" lead resistance is 0.026 ohms, and in the metrology laboratory/calibration facility, you'll see several sets of "standard leads" wherever DC ammeters and shunts are being worked on.

I agree that the meter movement operates on current, but as a practical matter all DC switchboard meters (over about five amperes) are really DC millivoltmeters, which respond to the millivoltage developed across the shunt.

I've worked in metrology for the Navy, doing just what I've described above, for quite a few years.

A meter will "read" whatever it responds to; it depends on the user to use the meter's indication correctly. #In the case of the 500 ampere DC meter and shunt, if you were to pass a constant 500 amperes through that shunt and you connected a digital voltmeter across it, you would read 50 millivolts and not some value of current. #It would be up to you to determine that if 500 amperes = 50 millivolts, then 300 amperes = 30 millivolts and so forth.

K9STH

05-24-2004, 08:09 PM

A meter can be "calibrated" to read voltage. But, it can only actually read current. You can also calibrate a meter to read ohms. But, in reality, all the meter is reading is current. That current is passing through an external series resistor (or, in some cases, just the internal resistance of the meter) when reading "voltage" (Ohm's law where E=IR). It is reading current when used either in series, or in series/parallel with a "shunt", between the source and the load. It can be calibrated to read ohms when a known voltage is applied and the meter scale calibrated in "ohms" rather than the true current it is reading (E/I)=R.

No matter what a meter "says" on it, all that the meter can really do is to read current. Now, by various combinations of outside components (outside meaning different from the meter movement itself, not that other components - like shunts or series resistors - cannot be included inside the case of the meter which is often done), the meter can be calibrated to read voltage, current, ohms, temperature, revolutions per minute, light intensity, sound intensity, and any number of other applications. But, no matter what the calibration on the meter scale, the meter itself is only reading current! It is the other components that determine what that current can be interpreted to mean.

As such, I stand on my previous statement that meters can only actually read current. However, they can definitely have their scales calibrated to read other things. But, in reality, all that they are reading is current.

Glen, K9STH

RH_tech

09-06-2010, 08:59 PM

My rule of thumb with meter problems is to turn it over to a shop (unless it's just bad electronic components -- resistors, etc. -- that I can easily swap out). The mechanics and technology of meters are simply too delicate and arcane for a non-pro to safely engage, I think. Screwing around with the fragile meter movement, microscopic springs, pinpoint pivots, etc. is an invitation to disaster.

Something as simple as nudging a spring slightly out of shape, or as inconspicuous as a speck of dust or flake of metal (attracted to the magnet), can trash the meter's accuracy, linearity or reliability -- often without you even realizing it -- and "simple, obvious" fixes often result in permanent damage to the movement. Stuff that "works fine" when you put it back together often has a way of deteriorating rapidly afterward.

After scrapping a few too many meters, and wasting way, WAY too much time, I've developed a healthy respect for the pros, and an admiration for how much they can do, fast, for a fair price.

I'd suggest getting a meter shop to look at it (or ask them to suggest a substitute). One of the shops that shows a lot of enthusiasm for repair of vintage meters is Design Development, LLC, in Wichita, Kansas (USA) (www.wilbacmeter.com (http://www.wilbacmeter.com/)).

They're the folks who have revived the Wilbac line of American analog panel meters, but their shop will work on just about ANY brand or style of analog meter.

They not only do meter repair, but also do really slick refinishing work, resulting in a meter that looks brand new, with really sharp, readable scales.

Design Development's operation also sells some good custom meters ("Wilbac" brand) that I can work on, myself, because the electronics (resistors, capacitors, etc.) are in a separate, screw-down-cover compartment, isolated from the meter movement. I can solder and swap components all day, and not screw up the mechanism (assuming reasonable heat-sinking, of course). They fit any old standard Wilbac or Simpson panel mounting hole, and most others it seems.

They have a bunch of other stuff, too -- shunts, transformers, and a huge variety of what they call "economy" meters (American) for "bulk orders", etc.

Nice folks with a good attitude, and a wealth of knowledge, it seems, eager to help.