Evaluating Analog Meters for use with MB-1

    This section provides some information to help you determine if a particular analog meter will work with MB-1.

    Any DC meter with a full scale rating of 1 mA or less will work with MB-1. It's important to realize that lots of meters with all sorts of ranges and units may actually fit this bill. For example, a meter with a "150 kilovolt" scale, when tested was found to be just a 1 mA Full Scale meter movement. No mods were required whatsoever to use that meter. Obviously, that meter was not meant to be connected directly to 150 kilovolts!

    By programming the 150 kV meter with three power ranges (15w, 150w, and 1500w), the 150 kilovolt meter face can be used with no modifications, and turned out to be a nice match for use with an AL80B amplifier, which is capable of peaking at approximately 1100 watts.

    The meter shown below was obviously used in some type of radio application. In spite of its scale markings, this meter is just a 1 mA full scale DC meter movement and will work perfectly with MB-1. This meter has a linear meter movement and also has a linear scale. If you do not want to design a custom meter face, this meter could also be used with no modifications. You would calibrate it using the Linear Scale option in the Panel Meter setup procedure. Linear scales are very easy to calibrate since you only need to calibrate them at a single point - namely at the full scale deflection point.

    If you wanted to use the meter face below directly, one option is to use the lower scale for power, and set it up for three power ranges: 30 watts, 300 watts, 3000 watts. If you are a QRP operator, another option would be to set up three scales with the following full scale values: 3 watts, 30 watts, 300 watts.

    The top scale could be used as an SWR scale with a full scale SWR value of 5. Although a bit unconventional, MB-1 can use 0-based scales for SWR readings.  Without this feature, you would not be able to interface any 0-based scales to MB-1 for SWR measurements. This is handled by having the software idle the needle at a reading of "1" for 0-based scales when the panel meter is reading SWR. (This is discussed in detail in the User's Manual.)

    There are lots of other variations that could also be used directly with the meter below without the need to create a custom meter face.



Determining the Full Scale Value of a Meter Movement

MB-1 provides three rear panel RCA jacks to aid you in determining the full scale current rating of a meter movement. There is a 50 uA current source, a 200 uA current source, and a 1 mA current source. Each current source is implemented using the 5 volt bus in series with a suitable resistor such that the short circuit current value equals the specified current value. But since the resistance of most meter movements is generally low compared the series resistor in the current source, connecting a current source across the meter movement will essentially generate the short circuit value.

The 50 uA and 200 uA sources are labeled directly on the rear panel jacks. To approximate a 1 mA source, use either of the two FR POT (Front Panel Pot) RCA jacks on the rear panel and turn the front panel pot to its maximum CW position. This will apply 5 volts through a 5.1K resistor to the FR-POT RCA  jacks yielding a short circuit current of approximately 1 ma.

The following are the full scale ratings of popular DC meter movements:

Meters with the above full scale ranges (or anything in between) will work with MB-1. Meter movements with full scale value of less than 30 uA will also work, but we have not come across many of those.

Most meters that you will get from eBay or other surplus sources will not have a clearly marked full scale current rating of the meter movement (although knowledgeable sellers may be able to provide you with this information).  To do a quick check on a meter, connect the 50 uA source across the meter terminals (the tip of the RCA connector is the positive side). If the meter movement reads approximately full scale (without pinning the needle), you have identified the full scale value (50 uA). If the needle deflection is half scale or quarter scale, you have a 100 uA or 200 uA meter movement respectively.

If you get very little deflection with the 50 uA source, try the 1 mA source.

If you get some needle deflection with the 1 mA source, but the deflection is very small, there is a good chance that there is an internal resistor in series with the meter movement (common on meters with a DC voltage scale) or a shunt resistor in parallel with the meter movement (common on meters with a DC amps scale).  If you are careful, you can disassemble the meter, and remove (short) any series resistors, or remove (open) any shunt resistors across the meter coil. After modification, recheck the meter with the MB-1 current sources to determine if you have converted the meter into one that will work with MB-1.

Even some meters with AC voltage and AC current scales may use DC meter movements internally. Such meters incorporate rectifiers and other components to convert the AC signal to a suitable DC signal to drive the DC meter movement. In these cases, you can remove those components and connect the meter lugs directly to the meter movement coil. 

Of the dozens of meters purchased on eBay and other surplus sources for experimenting during the development cycle of MB-1, most of the meters with a DC marking were found to be DC meter movements with a full scale value of 1 mA or less. We also found that many meters marked with AC voltage scales were also actually DC meter movements with additional components such as rectifiers. Most of these meters were suitable for use with MB-1. It is probably a good idea to avoid meters with AC current scales unless you have reason to believe that the internal meter movement is a DC movement.

There are some really large meter movements that will work fine with MB-1 and will give you unmatched readability and resolution. Of course, the needle inertia will keep the needle of these large meters from responding as quickly as the needle of a smaller movement, but for peak power, average power, key-down, and SWR readings, the slower needle response is not an issue.


Modifying Panel Meters for use with MB-1

A number of different surplus meters
are tested below and modified where
possible for use with MB-1.


DC Volt Meter

Initial Test - 1 mA source does not drive meter to full scale.

Internal View - Series Resistors were found and removed.

Final Test  - The meter now shows full scale deflection at 50 uA. The meter movement is therefore a 50 uA Full scale movement and can be used with MB-1.


AC Volt Meter

Initial Test - 1 mA source shows no deflection.

Internal View - A Rectifier and Series Resistors were found and removed.

Final Test  - The meter now shows 20% deflection when connected to the 50 uA source. The meter movement is therefore a 250 uA Full Scale movement and can be used with MB-1.


DC Ammeter

Initial Test - 1 mA source shows no deflection.

Internal View - A shunt (copper bar) was found and removed (cut). The series resistor was also removed for the final test below.

Final Test  - The meter now shows 10% deflection when testing it with the 1 mA source. The meter movement is therefore a 10 mA Full Scale movement and cannot be used with MB-1.



AC Ammeter

Initial Test - 1 mA source shows no deflection.

Internal View - There was no obvious rectifier and meter coil. Therefore, no attempt was made to modify this meter.



Meters that passed the test with no mods Required

Top Left - The huge Triplett Meter is a 1 mA Full Scale meter movement.

Top Right - The Micro Mhos meter is a 1 mA Full Scale Meter Movement.

Bottom  - The Meter Movements (FWD and REFL) in the Daiwa 801 are 100 uA Full Scale Movements (50% deflection when connected to the 50 uA current source).


 Analog Meters: eBay searches for Analog Meters