The 4EX3A11 test set
Eventually the red PTT button wears out and will need replacing. One common mod (while you have the housing open) is to add a SPST mini-toggle wired in parallel with the button. This allows you to use the red button for momentary PTT, and the switch when you want to key the transmitter for longer periods or when you need use both hands for something else.
Yes, the mechanical zero on the meter in the photo needs adjustment.
If you are only going to test Mastr II, Mastr Executive II, or Custom MVP radios, then you can use the 4EX8K12 test set kit (LBI4436) which came in a smaller box and with permanently-attached red and black test cables. The Model 4EX3A10 test set (LBI3491E) or 4EX3A11 test set (LBI-4454B) has a connector on the side for the radio-specific cables. The meter circuitry in the 4EX8K12 is identical to the meter circuitry in the 4EX3A10 or 4EX3A11.
All three test sets used a specially made analog meter calibrated as a -1/2 volt to +3 volt movement, and the entire meter box was configured as a 20,000 ohms-per-volt voltmeter. There are two good reasons for this:
1) ALL of the metering points on radios of that era were set up to use a 20k-ohms-per-volt meter, and the measurement circuit uses the input resistance of the test meter as part of a series circuit. Without a load of the correct value none of the metering readings were meaningful.
2) There were no inexpensive portable DVMs when these radios were being designed - every two-way tech had a portable VOM (a Simpson 260, a Triplett 630 or the WW2 surplus military equivalent), and all were 20K-ohms-per-volt. Modern DVMs have a input impedance that ranges from 2 to 11 megohms (depending on the manufacturer and model, most are 11 meg) and without the proper 20k-ohms-per-volt load none of the book measurement values will be correct.
This means that YOU CANNOT USE A MODERN DVM TO PROPERLY TUNE A PRO, EXEC, EXEC II or MASTR II.
Two additional "gotchas" are that most inexpensive DVMs go crazy in
the presence of RF energy due to the fact that they are sold by price,
so to make a profit the manufacturers scrimp on the shielding, and
lastly, in every case you are tuning for a peak or a dip - attempting to
do that with dancing numbers on a DVM is an exercise in frustration -
you are constantly trying to determine if the new number is higher or
lower than the last number and your brain can't keep up with the DVM
display update rate. You NEED an analog needle meter to see the peak or
dip (and some of the peaks and dips are 'way too small).
The three links in the text above will download the manuals for the
4EX3A10, 4EX3A11 and 4EX8K12 test sets.
Additionally, here are the schematics for the 4EX3A10 and 4EX3A11
metering cables for various radios:
Porta
Mobile, Mastr-Pro and Exec 74 kb
Mastr II,
Exec II and MVP 68 kb
Delta
mobile 84 kb
Metering
cables parts list 40kb
With the information from the above LBIs you could design a 4EX3A10 and
4EX3A11 metering cable for almost any radio... even a Micor.
