Thanks again to all who had suggestions regarding my
"sticky and sluggish" multimeter. The bottom line is
that the meter movement itself was bad. In the
process of "solving" this problem, I found that all
76A's were NOT created equal - at least in the "meter"
department.
METER DIFFERENCES
Apparently there were two (2) different types of
meters used in the 76A series. One type uses an "iron
vane" type of movement (moving magnet) and the other a
"moving coil" movement (D?Arsonval and a variation
known as "Weston").
The "iron vane" movement is very simple and
inexpensive, has an internal resistance of about 1000
ohms and a usual accuracy as good as 5% or so. The
"moving coil" movement is much more accurate (around
2%) and typically has an internal resistance in the
100 ohm range. Some (a few?) "early?" 76A's used the
"iron vane" type and mine is (or was) one of them! My
76A was apparently manufactured in May of 1978.
NOTE: In the 76A, the different meter types are NOT
directly interchangeable "physically" due to different
mounting brackets and, changing from one type to
another MAY require changing a resistor and
re-calibrating the wattmeter.
HOW TO TELL WHICH TYPE YOU HAVE WITHOUT TAKING IT
APART - of course - taking it apart is the only way to
know "for sure" but, as a pretty good guess try the
following:
Look at the front panel of your 76A. The "iron vane"
type meter USUALLY has the "zero set" hole drilled
ABOVE the label for the PLATE CURRENT and the
MULTIMETER and if you look in the hole, the set screw
itself has a single slot. In addition, the multimeter
face has a small red rectangle at the top between "DC"
and "AMP" and the red label at the bottom is marked
"RF WATTS." The red markings for watts (beyond the
left hand end) has the first mark at .1 and the second
mark at .5 and the third red mark is 1.0.
The "moving coil" type meter USUALLY has the "zero
set" hole drilled BELOW the label for the PLATE
CURRENT and the MULTIMETER and if you look in the
hole, the set screw itself has two slots perpendicular
to each other. The meter face itself does NOT have
the red rectangle at the top, is marked "RF KW" in red
at the bottom and the first mark is .03, the second
mark is .1 and the third mark is .5.
Please note the "usually" as there may be many more
differences than those noted and those noted do not
necessarily provide the final answer!
WHAT'S REQUIRED TO REPLACE THE "OLD" METER
Replacing an "iron vane" type meter with the better
(and apparently somewhat more available) "moving coil"
meter requires removing the front panel, drilling some
mounting holes in the rear mounting bracket and
enlarging the mounting slot in same. The front of the
"new" meter face is the same size as the old one and
fits perfectly into the hole in the front panel.
NO CHANGE to the front panel is required UNLESS you
want access to the "zero set screw" in which case you
will have to drill a new hole in the front panel. It
will be just about in the middle of the word (at least
in my case) "MULTIMETER." I did NOT do this as once
set to zero, the new meter did not require any
additional adjustment.
If you choose not to drill, the "zero set screw" will
have to be removed from the new meter otherwise it
sticks out in front of the meter housing too far and
will hit the front panel when it is attached. In my
case, if I need to re-zero the meter in the future -
I'll have to remove the front panel - not something
I'm really interested in doing very often but not all
that hard either.
In addition, one resistor (indicated as R27 on the
"old" schematic) MAY have to be changed from 1 k to
1.5 k in order to accurately meter the grid current
AND the wattmeter WILL have to be re-calibrated.
PHOTOS
Once the "conversion" is complete, it's hard to tell
that the meter has been changed and the front looks
pretty much the same as always - 'course now the meter
actually works and that's very nice! I took a number
of photos of the "conversion" process of the
multimeter in my 76A (including the rear mounting
bracket modifications) and am currently looking for a
place to post them on the web so those interested
would have easy access to them. Please drop me an
email if you have any suggestions.
My thanks to Bob Larson of Larson-Metercraft
(http://www.larsonmetercraft.com) for quickly
identifying the different meter movements (this guy
knows how to put the "wiggle" back in your meter!) and
to Anthony and Glenn at ALPHA POWER
(http://www.crosslinkinc.com) for their help in
obtaining a new meter.
The usual disclaimers apply: YMMV, no pecuniary
interest in either Larson-Metercraft or CrossLink,
etc. etc. 73 es gud dx --Don N4HH
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