The question was asked previously why the thermcouple heads
in the Boonton Q Meters were so fragile and prone to burnout
so easily. After looking around on the web, reviewing some old
notes in my library and speaking to a few retired techs and engineers,
it appears some answers may be available.
If you look at:
you will see more info on the Q Meter sensor head. The writer's
observation clarified a couple of misconceptions I had about the
sensor head. One was the mounting of the thermocouple. For
decades my assumption was the thermocouple was mounted
to the 20 milliOhm precision disc resistor. In reality it is mounted
on a bakelight disc shaped board. The writer at prc68.com
measured the wire diameter and reports it to be 46 gauge. That
works out to a wire diameter of just 0.0016" for the thermocouple.
Making the spot weld must have been fun in the 50's with that
small wire. It explains a lot about the sensitivity to shock.
(As an aside a PMEL tech told me that the HP/Agilent RF Power
Meters use a similar thermocouple with fine wire. As they were
smaller and more prone to shocked, the failures were and still
are frequent.)
Back to the Q Meter.
The thermocouple is heated by a short length of what is reported
to be nichrome wire. The writer at prc68 reports the wire is 37
gauge or 0.0045" in diameter. The nichrome wire heater and the
thermocouple both are mounted on the bakelight disc inside the
sensor head. and the thermocouple bead is welded to the heater
wire.
In looking at a chart which showed various nichrome wire gauges
and the expected temperature of the wire when a specified current
flows through the wire, 37 gauge Nichrome A wire would reach 1600
degrees F for 1000 milliAmps flowing through the wire.
Looking at the chart found here:
you can see a fairly wide range of temperatures for currents varying
from 350 milliAmps (400 degrees) to 1290 milliAmps (2000 degrees F).
The reported melting point for Nichrome A wire is 2,552 degrees F. The
current to reach 2000 degrees is only 29% over the current reported to
be flowing in the precision resistor in the Q Meter. If you examine the
scale of Boonton 260's Multiply Meter which is monitoring the current
through the nichrome wire, the meter is marked with a red zone that
begins at about 5% of the meter scale above the meter scale's "Multiply
by 1" mark. It appears that the heater operates at a temperature that is
was approaching 63% of the wire's melting point.
The "Multiply by 1" value on the meter scale appears at an estimated
80% position (1000 milliAmps) of full scale reading. If the meter? needle
position and scale is modestly linear, that suggests full scale reading of
the meter will be 1250 milliAmps. On my Boontoon 260, the "Multiply
by" factor control not only allows one to reach full scale, but? if careless
the meter can be pegged out. That may explain why so many meters blew
the heater wire and destroyed the thermocouple. If you look at the photo
(Figure 2) of the thermocouple sensor head at:
you can see the char marks on the Bakelite terminal board of an exemplar
thermocouple head.
Peter Olin, AI2V (reported to be a silent key) at one time provided repairs
and conversions of the 260 Q meter to include sensor head repairs. I also
remember that one conversion of the unit led to the vacuum tubes being
replaced with solid state active devices. From what I could determine most
conversions were driven by the lack of availability of the BR535 vacuum
tube (triode) used in the integral AC VTVM.? (I will repeat what I stated pre-
viously, the BR535 was a hand selected triode. A popular triode of the period
(I forget the industry part number for the tube) was purchased by Boonton
and each tube was tested for use in the Q Meter. Peter told me that culling
was dictated by an abrupt change in input impedance of the tube at 1 MHz.
Apparently there was a relatively frequent production error in manufacturing
the tube that caused the perturbation and Boonton's solution was to cull tubes
which presented with the undesired characteristic.
Another piece of information from my notes on the box is the resistors
R203 and R204 in the leads to the "Multiply by" meter are hand picked
at the time of installation of the thermocouple head. Apparently the
output voltage slope of the thermcouples varied and to compensate
for the delta between units, the resistor values were adjusted. Values
generally were between 30 ohm and 65 ohm. Equal value resistors
were used in each lead. The capacitors were in place to attenuate
RF that might couple into the sensitive "Multiply by" meter. The typical
0.01 uF value was chosen for service.
Many of the older units have a General Electric voltage stablilizer
block mounted on the floor of the box. In the last 20 years each one
I encountered either had failed or was in the process of failing. The
devices are enclosed in a stamped metal shell and filled with a 'GOO'
that makes a mess when the unit overheats. The GE boxes were
there to address line voltage fluctuations. Given the stench they
made when going up in smoke, I pulled the stabilizers out and use an
external UPS to provide well regulated AC power.
Please let me know if you notice any of the above info is in error and
also of any additions you might make.
Regards
Chuck WD4HXG
