[Collins] 312B-4 wattmeter query

[Dr. Gerald N. Johnson]

The RF wattmeter is symmetrical. The ranges are achieved with different 
values of resistance, set by selected resistors so each is individually 
calibrated.

Since a wattmeter can introduce some SWR, be sure to compare them with 
each driving and each driven. Even a Bird will introduce some impedance 
change (or especially).

The schematics tend to show a 290 microamp meter. That's very odd and a 
draftsman's error where the scale on the Leroy missed and he put a 9 
where there should have been a zero. The meter is really 200 microamps 
which is not very common but has been a standard movement.

The sticky meter is inside the meter itself and takes careful finesse to 
repair. Finesse that is about gone from meter repair shops. I have tried 
but the gains have been limited. Its possible its been open and there's 
been some iron filings that have snuck in its also possible that the 
pivots are blunted from it being dropped or the pointy pivot bearings 
have cracked. The mate for the sharp pivots are often doughnut shaped 
jewels. And they can be cracked by too much shock. A good meter man 
should be able to correct them.

You can exercise the sticky meter with a suitable variable voltage 
supply and series resistor. 5K ohms per volt series resistor. So for a 0 
- 10 volt supply, a 50K resistor will keep the meter from pegging. And 
can be a check of the basic meter's calibration. When its most moving in 
response to knuckle raps, its unlikely to be accurate.

I'm not sure why the meter that pegs on all ranges is doing that, could 
be its a replacement meter movement too sensitive, or the four factor 
selected resistors have been shorted or replaced with lower values to 
make it a handy meter for legal CB powers of 5 watts. Maybe the variable 
capaictors have (one or the other) have been shorted by lightning. That 
meter can be tested for sensitivity the same as the sticky meter.

The basic circuit was discussed in QST more than 50 years ago by Warren 
Bruene, then probably W0TTK, now W5OLY. ARRL's W1FB "simiplfied" the 
circuit which reduced its performance and that simplified circuit has 
been in handbooks for decades. The original article is titled: "An 
Inside Picture of Directional Wattmeters" April 1959 beginning on page 24
available to ARRL members on line or in good libraries and on CD-ROM.

There are two pages on how it works in the Collins Yellow Book titled 
"Fundamentsls of Single Side Band."

The two variable capacitors are set to minimize reflected power on a 
fine load each one set with the RF flowing the opposite direction. They 
could have been damaged or shorred by lightning. An ohmmeter check 
should show the coax line center conductor an open circuit to ground 
when not connected.

The coupler can be reversed with no problems, just reverses the switch 
indications of forward vs reflected. As far as I know, that's how the 
reverse power scales were calibrated by feeding it backwards, and the 
two variable capacitors were calibrated by feeding RF forwards and 
adjusting the one on the reflected circuit side for a minimum meter 
indication, then with the sensor reversed the "forward" was adjusted for 
minimum indication with the RF flow backwards.

Changing diodes would require new calibration resistors.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 5/6/2012 4:33 PM, Chris Kepus wrote:
>
> I have 2 B-4 units: one winged, one meatball.
>
> I haven't used either for some time so decided to test their wattmeter
> function before putting them "on-line".  Used a Valiant transmitter for RF
> power into a Bird into a 312B-4 into a Heathkit Cantenna. The results were
> very disappointing.
>
> The CCA site had nothing in the way of info to help.  The Collins list
> archives back through 2007 were also quiet on B-4 coupler problems or
> rebuilding. Google didn't turn anything up.
>
> Before I plunge into these units, I hope someone familiar with the B-4 can
> suggest some possible causes of the symptonms and/or the most efficient way
> to trouble shoot my #1 and #2 B-4's problems.
>
> First up to test was #1, the winged B-4.  The meter pointer at rest (no
> power in) is a cat whisker above zero.  With power applied, the meter
> pointer moved to indicate 75 watts (200 W mode) while the Bird indicated 110
> watts. Rapping the meter with a knuckle caused the meter pointer to advance
> to roughly 140ish watts. When power was removed, the pointer did not return
> to zero but stopped at about 30.  Hmmmm.  Rapping with a knuckle caused the
> pointer to return pointing near zero.  Repeated this about six times hoping
> the meter exercise would help but no joy. It appears the meter has a
> "sticky" movement.
>
> Second to test was #2, the meatball B-4.  At rest (no power in), the meter
> pointer is at zero.  With power applied (110 watts on the Bird), the meter
> pointer pinned above 200 watts (200 W mode).  Same result in 2000 W mode.
> Since the coupler is bi-directional with 200 and 2000 watt ranges for
> reflected power, I was tempted to input the power on the antenna side to see
> what the meter would read. Nothing on the schematic looked to be in harm's
> way if the input/output were flipped.  Anyone?
>
> As Al, W8UT, said in a very similar circumstance in 2008, "Any help greatly
> appreciated."
>
> Thanks es 73,
> Chris
> W7JPG
>
>