HM-2140 issues

[Kenneth G. Gordon]

On 19 Jul 2009 at 9:49, Glen Zook wrote:

> Stupid question, 

No. It isn't.

> but you are making this reading when connected into a
> dummy load?

Yup. I am using one of those B&W units that Fair Radio was selling 
for very low prices a few years ago. These are the units that were 
used with the T-368 transmitters, and there is a 600 watt dummy 
load included in it. I think I paid $25.00 for mine, but they cost 
$125.00 now from Fair Radio.

>  If not, then you have to subtract the reflected reading
> from the forward reading to get the true power.

Yup. I know. 

However, the main question I have is what, exactly, is the difference 
between Peak Power, and Peak Envelope Power...if any. 

As I remember it, those are NOT equal. In fact, for normal voice 
work (SSB) I vaguely remember that PEP "can be taken to be" 
twice the PEAK power.

Therefore, 600 watts PEAK power (carrier or CW) would be 1200 
watts PEP on SSB.

Is this correct, or am I all wet?

> If you are testing with a dummy load then I would suggest measuring
> all of the resistors in the unit with an accurate DVM.  "Shot gunning"
> is going to be the most productive way because there are possibly
> several resistors that have changed value.

Unfortunately, I agree with you. :-(

I was, fortunately, able to find a schematic and description, of the 
(formerly) sealed calibration unit.

There are 4 pots in there: 2 of those are used to calibrate the high 
and low FORWARD power (output) conditions of the meter, and the 
other 2 are used to calibrate the REFLECTED power.

Furthermore, the pot labels were handily silk-screened on to the 
circuit board in the calibration unit.

Some previous owner of my unit had broken the seals and for 
whatever reason, had cranked the high-power calibration pot, R-
106, over against the high-end stop. :-(

Using my B&W dummy-load/wattmeter as the standard, I adjusted 
R-106 until the two read the same at 600 watts, then checked it on 
80 meters on the low settings as I know that my SB-102 puts out 
almost exactly 110 watts in tune on 80.

Both meters read very close to 110 watts.

I don't know the calibration accuracy of the B&W unit, but at least 
now my HM-2140 is far more accurate than it was.

At some time in the future, I will fire up the (calibrated) oscilloscope, 
carefully measure the dummy load resistance, and get the 
calibration of the HM-2140 as close to accurate as I can.

But this still leaves me with the PEP vs PEAK POWER question.

The HM-2140 and units like them actually only measure peak 
(carrier) power, and not really PEP if I am correct in what I say 
above, since the calibration procedure has you set the various pots 
in the meter unit so that at full-carrier output (tune) the "PEP" 
reading is the same as the AVERAGE reading.

All the circuit does is compensate for the intertia of the meters.

Ken Gordon W7EKB

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