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Author Topic: Power/SWR meter issue.  (Read 3398 times)
KA0HVE
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Posts: 117




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« on: July 09, 2013, 08:29:52 AM »

I have a Swan SWR-1A that I bought new around 1980.  The power meter works but the SWR meter fails intermittently.  I checked my coax connector solder joints and continuity and they're good.  Last time the SWR meter failed I ended up bumping it on my desk and it began working again. (  Grin  ) Just before that I opened up the case and hit the SWR meter with a tiny voltage and the meter bounced so I think the meter itself is fine.  Bad component on the SWR side?

Anyway, another thing I was wondering about is that while adjusting the tuner for minimum SWR I would expect the forward power to increase somewhat but the power meter shows the power decreasing a bit when the SWR decreases (reflected power is reduced so it appears to me that forward power should increase).  Can someone explain that to me?  Does the power meter side of this particular model meter add the forward and reflected power together for the power meter measurement?  It doesn't say 'forward' power on the meter.

I'm in the market for a new SWR meter.  Any recommendations for one that also measures power up to maybe 10 watts?  (The OHR WM-2 sure looks nice but it's a watt meter and doesn't directly measure SWR.)

Thanks.
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K8AXW
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Posts: 3672




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« Reply #1 on: July 09, 2013, 10:10:12 AM »

HVE:  One thing at a time.

The intermittent - That type of problem usually means dirty contacts, poor solder joint, poor ground connection of a PCB, etc..... or a bad wire.

I know this looks like the old saying, "I have it narrowed down to a fuse or something else."

The best thing to do is clean all contacts....switches and pots.  After that, leave the box open and apply power to a dummy load, and take a pencil and start probing and pushing (with the eraser end) on everything in the box, starting with the wires then the ground connections and so forth. 

If banging it on the desk takes care of the problem, probing with a pencil will usually show you where the problem is. (This doesn't work for profanity fixes)
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KA0HVE
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Posts: 117




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« Reply #2 on: July 09, 2013, 10:29:31 AM »

Thanks AXW!  The pot seems to be working fine because the Power meter adjusts well with it.  I had been thinking there is probably a bad solder joint other wise bumping it on my desk should have done anything to fix it but maybe it's something elise.

I'll open it up again and nudge some things around with a pencil eraser like you said.  Definitely worth a try.

Thanks again.
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WB6BYU
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Posts: 13028




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« Reply #3 on: July 09, 2013, 11:08:39 AM »

Likely there are two ganged pots - one for forward and one for reflected.  You may
have a dirty contact in the reflected pot.  I have two such devices (basically the
same but with different labels) and I really like them because I can see both forward
and reflected readings simultaneously.  Intermittent contacts in the pots are certainly
a problem at certain settings, though not the only possible cause.

Forward power often reads higher at a high SWR:  if you have a constant 10 watts
output, for example, at a high SWR then you might see 25 watts forward and 15 watts
reflected - the difference between them is the actual power delivered to the antenna
(if we ignore coax losses for the moment.)

At any point along a feedline there will be a voltage, a current, and a phase angle
between them.  These determine the impedance at that point.  They also will
resolve into forward and reflected power relative to the characteristic impedance
of the line
.  It's a fairly simple mathematical exercise that gives a lot of insight
into impedance, SWR, etc.
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KA0HVE
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Posts: 117




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« Reply #4 on: July 09, 2013, 01:06:45 PM »

Yep, you're right.  There are ganged pots.  I'll squirt in some contact cleaner and see if the issue reappears and I'll still try the pencil eraser idea too.

Thanks guys!  I may be back with more questions.
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KA0HVE
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Posts: 117




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« Reply #5 on: July 10, 2013, 06:35:09 AM »

Well...... the thing was working 'til I fixed it.  Cheesy

I used contact cleaner on it and now it's not working again but other times when I've disconnected it and reconnected it the thing stopped working too.  I also tried the pencil eraser technique to see if I could get the SWR meter to bump but nothing happened.

Until I get it working or get a new one I'm not transmitting.  I don't like operating without an SWR meter so I think I'll buy a new one.  I'll see if I can fix it and, if I can fix it, I'll keep it for a backup.  Any recommendations for a new SWR meter?

Thanks guys for helping me out.
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WB6BYU
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Posts: 13028




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« Reply #6 on: July 10, 2013, 08:21:08 AM »

In the meantime, hook it up backwards.  Then the meter marked "Forward" will read
reflected power, so you can monitor your system and adjust your tuner even if you
don't know the exact SWR.
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KA0HVE
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Posts: 117




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« Reply #7 on: July 10, 2013, 08:37:15 AM »

Great idea!  I like it.  It may take a while to replace since I'm thinking about replacing it with a Ten-Tec 1225 SWR Bridge Wattmeter Kit.  I've never built a kit and it may take me a while.

But I'm open to suggestions for a replacement.
« Last Edit: July 10, 2013, 08:42:03 AM by KA0HVE » Logged
WB6BYU
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Posts: 13028




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« Reply #8 on: July 10, 2013, 08:52:26 AM »

Assuming you see no movement on the meter, it is also possible that you blew a
detector diode (or the meter movement itself.)  The former is easy to replace, while
the latter may require finding a similar instrument at a swap meet.

However, one problem with such meters is that the sensitivity depends on frequency:
I can drive mine to full output with a signal generator on 2m, but it needs about 25 watts
on 80m.  Getting a more modern design isn't a bad idea.  I like the dual-needle type,
which allow simultaneous monitoring of FWD, REV and SWR on a single meter face:
sometimes you can find an inexpensive MFJ tuner with such a meter built in for less
than the stand-alone meter.  (They typically have a range with 30W full scale, but you
could readjust the calibration pot for 10W full scale if you want.)
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KA0HVE
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Posts: 117




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« Reply #9 on: July 11, 2013, 06:38:32 AM »

The meter appears to be fine when it works.  Can the diode work intermittently?
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WB6BYU
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Posts: 13028




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« Reply #10 on: July 11, 2013, 09:11:59 AM »

Could be.  A quick check with a voltmeter should give you some idea where the problem is.
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K8AXW
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Posts: 3672




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« Reply #11 on: July 11, 2013, 09:52:19 AM »

HVE: I've never seen a diode work "intermittently."  A diode either works or doesn't.  No doubt some parameters might change but intermittent.... I've never seen it.

However, I have seen bad solder connections involving diodes.  This is usually caused by someone being over cautious when soldering the diode into the circuit and not getting the solder to flow properly.

When re-flowing such joints, use something like a hemostat, needle-nose pliers (with rubber bands around the handles to keep the jaws closed) or something similar between the diode body and the soldering iron to sink away excess heat.

As a side note, one of the greatest items to add to your tool collection are hemostats.  They can be found at any ham fest or surplus outlets like Allelectronics.com. 

They are invaluable for heatsinks when soldering transistors and diodes into a PCB or P-P circuit.  They can also be used as a third hand.
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AD6KA
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Posts: 2232




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« Reply #12 on: July 21, 2013, 05:27:56 PM »

Quote
I'm thinking about replacing it with a Ten-Tec 1225 SWR Bridge Wattmeter Kit.


FWIW: I built Ten Tec's previous version of this kit.
Single backlit meter and covered 2m too.
GREAT performer, fun to build, easy to operate.
Go for it, you'll be glad you did.
73, Ken  AD6KA
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