Posted: 11/30/2011 4:06:09 PM EDT
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I just picked up an URM-120 (technically a TA-1285A/URM-120) and either got a bad one or am doing something wrong.
Here's what I did: Removed cover Inserted slug (25-250 MHz) set on highest power setting (500W) Connected cable Replaced cover Hooked up coax (2 meter with a max output of 50W, probably closer to 30W now.) I tried keying up on low power & high and at the various power settings, forward & reflected, but barely got the meter to move. (2, maybe 3 ticks from 0) This meter has a knob on the top right that clicks into the "CAL" position and turns almost 360* for "VSWR" I tried the 200-1000MC slug on 440 with the same results. Observations: The power adjustment knob only turns clockwise (?) and is a lot tighter than I expected it would be. I expected it to click very noticeably into the 4 power setting positions, but instead there are many very small clicks, like a scope turret. Since there are around 5 clicks in each power setting range, It makes me wonder if I have it set right. The meter came with 2 slugs, with matching serial numbers, and all have calibration stickers dated 11-26-97. The meter & slugs came in the military case with the same serial number on its plate. Contractor: Struthers Electronics Corp. Am I doing something wrong? Faulty meter? Suggestions? Thanks! 
ETA: Pics of the meter's innards. 
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Quoted:
Inserted slug (25-250 MHz) set on highest power setting (500W) ... Hooked up coax (2 meter with a max output of 50W, probably closer to 30W now.) ... barely got the meter to move If you're only putting 30 watts into a wattmeter that displays 500 watts full-scale, would you really expect the meter to move much? That's only 6 percent of the full-scale power... |
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What are you transmitting into? Antenna, dummy load?
If you have a high SWR condition your radio may be at radically reduced power output due to an internal protection circuit. For power measurement you need to "disengage" the SWR adjustment knob, it probably has a click position for "power" or something like that, can't read the top in your photos. For SWR measurement, you adjust the knob to put the needle at the specified point on the scale, then switch to reverse power direction and read the SWR from the scale on the meter. The meter will only read when the element is seated correctly and all the way at the forward or reverse position. And of course will only read power going in the correct direction from the arrow on the element, either forward from the transmitter to the load, or power reflected from the load. I'm not familiar with that exact instrument, just going off of general information about the similar Bird wattmeters... those elements however look like they are adjustable power. Quick google search indicates that that is indeed the case, each slug has an adjustable power range. |
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You will need to connect a reasonably matched dummy load or antenna to the 'output' conx of the wattmeter, otherwise IIRC, it won't indicate because of the high VSWR.
And good for you buying a decent and very useful piece of test equipment that once you figure out how to use it will serve you for a long time. I wish more folks would see the utility of doing this! |
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To answer a few questions...
I'm transmitting into an antenna. I tried all power settings, starting at 500w, all the way down to 10w and still had very little needle deflection. I tried three different bands, 2m, 440, & 28m, on two different radios, on antennas with known good SWR's. I keyed up with the SWR knob in several different positions without any luck. |
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Quoted:
To answer a few questions... I'm transmitting into an antenna. I tried all power settings, starting at 500w, all the way down to 10w and still had very little needle deflection. I tried three different bands, 2m, 440, & 28m, on two different radios, on antennas with known good SWR's. I keyed up with the SWR knob in several different positions without any luck. Try the other slug, but be sure to start on the highest pwr setting. If neither slug works and if they were said to be good, then the meter may be bad. Look inside the meter's glass face and you may see something like 50 uv full scale. Whether or not you see anything... ...take the unit apart far enough to clip on a DVM to the meter terminals and see if you get a reading. If you get no reading, suspect the diode in the slugs. I'm not telling you to do this, but I would -after checking the p.n. of the diode and making a determination if it were safe, check the diodes with a DVM. Or substitute a physically SIMILAR diode for diagnostic purposes to verify the meter is working. Here's a manual that shows how to access the diode. http://128.205.126.49/images/PDF-Files/AN-URM-120.pdf If the diode is bad, there are a wide variety of dirt cheap SMT diodes available now that would have a different form factor but some should work OK at lower frequencies at least, if expediently installed, with likely wrong calibration [that could be tweeked] but still make the instrument very usable. Modern diodes can have far greater perf and reliability for literally pennies. IIRC, those cartridge diodes can sometimes be made to work again by bumping them, or barely twisting the setscrew [sealed w/ lacquer] some have in the end. I think they are point contact diodes using a tiny wire whisker attached to the setscrew. [I used to play with them when I was a kid and still have a bunch in a different location from various diode detectors, etc, not sure abt the part numbers, 1N21, 23, 58, etc rings a bell] Used one in a crystal set my Father made for me. They used them in the uw relay stations. |
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Quoted:
Quoted:
To answer a few questions... I'm transmitting into an antenna. I tried all power settings, starting at 500w, all the way down to 10w and still had very little needle deflection. I tried three different bands, 2m, 440, & 28m, on two different radios, on antennas with known good SWR's. I keyed up with the SWR knob in several different positions without any luck. Try the other slug, but be sure to start on the highest pwr setting. If neither slug works and if they were said to be good, then the meter may be bad. Look inside the meter's glass face and you may see something like 50 uv full scale. Whether or not you see anything... ...take the unit apart far enough to clip on a DVM to the meter terminals and see if you get a reading. If you get no reading, suspect the diode in the slugs. I'm not telling you to do this, but I would -after checking the p.n. of the diode and making a determination if it were safe, check the diodes with a DVM. Or substitute a physically SIMILAR diode for diagnostic purposes to verify the meter is working. Here's a manual that shows how to access the diode. http://128.205.126.49/images/PDF-Files/AN-URM-120.pdf If the diode is bad, there are a wide variety of dirt cheap SMT diodes available now that would have a different form factor but some should work OK at lower frequencies at least, if expediently installed, with likely wrong calibration [that could be tweeked] but still make the instrument very usable. Modern diodes can have far greater perf and reliability for literally pennies. IIRC, those cartridge diodes can sometimes be made to work again by bumping them, or barely twisting the setscrew [sealed w/ lacquer] some have in the end. I think they are point contact diodes using a tiny wire whisker attached to the setscrew. [I used to play with them when I was a kid and still have a bunch in a different location from various diode detectors, etc, not sure abt the part numbers, 1N21, 23, 58, etc rings a bell] Used one in a crystal set my Father made for me. They used them in the uw relay stations. Here's what I've done so far: ============================================ I put a DVM (set to ohms) on the terminals of the meter & the needle slammed to the right. The SWR adjustment knob, turned fully clockwise (highest resistance), slowed down the needle, but it still deflected all the way right. Range of resistance across the SWR knob terminals was (approx) 156 to 1680 ohms. 40 watts input only shows 2 watts on the meter, no matter the range selection on the slug. ============================================ MOAR suggestions?????? eta - new meter innards pics in first post. |
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I wouldn't go digging into the slugs, replacing diodes, etc etc until you've substituted a known good meter in its place and confirmed that the meter is the issue. Find someone local with a Bird 43 or similar and try it in place of your URM-120, and make sure there isn't some other problem or misconfiguration going on.
The meter element on one of these type wattmeters is a sensitive millivolt meter, it's displaying the output from a simple directional element. I wouldn't try to put another meter onto it or feed it substantial voltage. |
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Quoted:
I wouldn't go digging into the slugs, replacing diodes, etc etc until you've substituted a known good meter in its place and confirmed that the meter is the issue. Find someone local with a Bird 43 or similar and try it in place of your URM-120, and make sure there isn't some other problem or misconfiguration going on. The meter element on one of these type wattmeters is a sensitive millivolt meter, it's displaying the output from a simple directional element. I wouldn't try to put another meter onto it or feed it substantial voltage. I didn't really want to go digging in the slugs at this point. I've read that the diode is usually the problem and that they are hard to find. The setup is good and has been tested with another meter. Power out is 40 watts with an SWR of 1.3 on 2 meters and 20 watts with a flat SWR on 70cm. This is with LMR400 going into a "Copper Cactus" antenna I built. Anyone out there actually repaired one of these? Suggestions? Thanks for the replies so far! |
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Got it working!
A buddy gave me a couple of slugs he had laying around and the first thing I noticed was that the knobs turned easy and there were 4 noticeable detents, one for each power setting. I pulled the top off of one of my slugs and determined that the knob was just spinning in the cam (item 14) because the cam was binding were it goes through item 13 & would not move. I don't know if the plastic cam swelled up over the years or something else was the problem, but I chucked it up on a drill & spun it while holding a bit of 400 grit on the shaft until it was a slip fit then added a drop of glue to the metal knob shaft to keep it from spinning.  
Once I got the cam moving freely I put it all back together and tried it out and, other than needing a calibration, it worked like it was supposed to. Calibrating looks easy enough; just hook it up to a known power source and adjust the calibration screws (item 20) until the meter matches the input power. Now I just need to get my hands on a good HF slug...... |
