looking for trouble

part last: how to look

de Bill, K7WXW

Generally, when something in my shack stops working, I start with the basics: are the cables right? Have I turned on the power? Operator error is always the first hypothesis. When I’ve worked through the obvious stuff and don’t have a clue, I turn to the interwebs. Whatever I am trying to fix has probably been used, torn apart, repaired or modified by someone somewhere who subsequently wrote an email or blog post about it. A thorough search usually results in a pile of helpful files, bookmarks and contacts.lookingfortrouble2

Today, unfortunately, my first search didn’t reveal much so I went to plan b: asking for a little help from my friends.

Email lists and forums – what we used to call bulletin board systems and newsgroups – are my connection to hacker/maker/ham communities that always know more than I do about whatever piece of gear, technology, or software I am trying to make work, repair or hack. There’s a vast store of tribal knowledge in such places and on most lists, people are happy to help you if you do your homework and ask specific questions. In this case, I distilled what I needed to know into two questions, wrote a paragraph describing the symptoms and what I’d done so far, and sent it to a QRP list where I hang out. I write an email like this once or twice a month and the result is almost always helpful.

My email netted me a schematic of the base unit, good troubleshooting advice, and references to several articles about SWR meters. The articles helped me understand the theory and gave me pointers to other articles, along with the callsigns of hams interested in SWR meters and directional couplers. I used this info to do another web search and soon had a stack of articles, schematics, and photographs, more than enough to figure out how the WM-1 works and how to fix it.

It took a couple of hours to go through all the material. Along the way I learned a lot about directional couplers and RF measurement.  I was able to make some good guesses about nature of the problem, and come up with probable fixes. This is one of the benefits of I-have-to-fix-it-myself gear: figuring out what’s wrong with something is a great way to learn how it works.

I made a repair list and a plan for home brewing a new RF detector. I should be able to fix the old meter but building a new one will give me a backup and help me convert all the theory I’ve been digesting into a real piece of gear. Another perk of fixing-it-myself: the satisfaction that comes from using gear that I’ve repaired, modified or built myself.

I am glad I lost the habit of immediately replacing stuff that doesn’t work. My unusual meter spike has turned into a file of interesting articles, a repair project, a cool home brew project and a must-to-read list of RF design tutorials. I know a little more about SWR, RF power measurement, and directional couplers and I have had interesting conversations with three long time hams. If, three days ago, I was wondering about the wisdom of buying a flaky third-hand meter, today it appears to be a pretty good deal.

looking for trouble

part two: all the things it is not

de Bill, K7WXW

The sun is almost up when I let the dog out, make coffee for Amy, and sit down to check my email. I have three replies to my query. Someone sent me a schematic of the main board, someone else, three or four references to directional coupler designs along with technical data and photographs from three or four watt meters he built. The last email is a reminder not to rush through troubleshooting. I get my notebook.

My friend’s reminder is timely. Like most fools, I tend to rush in. Making a plan attenuates this tendency. My curiosity helps, too. Why doesn’t this work? is such a great question. I don’t just want to get this meter back on line, I want to know what makes it go. Troubleshooting is one way of figuring that out; maybe the best. Besides, I don’t have a spare meter. Until this one is fixed, I am without. Using the emails, schematic, and a little common sense, I make a list of things to do, then head downstairs.

After disconnecting the power and undoing the wing nut that holds the ground strap, I pull the desk away from the wall and start working through my list. I remove and test the coax cables and feedline. I examine the ground connections. Everything checks out. I make a note to look at the cables and connections at the outside panel, too. So far, though, no obvious problems.

I have two antennas, each going to a matchbox tuner. Between the rig and the tuners is a coax switch. The configuration is a bit complicated, so I take the switch out and test each antenna path separately. A basic troubleshooting rule I learned as an engineer: keep it simple. Complexity is hard to understand, let alone test. The corollary: change one thing at a time. Two variables changed means four possible outcomes, three means nine, and so on. Don’t make things harder than they already are!

I check the coax switch with an ohmmeter. It is good, too. I put the switch in line with the two tuners and test again. The antennas load on all bands. I make more notes. I’ve eliminated almost everything but the meter. I make sure I have cabled the radio correctly, push the desk back, and head upstairs. I haven’t completely nailed the problem down but I have eliminated a lot of possibilities. That is a good morning’s work; time for breakfast.

looking for trouble

part one: when stuff breaks

tshoot_040717

de Bill, K7WXW

While tuning through the 40 meter band, I notice the SWR meter spike and return to zero. Ten minutes later, it happens while I am not touching the rig. That’s weird, I think, but the net starts in twenty minutes. I don’t want to start taking things apart. I decide look into it later.

I drop to 80 meters, lower the RF power to five watts, and switch to AM. When I key up, the SWR meter goes to max and stays there. Adjusting the matchbox makes no difference. I move back to 40 meters. The same thing happens. Weirder. It worked ten minutes ago. Looks like I am taking things apart. With the meter out of line, I match the dipole on both bands. Putting it back in, I don’t.

Bad cable? I swap in a new one and check the old one with a ohmmeter. The new cable doesn’t make a difference and the old one is good, at least at DC. While I am switching the cables, I bump the meter and the receive audio gets a lot louder. I try to match the antenna again. Success. Could my third hand meter, an Autek WM-1, be the source of my woes?  I take it out of line and get ready for the net. For now, I can use the rig’s SWR meter.

I am embarrassed to admit this, but in an earlier life, I would have chucked the WM-1 and ordered something shiny and new. I was a busy guy.  Who had time to figure out why something didn’t work, right? In this life, I finish the net, put the meter on the bench and start taking the case apart.

I look for obvious problems: loose wires, cracked components, bad solder joints. The solder work is questionable and the sensing toroid isn’t glued down but there are no glaring (that is, easy to see) faults. I make a note to reflow the solder on all the connections and glue the toroid into place.

The instruction manual notes that unexplained SWR spikes aren’t anything to worry about. I speculate for a moment about career path of whoever approved the manual. There’s a schematic for the detection circuit but none for the main board and no explanation as to how it works. Not helpful.

I do a quick web search and turn up a few posts about the odd behavior I am seeing. Unfortunately, none include a fix or schematics of the base unit. I can’t call customer support; Autek disappeared years ago. I am not sure what to try other than cleaning up the bad soldering, so I go to plan usual: asking for help from people who know a lot more than me.  I send an email to the QRP list and set the meter aside for the night.