For those that do know me in real life, I'm not an electrical engineer, I'm a traffic engineer (more specifically, I do traffic forecasting, which also makes me somewhat of a programmer and a transportation planner).  I can build electrical things when they're easy or when there's a schematic.

For those that have been following this blog, I just put my only real* HF rig in my truck.

For those of you that have been following me on Twitter and have a damn good memory, you may remember my first rig was a Ten Tec Omni.

So enter my issue.  I have a broken (but otherwise great) rig at home, I'm too cheap to send it to Ten Tec**, and I want to learn more about repairing my equipment.  This is a lot harder than Arduinos and Raspberry Pis and Beagle Bones!

The Transceiver

The transceiver is a Ten Tec Omni C Model 546.  It has all the options - three crystal filters (2.4 KHz, 1.8 KHz, and 500 Hz) and three audio filters (10 KHz, 500 Hz, and 150 Hz of audio) PLUS the WARC band (12 meter and 17 meter) crystals installed.

The guts of the rig are modules - each plugs into the chassis.

These are the guts of the Omni C.  Note the modules, each of these can be removed pretty easily.

These are the guts of the Omni C. Note the modules, each of these can be removed pretty easily.

The Problem

While running around 25 watts out on the NAQP RTTY contest a few years ago, the rig and power supply died mid-transmission.  I tried throwing the switch off then on, and the I saw the lights (the meter and the frequency display) light up and immediately die.

I tested the power supply, and it has 13.8 volts out.

I put an ohmmeter on the rig's power connectors and saw very low ohms (something like 200 ohms).  Seems to me that a part failed and opened a path to ground.  With that thought in mind, I did check the 1 uF capacitor and the diode that connect between the +V in and ground (those seemed like the easy tests.  I basically disconnected both from ground and tested the resistance betweren +V and GND and there was no change in resistance.  However, the rig is modular, and there are dozens of places where there is a capacitor between a +V and GND connection.

My Thoughts

One thing about this rig is that everything is modular.  I can remove parts very easily and test the resistance to ground (which I'm guessing should be very high).  So if I find something that has a low resistance to ground (in the ohms instead of kilo-ohms), I should probably start looking there.

One thing I did do is remove the meter light bulb and the resistance went up to 450 ohms.  It seems to me that the resistance should be somewhere in the 10 kilo-ohm range, but again, I'm not an electrical engineer.

Reader Participation

Let me know in the comments, via twitter, Google+, or email - am I on the right track?  Anyone have one of these and know of a common failure?  I'll gather a work-week's worth of comments and do tests on the weekend (or through the week, if I have the time).


Category: Equipment

About the Author

Andrew is the owner of this blog and enjoys computer programming, building things, and photography. He's a pretty busy guy, which explains why updates to this blog are so infrequent.

4 Responses to Radio Repair Thoughts and Questions

  1. Russ Hines says:

    Hi Andrew:

    Basic troubleshooting suggests a "divide and conquer" approach, so here's a few thoughts:

    First, what on the radio DOES work properly, if anything? For example, if the radio still receives well and control (band and mode switching, etc.) works as it should, then concentrate on the transmitter.

    Second, RTTY is a 100% duty-cycle mode. Most manufacturers recommend reducing the output to 1/4 power (25w in the case of a 100w rig). Even then, and depending on the design, some rigs just don't like that high of a duty-cycle.
    So, I would suspect the PA, the power supply, or both. Just because you have 13.8 VDC on its output, power supplies can fail to deliver the necessary current the rig needs for transmit due to other failures... pass transistors come to mind here. If you can substitute with another supply, and the problem persists, the problem is likely in the rig.

    BTW, an RF probe for your voltmeter comes in handy troubleshooting lower-level transmitter stages. Easy to make one, a simple diode and capacitor will work, Google or an ARRL Handbook should turn up a schematic.

    Where things quit working properly is a likely failed stage or component. Continue to "divide and conquer" and you should have it whipped in no time.

    Good luck.

    73,
    Russ
    WB8ZCC

    • Andrew says:

      Thanks for the reply. I was running it at 25 watts (it's a 100 watt rig) for the RTTY contest for the exact reason you state. I'm going to try it with a different power supply this weekend and see what happens without, and then (presuming success) with the finals. We'll see and I'll post an update next week.

      I pulled the plans for an RF volt probe (I'm guessing that's the same thing) from the ARRL Handbook, and it's a pretty simple build, so I'm going to see if I can build it this weekend.

      TNX & 73 de KE8P

  2. Dan KB6NU says:

    I second Russ's comments.

    The first thing I'd do is to either try another power supply with the Omni or connect another radio to the power supply. Either should tell you whether the power supply is really good or not.

    Another thing you might do is to check the power amplifier board to see if there's any evidence of overheating. Then if you can disconnect that board from the power supply, see if the radio will power up.

    Another thing you might do is to send an e-mail to TenTec service with the text in your post. They're supposed to be cool about such things and might give you some ideas as to where to look first.

    • Andrew says:

      Dan, thanks for the comment. I didn't see any evidence of overheating on the PA board, but that of course doesn't mean that there was none OR that there isn't a problem there (after all, I was running 25% power, so I may not see overheating since it would likely be able to take a lot more heat... I guess?).

      After the weekend and gathering more data, I might just drop a note to Ten Tec. I've heard wonderful things about their service department as well, so maybe they can get me pointed in the right direction.

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