EL rotator back in action, and lessons learned

In my first posting, I said that one of my ongoing projects was finishing the elevation rotator (EL rotator) that is part of my amateur satellite setup. Well, it's still not finished, but at least it is back in action after damage (accidentally caused by me) had stopped it elevating for several weeks :-)

I am going to write an article about the design and construction of the EL rotator that, I hope, will be published in Oscar News. I’ll also put the information on my website. But, in brief:

• A satellite TV linear actuator rotates the antenna boom in the vertical plane.

• A potentiometer, housed in a metal box and connected to the boom by a pantograph arrangement, senses the amount of elevation.

• The ARS hardware and software (from EA4TX) in my shack measure the elevation and commands the linear actuator up or down as required.

The EL rotator had worked well for several months, but I had left three small jobs uncompleted:

• Calibrating it more precisely than I had when first getting it going.

• Trying to cure RF interference coming from the actuator motor when it is in motion (noticeable and annoying but not a showstopper).

• Completing the waterproofing by putting some sealant around the lid of the metal box in which the position-sensing potentiometer is housed.

A few weeks ago, I decided to tackle the third job, using RTV silicone sealant. I put a bead around the joint between the box and its lid, and ticked the job off as done.

A week or so later, I noticed that the EL rotator was stuck at an elevation of about 20 degrees, even though I had used the ARS software to park it horizontally. It quickly became clear that the elevation reading as measured by the EA4TX hardware was erratic below about 40 degrees and dropped to zero even when the rotator was still elevated.

More tests showed that the position-sensing potentiometer was at fault. So I had two questions to answer: why had it failed, and how could I fix it?

I concluded that the only thing that could have happened to cause such a sudden failure was acetic acid fumes from the sealant somehow affecting the carbon track or the wiper of the potentiometer. When I opened the metal box to test the potentiometer, there was a very strong vinegar smell despite the small drain holes in the box and the sealant being fully set. (Without knowledge of chemistry, this is really a guess rather than a proven theory).

With hindsight, a number of options were available for fixing it:

• Replacing the potentiometer.

• Rotating the potentiometer so that an undamaged portion of the track is used to measure the 90 degree swing of the elevator.

• Using PCB cleaning spray to flood the carbon track in the potentiometer to see if this cleared whatever was causing the problem.

• Blowing a strong jet of air from an aerosol into the body of the potentiometer to achieve the same thing.

My design should have let me just loosen some grub screws on the shaft that links the potentiometer to the pantograph arm, and then easily remove and replace the potentiometer. So, at the time, I only thought of the first two options, and chose the first as being the most certain of success.

However, when I started the repair, I discovered that I could not shift the grub screws, due to a combination of corrosion and the Loctite applied to them when installed. In fact, I stripped the slots on the screw heads trying to free them.

So removing the potentiometer then involved:

• Drastic surgery (using a grinding wheel on a Dremel) to separate the potentiometer assembly from the pantograph arm.

• Soldering a new socket onto the pantograph arm.

• Cutting and fitting a new shaft extension for the potentiometer.

• Fitting the new potentiometer itself.

This all took time but went pretty smoothly. I waterproofed the grub screws (using self-amalgamating and PVC tape) to avoid a repeat of that problem. And I have not used sealant on the metal box, relying instead on what is a fairly snug mechanical joint.

I still have to sort out the other two jobs (final calibration and dealing with the RFI), but at least I am back in action.

Ugly VFO circuit diagram

Here it is, a combination of the "Practical 7MHz Hartley Oscillator" from Experimental Methods in RF Design (page 4.3) and a buffer amplifier from the website of Alan Yates VK2ZAY. Click on the diagram to enlarge it.

The inductor is 19 turns on a T50-2 toroid, tapped at four turns up from ground. Supply voltage is 12V. The variable capacitor is one half of a dual-gang capacitor from www.6v6.co.uk.

Ugly VFO works

After a couple of hours of drilling, winding and mostly soldering, the "ugly construction" VFO and buffer amplifier is working. The output on the oscilloscope is fairly stable and the tuning range is roughly what I calculated it would be.

The frequency is on the low side (6.7MHz rather than 7.0MHz). When I tested the toroid inductor before putting it into the circuit, I noticed that the inductance varied greatly depending on how close or wide I pushed the windings. However, since I don't actually plan to use the VFO on the 40m band, I am not too fussed about this. I didn't build any trimmer capacitors into the circuit.

Here are a few photos of the VFO in its current state. I will add details of the circuit in the next post, plus I want to do a proper test of the stability and tuning range. Then I will build and add the Cumbria Designs X-Lock "huff and puff" stabiliser that I have in kit form and see how that works.

Three that got away

Tidying up my workbench, I realised that I have three other projects on the stocks:

• an S-Band downconverter from Bob Seydler K5GNA that I want to try out on AO-51
• a signal source to help check that the downconverter is working - I have my eye on a design from Bertrand Zauhar VE2ZAZ
• a "huff and puff" VFO stabiliser from Cumbria Designs that I bought to try with the homebrew VFO project
  

Both of these are at the back of the queue. I just started the homebrew VFO. More on this soon.

Now I'm confused

Did some testing with the IMD Meter in my garden near my HF antenna. Field strength was either zero or full scale. IMD was either 'no signal' or very, very good (-34).

I notice from reviews on eham.net that at least one other user had similar results. After discussions with the supplier, they concluded that the meter was working and that they were putting out a consistently good signal. Maybe I'm just too pessimistic.

Anyway, KK7UQ's website indicates that he is 'offline' for a while due to medical treatment, so I will pursue this via email when he gets back.

Now I need to decide what to do next: begin the major K1 build, or have some fun with the oscillator.

IMD Meter is working. Or is it?

I do a fair amount of PSK-31 operation. Good transmitted signal quality is important to keep bandwidth to a minimum, which, after all, is one of the impressive features of this mode. Quality is measured in terms of IMD (intermodulation distortion) and there are basically two ways, I think, to check it. One is to transmit an idle signal and get the person you are in QSO with to read off the IMD from their PSK-31 software. The other is to use a device at the transmitting end to measure the IMD.

One such device is the IMD Meter from KK7UQ. It samples your transmitted signal through a telescopic aerial and has a microprocessor that measures and displays the IMD. The fact that it is available as a kit also appealed to me. I ordered one from the USA and, within a week, it had arrived.

Construction was pretty easy (good documentation), even though there are a lot of components on small PCB and my eyesight is not as good as it once was. And I was delighted when, switching on the meter for the first time, it ran through its computerised self check without a hitch: everything flashed and bleeped like the manual said it should.

Then I tried to measure my PSK-31 transmitted signal. There are two steps in this: set the meter to measure field strength (to make sure you are close enough to the transmitter) before switching to IMD mode for the actual measurement. The meter gave some weird results. The field strength flipped between zero and full scale without any clear correlation with output power from my transceiver, position of the IMD Meter and so on. It just felt a bit flaky.

Back to the manual, which has an excellent step-by-step troubleshooting guide. This pointed me to checking various voltage points on the PCB and making sure that there were no dry solder joints in the RF area of the board. I couldn't see anything glaringly wrong, but I resoldered the joints just to be sure.

When I retested the meter, things had settled down. The field strength reading was a lot more stable, and I got a valid IMD reading when I switched to that mode. Only problem is, I get the same very good IMD measurement whichever band I test on and no matter how badly I try to generate a poor signal by increasing audio drive from my PC to the transceiver. Call me pessimistic, but I can't believe that my setup is that perfect :-)

Anyway, next stage is to try the IMD Meter further away from the transceiver (so far, I have been testing on my shack desk) and see if I get some variation in results.

Two things I have learned so far. Picofuses are expensive. These are little in-line fuses, about the size of an old-style 0.5 watt resistor. The PCB includes one to protect the power supply from a short on the board; for example, a short caused by measuring spot voltages when troubleshooting and shorting two IC pins together :-) Obviously, there wasn't a spare in the kit, so I checked out various suppliers to see where I could get a replacement. I saw one site offering them for close to £7, so I decided to take my chances and replace the blown fuse with a wire link. Fingers crossed.

The other thing I have learned is that QuickMix is a really handy piece of software. I installed this a while ago when setting up my soundcards to work PSK-31, RTTY and so on via the Tigertroincs interfaces. It allows you to save multiple different profiles of sound control settings and reload them at will. Ideal for returning to optimal settings after deliberately trying to overdrive the transceiver.

I'll post the results of the further tests when I have them. Meanwhile, here are some photos of the IMD Meter (internal and external) and the mod I made to attach a hinged, detachable aerial in place of the fixed one supplied with the kit.

Welcome to my projects blog

Welcome to my first blog. You can find out more about me and my amateur radio interests on my website. This blog gives me a chance to record blow-by-blow my (sometimes faltering) attempts at homebrew construction.

You will quickly discover that I am not a great 'completer finisher' when it comes to amateur radio. I get a buzz from buying a new kit, or sourcing the parts for a scratchbuilt project; I enjoy the rush of construction; but it can take me a while to get around to the final, final steps.

Anyway, to begin with, here is a quick list of the projects I have on the go at the moment:

• completing the IMD Meter kit from KK7UQ
• final tidy-up on my homebrew elevation rotator for my amateur satellite activities (to be honest, I managed to damage part of it when I was 99% finished, so now I have to work out a way of fixing it without doing even more harm)
• starting on an Elecraft K1 transceiver kit
• building an 'ugly style' oscillator and buffer amplifier (which I researched and got parts for just because it looked like something new to try, not because I need a standalone VFO)

The IMD Meter is built but not working properly; so my next posting will be on trying to troubleshoot it, and some thoughts on the build overall.