Wednesday, 12 February 2014

Meters, OCXOs and strange goings on in the workshop

Mark G0MGX has returned Wispy to me, and now while im sat here in windy rainy Yorkshire, he's off working in A71 land! Im very grateful to him for the tests done on Wispy. I now know where to start on improving it and getting it working properly, and I also know to beware of the odd behavior of test equipment at times!

This occasional odd behavior showed itself again this afternoon. The photo below shows my mock-up 28MHz oscillator, and my frequency counter

So, doesnt seem much wrong there, does there? But look again. Look at the connections to the oscillator...

Apart from the scope probe, there are no connections! It ISNT POWERED! This lasted while I took the photos, and checked about the bench to see if I could fathom out what was happening. Then The counter suddenly went to zero, as it should have been! How very odd!

I put this bizarre occurrence behind me and cracked on with the tasks in hand. I didnt want to spend too long out in the workshop as the weather was really bad. The first thing was to test my various panel meters and determine their FSD currents. Having ordered a 1mA panel meter, I knew of course that some of them would prove to be 1mA, and I had a suspicion that the biggest and best for use as a meter for very low power levels, where I want a nice big movement, would be the ones to do so.

I rigged the variable PSU to 5v, and with a 100k linear taper pot in line with the multimeter set on its 2mA scale, proceeded to turn each meter up to full scale deflection (FSD) and read the required current.

Three of my best and biggest meters, all dB scale, and two of them being very big movement and anti-parallax mirrored, all proved to be 1mA! The trick now will be finding or building a box for the 50mW power meter that one of these beasts will fit! Were talking 3inch high and 4 inch wide meters!

My final task was discovering how to connect up a GFS-203A 10MHz OCXO (I think I referred to these at TCXOs last time). I tried to work the connections out from an old RC-690 processor board but the only one I could be certain of was the output, as this went to a coax connection. I was fairly confident though that I knew which was Ground, as it also connected to the devices metal case. But what voltage? A bit of research led me to the idea that 5V might be enough, and so I rigged it up to try

With a 5V supply, this little wonder fired up and put out an (excuse the pun) rock steady signal at 9.99994MHz. Just 60Hz off. A tweek of the adjustment trimmer and I found it can be adjusted +/- about 200Hz. Adjusted properly, it just sat there spot on 10.00000MHz.

Unfortunately, this means my idea of multiplying it up to 50MHz for WSPR is out, as there isnt the frequency swing needed. So i'll have to think of another way to get 50.294MHz.

Although still waiting for most other things ive ordered, my items from G-QRP club sales have arrived. I can now try a frequency doubler oscillator for 10m WSPR from 14.060MHz (which I know works) to feed as LO for TennaLady.

Ive rerigged the station back to HF + 6m, which means for now 6m WSPR is out. This is due to in this configuration the 50MHz port of the rig goes direct to the beam, without any metering, so I cannot set Tx powers for WSPR on 6m at the moment.

Anyway, thats enough radio for now. Ive ironing to do. Both mine and Sams Gi's need a press before tonights Karate session.

UPDATE - Odd frequency counter indication SOLVED!

It never occurred to me  at the time. I was sat here thinking, shall I turn off the 10m WSPR beacon? All while I was in the workshop, this has been running - on 28.126MHz! The workshop, and the counter, are about 16ft underneath the antenna...

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