{Collins} 75A3 15 M update

I just wanted to update everyone, and thank all of those who offered advice in tracking down the problem on my 75A3. 15 M was 'dead' so with help I started poking around in the crystal oscillator circuit, variously suspecting the crystal or the coil or mica cap.  Not having a lot of test gear or a lot of practical experience in how to use it I ran out of steam, but a local member of this list stepped in to help me out. Colin Lamb,K7FM, took the radio to check it out. Here's his email to me this morning :


Hi Dan:

Had a few moments yesterday to put your radio on the bench.  Checked crystal - it is fine.  Put a scope on the output of the oscillator circuit.  Nothing on 15.  Tuned the oscillator coil and it began oscillating. Checked all other bands and seemed ok, although the output on 15 and 10 were down considerably from 20.  Went to dinner and spent the rest of the night with my wife.

This morning, I flipped the receiver on to fine tune things.  Warmed it up good.  Nothing on 15 meters.  Checked on other bands and nothing on 10 meters either.  This is normally indicative of a bad tube.  Pulled tube and put in tube checker.  It was warmer than I expected it to be.  Put in tube tester and checks ok.  But, I was convinced the tube was bad.

Went to tube supply and found a new 12AT7 tube.  Installed it and all bands are working nicely.  I will check other things out over the weekend, but I am listening to the 15 meter oscillator and watching it on a scope and it is solid as a rock.

73,  Colin  K7FM

I also want to thank Bill Carns for letting me call him on the phone and walk me through some steps early on, and all of the others who offered good advice. Nothing beats experience! I had tested the tubes not long after I got the radio and thought, 'OK-they all seem pretty solid', and usually tubes are pretty rugged. Also it's been really fun to get better acquainted with my new (to me) VTVM, frequency counter and signal generator even though I couldn't get this on my own. A grid dip oscillator is on its way to me which will be fun to learn about. This also pushed me to build a little crystal checker. I semi retired from my line of work which is about as far removed from radio and electronics as you can get, but my goal is to learn about these great old rigs and put them to use. Next will be the 32V2 I picked up with the A3, and hope to get it on the air for straight key night. I'm also building a solid state T-R switch for tube rigs that was outlined in QST about 2 years  back.  I do remember one adage from my elmer of many years ago-'keep one hand in your pocket while poking around :) Thanks everyone.

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