Re: {Collins} 75A3 15 M update

["Charlie T, K3ICH" <pincon@xxxxxxxxx>]

Just as a general comment,  and not to insult anyone's trouble shooting 
abilities,  a "Warmer-than-Normal" tube sometimes indicates a leaky coupling 
cap which is applying a little + voltage to the grid along with the desired 
signal, driving the tube into higher than normal plate current.

73, Chas


----- Original Message ----- 
From: "Dan Presley" <n7cqr@xxxxxxxx>
To: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Sunday, November 09, 2014 2:21 AM
Subject: {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 :
>
> MYSTERY SOLVED!
>
> 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  b
> ack.  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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