[ARC5] ARC-2: A Tale of Chasing One's Tail.

[David Stinson]

More adventures with the AN/ARC-2:

Now that the PTO drift problem is fixed, I began 
working on non-destructive means to tighten 
the IF bandpass.

I removed all the little shields on the side with the 
RF and IF stages and reduced the values of the 
IF stage coupling capacitors, hoping to reduce
the load on the tuned circuits and thus increase 
their effective "Q."   Turned the rig back on and...
the IF selectivity did seem better, but I had low 
audio out and low signals.   Well I expected that,
since these changes would change the IF alignment.
So I set about realigning the IF.  Things looked 
better, but the audio was still low.   Hmmm.

Tested the transmitter and found it was working 
well as usual, until I noticed something very odd.
The RX and TX freqs were off each other by 
a couple of KCs.   That shouldn't be possible.
I found-out why:  You can't leave those little shields
off the sides because the stages will oscillate,
and that mixing product was pulling the receiver.
So I put the shields back on, leaving off only the one
over the stage I was currently testing.
I figure OK- the stages were oscillating an blocking
the rig, railing the AVC, right?  Wrong.  The stages
settled-down, but there was still low audio out.

I've been burning solder for 47 years.  One rule
I learned early:  If something suddenly stops working,
look in the last place someone stuck a screwdriver.
So I start testing voltages and resistances stage-by-
stage.  And everything checks "Normal."
I check the IF alignment to see if I've boogered it.
Nope.  Then the RF to see if I twisted the wrong 
slug.  Nope.  If I kept scratching my head I was going
to scratch-away what little hair I have left.
I did find and replace some weak tubes, 
but that wasn't it, of course.

I remembered the old rule and figured I must have
diddled when I should have dabbled somewhere,
so I put everything back as it was with the original 
parts and... no help.  OK... realign the IF back to 
where it belongs.... Nope.   I'm getting cross-eyed
and befuddled about now, most of my "play time" for
the weekend being nearly gone by.  I'm stumped, so 
I whispered "Lord, I could use a hand with this one"
(That's OK- Roll your eyes.  It works for me ;-).

I tested the transmitter again and it was still a happy boy.
Then a tiny candle flame flickers in the cob-webby
cavern between my ears.   Keyed the transmitter and
"testing one, two, three...."   The ARC-2 rectifies 
a sample of the transmitter RF and pipes it back to 
the receive audio driver stage as a "sidetone."  
It's usually loud (in fact, I have extra padding in the 
feedback line to tone it down), but the sidetone 
was just as weak as the received audio. ohhh noooo...

I'd been chasing my tail, assuming I'd made some 
goof in the IF stage when, in fact, the problem was
in the Receive Audio stages all along!
Took me until 10 PM to find it.
There is a parts-mounting board right next to the 
big 5 uFd 260V buss filter cap.  A bare wire
runs along the underside of the board.
There is also a dome-headed screw in the chassis
under the board.  In moving this big heavy beast
around on the bench, the screw shunted the wire 
to ground and killed the input to the 12SA7 
Audio Driver (yep- a pentagrid converter for an 
audio driver.  And a cool reason, too.  
Ask if you care to know why).
Fixed that and all is well. 

So... I spent all my bench time this weekend chasing
phantoms of my own making and here I am,
right back where I started.   Such is life! ;-)

73 DE Dave AB5S