[Milsurplus] TCS: Fixing a "Thunderstorm."

[David Stinson]

A TCS receiver repair:

Turned-on the TCS-12 receiver the other day and
heard what sounded like "thunder storm" static.
Quickly pulled the antenna and it remained.  When
you hear this from a TCS receiver, pull the power
at once, because B+ is leaking somewhere.  If one
has not dried-out and sealed the phenolic bases of
the IF transformers, B+ can be leaking from a
plate connection to ground through the trapped
water and can quickly blow-open an IF coil.

Removed the receiver and off she went to the
repair bench.
To isolate the "thunder," removed the cover over
the band switch and connected filament power to
the buss bars connecting the removable modules and
a variable B+ supply to the third terminal inboard
on the module nearest the front:
https://photos.app.goo.gl/qCSCjbkb62MHTsJm6

Connected a speaker (no antenna), turned RF and AF
gain up full and brought the B+ up just enough to
hear some "thunder."
Pulled the RF amp tube- still thundering.
Pulled the Mixer tube- No thunder.  This is good
news, since it means it's not in the IF.
Reinstalled the Mixer and pulled the Oscillator
tube- still thundering, so the problem is in the
Mixer.

Pulled the mixer tube and started scoping each of
the tubes socket pins.  Pin 8, the signal grid,
showed the jumping, jagging line of sparking B+.
A look at the diagram shows this pin is coupled to
the Plate circuit of the RF amp via a 100 pFd mica
cap:
https://photos.app.goo.gl/TXQ2XJ5bme4CbcQw9

Lifted the pin-8 end of the cap and sure enough,
the scope shows the open end of the cap with the
"jitter jives;" the cap is leaking, probably due
to silver migration.  Found a 91 pFd mica in the
junkbox and installed:
https://photos.app.goo.gl/4hK3TncUm3BmMXzeA
Oops that's the wrong photo.  Sorry.  Here it is:
https://photos.app.goo.gl/kbyNrm2z8r2z5Web7

Receiver back to its excellent self.  Total repair
time from out-of-case to back-in-case: under an
hour.  Had it been the IF, it would have been much
longer.


GL OM ES 73 DE Dave AB5S

"You're always working at the margins
 of what you don't understand. 
That's the only exhilarating place to be. 
To just illustrate what you already know
 is condescending and a waste of your time."
--Emmet Godwin--