[Milsurplus] TCS: Fixing a "Thunderstorm."

[AKLDGUY]

Good repair job. I like the oops photo, she's cute.

Just by coincidence I had a bad 100pF mica in my ARA 3-6 Mc receiver. The
BFO never worked, but would burst into life if a screwdriver was touched to
the grid of the 12SR7 or if the 100K/100pF grid leak was wiggled. The 100K
was tightly connected across the 100pF and my first thought was that
heating/cooling cycles had caused a hairline fracture or one of its leads
had pulled away from the carbon internally. Replaced it but nope, wouldn't
even start at all. Everything came right when I replaced the cap with a
120pF silver mica.

Neil ZL1ANM


On Saturday, July 21, 2018, David Stinson <arc5 at ix.netcom.com> wrote:

> 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--
>
>
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