[Boatanchors] [BoatAnchors] TCS: Fixing a "Thunderstorm."

[Ronnie Hull]

And it would have taken me a week to do this brave Dave! You still amaze me after all these years!

It ain’t hot in Louisiana yet... just a little brown on the edges

73 and GL
W5SUM

this message came from the occupied south

Sent from my iPhone

> On Jul 20, 2018, at 12:15 PM, David Stinson via BoatAnchors <boatanchors at lists.theporch.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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