[Milsurplus] [MMRCG] TCS: Fixing a "Thunderstorm."

[Ray Fantini]

Good job on posting the procedures, surprising today how many have no clue how to isolate a problem in a superhetrodyne! How many of us would have just waited until the week component failed completely and then becomes easy to spot?
Possibly the only good thing about Tantalum Capacitors is that when they fail you can't miss it, but that's on newer radios.

Ray F/KA3EKH


-----Original Message-----
From: MMRCG at groups.io [mailto:MMRCG at groups.io] On Behalf Of David Stinson
Sent: Friday, July 20, 2018 1:15 PM
To: boatanchors at mailman.qth.net; milsurplus at mailman.qth.net
Subject: [MMRCG] TCS: Fixing a "Thunderstorm."

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--



-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.

View/Reply Online (#181): https://groups.io/g/MMRCG/message/181
Mute This Topic: https://groups.io/mt/23755967/1043670
Group Owner: MMRCG+owner at groups.io
Unsubscribe: https://groups.io/g/MMRCG/leave/2381741/1260982536/xyzzy  [RAFANTINI at salisbury.edu]
-=-=-=-=-=-=-=-=-=-=-=-