There are only three things in this circuit where the static begins, the tube, the secondary I-F coil, and the 75PF cap C8 that is inside the I-F can. I have tried different tubes, thus eliminating it as the source. Does this sound like the silver mica disease mentioned in this forum on other postings?
If so, how can this be corrected, it is unlikely I can get an exact replacement I-F can?
Schematic is from The Schematic Bank under GE 574.
Thanks in advance for any help.
Listening to your signal flow analysis, the problem certainly sounds like that 2nd if transformer being at fault. Personally, I would just lift ALL the leads off from the OEM unit and sub in any other available transformer of that frequency, be it either a 2nd, or even a 1st IF unit that you have available as a stock unit or borrowed from the bone pile, or even temporarily from a working unit. Since we are merely wanting an initial definitive test
That would definitely establish the culpability of your original unit and then you could worry about the possibility of repairing that original transformer with the gutting / disconnect of its original variable cap scheme and the utilization of modern dipped silver mica units. Most of those old original units are failing on the plate side where there is actually some B+ level across the circuitry.
The usual failure mode being a loosening of the clamping action to the conductively coated mica infrastructure, an oxide buildup on the connection parts…thus
the inability for positive connectivity action.
Once I even pinpointed….under a 300 power stereo microscope.. the gradual lateral migration of a silver oxide path buildup across its porous porcelain cap, to the point where it had a path between its terminals and the adjunct grounded metal housing of the IF transformer. Eventually there was B+ to ground leakage across a spotty-conductive path.
I would guesstimate its growing continuance being from occasional high humidity in the air abetting the traveling of the deposit. Back in its major lifetimes use, it was subjected to those extreme / wider swings of room conditions, with it not being in a more temperate controlled room environs as we now usually have present.
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Can't access my CD with the schema on it now, but might possibly view it later on in the day.
73's de Edd
:This Ge 574 radio gives nothing but static, and I can control the volume of the static with the volume control. I used my signal tracer to trace the signal, and get a good signal,no noise or static in the tracer speaker,up to pin 5 of the 12BA6 I-F tube. When I use the tracer on pin 5 of the Detector 12AV6 tube, I get the noise and/or static on either RF or AF setting on the tracer.
:
:There are only three things in this circuit where the static begins, the tube, the secondary I-F coil, and the 75PF cap C8 that is inside the I-F can. I have tried different tubes, thus eliminating it as the source. Does this sound like the silver mica disease mentioned in this forum on other postings?
:
:If so, how can this be corrected, it is unlikely I can get an exact replacement I-F can?
:
:Schematic is from The Schematic Bank under GE 574.
:Thanks in advance for any help.
From the schematic I see, and can now confirm my suspicions, that you have those slug-tuned IF cans that probably use the integrated capacitors on a mica wafer.
From your description of the symptoms I would bet that you have a classic case of " The Dreaded Silver Mica Migration Disease"
That is where the silver migrates causing an electrical path from the B+ side of the primary coil to the grid input side of the secondary allowing high voltage to occasionally or often appear on the grid of the secondary side causing loud crashing static noises..
You'll have to disassemble the can and disconnect any connection to the wafer inside and substitute NEW 75-100pf mica caps either inside the can or externally.
Not easy... but certainly VERY do-able.
So how did you make out?
:So how did you make out?
:I set this one aside for now, I have another one of this model coming, and between the two, shouild be able to make one good one. This one has a broken clock glass as well, but the other one coming has a good clock.
The value of these cheap radios is not much, but the experience for me is invaluable. I am still in the learning stage, this is only the fifth one I have attempted.
The other four are working great.
::So how did you make out?
::I set this one aside for now, I have another one of this model coming, and between the two, shouild be able to make one good one. This one has a broken clock glass as well, but the other one coming has a good clock.
:
:The value of these cheap radios is not much, but the experience for me is invaluable. I am still in the learning stage, this is only the fifth one I have attempted.
:
:The other four are working great.
Sorry, I forgot to say thanks ever so much, Doug
Here's a link showing step by step pictures and how to repair that bad IF can.
http://www.oldradioz.com/john/Tech/IFcapRepair.pdf