Postby Sauerapple » 28 Jun 2015, 09:28
Okie dokie. After a little more shooting of trouble, here is what has transpired. Flip mains switch, mains fuse blows. Checked solders connections at plug, mains fuse holder, mains switch and primaries on PT. All good, no connection had continuity to ground. Hmmmmmmm. Un soldered the secondaries of the PT, no fuse blow (Hooray). Re attach all of the secondaries, until the purple wire (its always the purple wire) recreates the symptom. Hmmmmm x2. Follow the purple wire to the board and through the traces (with a continuity test, courtesy of my Fluke DMM) until it exits the board as a red wire bound to the rear Filter Cap. Checking (the drained) filter cap, it does not have continuity to ground. Hmmmmm x3. Solder connections appear in good order. The red wire is tied to lug "3" on the cap which is tied to lug "2". A brown wire exits lug "2". Lug "1" has green/yel (chassis ground) and an orange wire attached. The brown and orange wires run and both terminate at the OT, with the brown wire representing B+ and orange COM (ground). Hmmmmm x4. Unsoldering the B+ supply from the center tap and firing the amp up = NO BLOWN FUSE.....HOORAY!!!!
Now I am no expert on an OT. I know what it does, converts high voltage/low current signal from the output tubes to low voltage/high current signal that is safe to run into a low impedance load that a speaker can use. But I am not sure how it does it. Which I guess is acceptable for my purposes up to this point. Now logic tells me that there is a short or open coil inside the OT. But I do not know how or have a means that I know of the test it, short of throwing parts at it, which is neither economical or practical from a monetary/pride standpoint. What I can confirm is that with the COM wire removed from the OT, every lug test continuity to the ground lug. Primary from the output tubes, to the secondary lugs that go to the speaker jacks to the lugs not being used. They all test continuity to the COM ground lug. Does not seem that that is a good thing, and should be pretty much conclusive proof that I have a bad OT, most likely caused by a bad/shorted tube or who knows what freak one in a million event that might have taken place.
Any ideas? Have I missed anything? Any suggestions?
Any thoughts or criticism (constructive or otherwise) I welcome. (also forgot to add, output tubes were removed for all test performed)
Cheers!