I recently burned up some secondary windings on an Edcor 131. The fuse didn't blow. I'm not a 100% sure, but I think it's the 6.3V taps, at least the ohm readings were off and they weren't on the other taps. I notice on the board that the high voltage center tap is grounded to the board ground and then to IEC ground. Not so with the 6.3V center tap....which might explain why the fuse didn't blow? Could somebody educate me on this? I can see that 6.3V isn't a threat.
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Jon often answers questions in the Tubelab forum without looking at the amp design or schematics. In the case of the TSE there should be NO EXTERNAL CONNECTION from the center tap of the 6.3 volt winding to ground or anything else. It should be connected only to the designated pad on the board.
The connection of the center tap is configured by the jumpers on the board in the filament regulator circuitry.
For 2.5 volt tubes the jumpers will actually ground the CT, so an external ground should cause no harm, but may create a ground loop.
For 5 volt tubes like the 300B the CT not used by the filament regulator circuitry and is left unconnected by the jumpers. External connection to the center tap can cause damage. The usual damage is blown diodes, and possibly a burnt trace on the PC board. I am 1000 miles away from my lab so I don't have a board or schematic to look at, but it's the large pair of single diodes, not the Schottky pair that gets fried. Grounding the CT connects the diodes DIRECTLY across the transformer output.
The filament winding can produce several amps under normal conditions. A short can produce 50 to 100 amps for a short time, blowing parts. A fuse can not protect against this since the total primary current is probably still at or slightly above normal.
I have done this myself, and some smoke did come from the transformer, yet it still worked fine and still does. Power up the transformer with the secondaries disconnected and see if it gets hot or blows the fuse. Measure the secondary voltages, and if OK, replace the diodes and try again. Inspect the PC board for any burnt traces near the transformer / diode connections and repair if needed.
The connection of the center tap is configured by the jumpers on the board in the filament regulator circuitry.
For 2.5 volt tubes the jumpers will actually ground the CT, so an external ground should cause no harm, but may create a ground loop.
For 5 volt tubes like the 300B the CT not used by the filament regulator circuitry and is left unconnected by the jumpers. External connection to the center tap can cause damage. The usual damage is blown diodes, and possibly a burnt trace on the PC board. I am 1000 miles away from my lab so I don't have a board or schematic to look at, but it's the large pair of single diodes, not the Schottky pair that gets fried. Grounding the CT connects the diodes DIRECTLY across the transformer output.
The filament winding can produce several amps under normal conditions. A short can produce 50 to 100 amps for a short time, blowing parts. A fuse can not protect against this since the total primary current is probably still at or slightly above normal.
I have done this myself, and some smoke did come from the transformer, yet it still worked fine and still does. Power up the transformer with the secondaries disconnected and see if it gets hot or blows the fuse. Measure the secondary voltages, and if OK, replace the diodes and try again. Inspect the PC board for any burnt traces near the transformer / diode connections and repair if needed.
Pads T1-1 thru T-1-8 are connected to the transformer, just like I did in the 45 amp. I jumped Pad 5 to Pad 6. I jumped pad 3 to pad 4 also. I powered up the board with no tubes in it and after 20 sec. there was some smoke, so I pulled the transformer and checked the volts, unattached to the board and it was OK on the 660v and the 5v and I think it was even right on the 6.3v. I then checked the ohms across all leads and that was OK except for the 6.3v taps, the ohms were kinda all over the place and not at all matching from center tap to each leg. Then I pulled the covers and there was melting on the wraps, on one end. so I called Edcor and sent the thing back, getting another when they wind it. I sure don't want to fry the next one. Oh and I couldn't find anything blown on the board, the D1 Schotsky measures 113ohms, center leg to both outside legs 0 ohms in reverse.
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