Question on power transformer - troubleshooting

bjlefebvre

Super Member
As you may have already seen, I'm building a Decware SE84C clone using a Edcor XPWR107 power transformer. I wasn't measuring any voltage out of the secondary tap (well, technically, 0.8V where the wire first entered the PCB board and then 0.0V at the tube heater pin, which was the very next point on the board) which led me to believe that I must have twisted the wires too hard or otherwise accidentally yanked them too hard when I was fitting it onto the top plate. But I just removed the transformer from the amp and did a continuity test on the two secondary tap wires - and heard the beep. If there's continuity, wouldn't that indicate the secondary tap is just fine and the amp's problem lies elsewhere?
 
On the face of it, this reads like you have a short in the heater circuit. I hope you're using a fuse on the primary side of that transformer.
 
On the face of it, this reads like you have a short in the heater circuit. I hope you're using a fuse on the primary side of that transformer.

Yup, using a 3 amp fuse on the primary.
So just to clarify - a short on the heater circuit would be a problem somewhere on the PCB, correct? In other words, not a problem with the transformer itself.

And just FIY, here's the schematic. The problem I've been having is the aforementioned voltage issue, with the left-channel output tube getting scalding hot while the right-channel output tube and 6922 input tube normal temperature. The voltage test point at the junction of the 1K-ohm 6W and 6.8k/ 2W resistor was also too high.
 
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Some of your assertions are confusing. You should have 3.15VAC relative to ground on each of the two green wires from the transformer. These voltages cannot be zero if the tubes are heating normally. With the transformer operating out-of-circuit, you should have similar voltages (a bit higher) relative to the green/white wire.
 
Thanks. That's what I'm saying - the tubes are not heating normally. The left channel SV83 is waaaaaaay too hot - like I feel it would red plate if I left it plugged in for too long. With that happening, the secondary tap, when soldered into the PCB board, is showing no voltage on the multimeter relative to ground. I'll see what the voltage is with the transformer operating out of circuit tomorrow.
 
If the transformer heater winding doesn't have a grounded center tap, you're not going to measure anything to ground. You'd have to read across the heater pins directly.
 
If the transformer heater winding doesn't have a grounded center tap, you're not going to measure anything to ground. You'd have to read across the heater pins directly.

Ok, did that. Thanks for pointing that out. With the multimeter measuring from one heater pin to the other, it's 6.7 VAC - which is close to the secondary tap's rating of 6.3 VAC. So the good news seems to be that the transformer is in fact a-ok. You saved me $68 plus shipping - I owe you a beer.

The bad news is that I'm again stuck on what the actual cause of the problem is here. Argh. Back to the trouble-shooting board.
 
Got any pics of what you are working on?

It sounds like a bad tube, red plating issue, or both. Switch that tube to the right channel and vice versa and see if the left channel tube either stays cool or heats up.
 
Well, looks like what we have here is an example of the importance of good fundamentals. I spent today cleaning up some of the less-than-stellar solder joints that were on the board and that - oddly enough or maybe not - did the trick. Problem solved. Thanks everyone - y'all saved me the cost and wait of a new transformer. I gotta fix a ground hum, but that should be peanuts compared to the headache that had been tracking down this problem.
 
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