Mesa Nomad 45 - Strange Power Transformer Conundrum

[DominoTree]

Very odd issue - across the 3VAC outputs on the power trans with nothing hooked up, I am seeing 3vac between the two, as well as 3VAC between each output and GND. However, *AFTER* the rectifier for V1, I am actually seeing 6VDC... Any thoughts as to how this is happening?

[Steve Conner]

That's how it's supposed to work...

[DominoTree]

Pardon my ignorance, but shouldn't the 3VAC outputs give 6VAC between eachother? 3VAC isn't enough to run the heaters on a 12AX7 or EL84, and according to this schematic, at least, that's how the tubes should be wired up... And why would I be seeing 6VDC rectified when I only see 3VAC unrectified?

[Tom Phillips]

Pardon my ignorance, but shouldn't the 3VAC outputs be out of phase with one another and give 6VAC? 3VAC isn't enough to run the heaters on a 12AX7 or EL84...

My guess is measurement error. I suggest that you measure again. Try connecting your voltmeter at the tube socket heater pins. You should measure 6 VAC as you expect.

By the way, I don't think of it as "...3VAC outputs out of phase with one another" It is a single 6.2 V winding that is center tapped and it is in phase with itself.

Let us know what you find.

Regards,
Tom

[DominoTree]

Out of phase was definitely the wrong wording.

I've measured and measured again at the transformer with nothing else hooked up and gotten 3VAC between ANY pair of the three wires on that winding. Perhaps my multimeter has issues with low AC voltages?

The plug for the power transformer *was* very well-cooked on both of the two 3VAC wires. I'll dig a little deeper when I get back infront of it and see what I find. I was getting +3VDC and -3VDC on the terminals for the first tube's heater, but I don't recall what I found on the other heaters. Definitely not 6VAC though (or they'd be working :P)

The part that was really confusing me is where the "phantom" 6VDC could come from if I only measured 3VAC across the wires that were later rectified.

[Tom Phillips]

OK. Additional new information is that something was cooked, which you cleaned up, and the amp is not currently working. Is that correct?

[Jazz P Bass]

Nomad 45 power supply

Is there any way that you can disconnect the transformer secondary from the rest of the circuit?
A resistance measurement across the three legs would help
The rectified portion of the circuit is for the footswitch.
As indicated, you should read +3Vdc & -3Vdc across the two filter caps (6800uf/10V)

[Bruce / Mission Amps]

Any chance that the difference between the FWB not being ground referenced and the center tap of the PT winding is grounded has something to do with this.....
How does the four diodes conduct with respect to the grounded center tap?

[Tom Phillips]

...How does the four diodes conduct with respect to the grounded center tap?

It is easier to see the circuit operation when you draw the equivalent circuit.
The attached diagram shows the equivalent circuit during the positive half cycle of the signal from the transformer winding.

Voltage V1 is formed by both halves of the secondary which provides 6.2V to all tube heaters except the first stage pre-amp.

Voltage V2 is generated by the full wave bridge rectifier across the full winding. Only two diodes are shown because only two are in play during each half cycle. V2 is theoretically 7.07VDC but will be a little lower due to various losses and less than perfect filtering.This voltage is floating with respect to ground.

Voltage V3 results from half wave filtering (Just the top diode) of the top half of the winding. Theoretical 3.54VDC max referenced to ground.

The equivalent circuit for the negative half of the cycle is similar. The other two diodes conduct and the "-3VDC" rail is formed by the other equivalent half wave rectifier.

This doesn't explain the problems that DominoTree is having. I'm still curious to find out what's up with that.

Regards,
Tom

[DominoTree]

Okay, snipped off the connector altogether and measured between the 3VAC leads and got 6VAC as expected. Put a new molex connector on and all is right with the world. The original connector was discolored so "cleanly" that I thought it was part of the manufacturing. I only noticed an issue when I unplugged it and some of the sheathing on one of the wires flaked off. Went up to some higher gauge wire and let it run for a couple hours and everything looks A-OK now.

[Jazz P Bass]

Sounds like the infamous Bugera heater connector.