Determine the impedance & winding ratio of a bad output transformer - Help Request

[TankAudio]

Thanks to JMFahey and Enzo for explaining the use of aftermarket output transformers and your suggestions!

You found an oscillation - but how? The output transformer has different resistances in its primary windings - but how is this related?

I mistakenly thought that having a shorted winding was like having "an additional" set of windings that were no longer part of the total winding resistance measured at the primary inputs. Along this same line of thought (this sounds kinda goofy as I write it, but it honestly happened), I wondered if the messy scope screen was a result of some kind of interaction in the shorted winding. As if the shorted winding produced a ghost/mirror/reflection, etc of the input signal. Someone will get a laugh out of that. 🙂 Besides, I did have some kind of oscillation as well as an asymmetrical/irregular waveform, and this was the first time I had encountered such an issue in an amp.

"if one winding is shorted, ALL windings are"

Thanks again JMFahey. The original post was formed to weed out all of the incorrect or inefficient ways to "fix this wheel."

Does this mean that a replacement transformer was the only route of repair, and that a windings ratio could not be constructed from the data I have? (new OT primary = 50/51ohms, damaged original = 59/29 ohms)

Slightly off-topic, in that tone is being discussed rather than "get it up and running again" repair - If the OT was replaced with a transformer having a lighter or heavier iron core (or other subtle differences) wouldn't the tone be affected? I have listened to back-to-back demo's of Fender and Marshall amps with Mercury Magnetics transformers being compared to the stock units. If someone has a great sounding amp, is it worth the extra effort or cost to pursue rebuilding of the original transformer?

One more thing, that transformer test with the neon (read the Super Secret...) and the 100k resistor has some merit I believe. I tested a good mains to dual 30V rail and the bulb lit up when the 6VDC 100ma regulated current power supply was removed. Nothing happened with the 1985 OT. I will test again with the good OT and see what results.

 

[kevinkr]

Guitar amp thread so will move it to I & A per forum policy.

 

[BinaryMike]

I'll just point out that the "Super Secret Transformer Tester" is uncomfortably risky to the OPT. Primary side spike voltage can easily exceed insulation rating as battery current is interrupted, especially when using anything bigger than a single AA cell on the secondary. I personally destroyed a handful of small single-ended audio OPTs in the attempt to use a similar technique for generation of high voltage in a Geiger Counter project several years ago. This should be a really great way to destroy expensive scope probes, too: Just hook 'em up to the primary winding and pulse that battery on the secondary side!

 

[TankAudio]

Applying say 5VAC to 12VAC from a power transformer secondary to one of the PP half windings, and then measuring the AC voltage on the secondary windings, and on the other half-primary winding, should identify an internal winding short circuit problem. Swap the signal source to the other half-winding to cross-check. Measurements of the applied signal voltage and the voltage on the other half-winding should be quite close, and are normally a little different due to excitation current in the driven half-winding.

Could we go over this one more time? I want to make sure I understand what to do.
1. Apply 5-12VAC to a power transformer secondary (any, or is this why Petertub suggested using the filament winding and 6.3VAC?).
2. What is PP?
3. On which secondary do I measure?
4. What will the measurements look like when I do identify an internal winding short circuit problem?

Mainly, I need to be clear when we are talking about the power or output transformer, and who's primary/secondary we are working with. Thainks

No real problem here, the winding could be done in one operation, and the
innermost ( that has equally many turns) is shorter.
The important is what AC impedance one can measure.

Wow. My earlier question about the 1ohm resistor is ridiculous. To be clear, if a transformer measures 50 ohms on one side, and 51 ohms on another side, it is not a manufacturing error at all. Between the center tap and each anode exists the same number of turns, but as the core is wound, it gets bulkier, and you will need more wire to produce the same number of turns.

Thanks so much for sticking around until these lessons "sunk in" for me. I feel much better! It's hard to believe how much I misread from 4 months ago. It is welcome to accept that I need to ask more questions in order to straighten up what's logical.

 

[BinaryMike]

"if one winding is shorted, ALL windings are" , since all windings are *magnetically* in parallel, because all are wound around the same core.

This reminds me of the method I used to confirm bad OPTs when I was in the service business. I just connected a 6.3VAC source through a small resistance (several ohms) to the OPT secondary, then measured AC voltage across the resistor to see how much current was flowing. I don't recall measurement details, but an OPT with even a single shorted turn in any winding will suck down a LOT more current than a good one. If you have a few good OPTs lying around, then it's easy to calibrate your expectations.

 

[trobbins]

PP is push-pull or plate-to-plate. My comments were related to output transformer testing.

Measure on the highest voltage secondary (ie. highest impedance secondary). That gives more voltage for your meter.

A shorted winding will cause the turns ratios to be different from expected. There are many good references on calculating turns ratios from impedances (you likely know the speaker impedances and can guesstimate the primary side PP impedance).