My output or power transformer is dead, isn't it? How do I test it to find out?

[R.G.]

First, it's probably not dead.

It could be, but it's not the first thing to guess. Transformers are in general quite durable. They fail two ways: being overheated and being punctured by too-high voltage.

When they fail, there are three kinds of failures possible. These are:
1. A winding burns open and will not conduct at all.
2. A winding shorts, possibly (a) to an adjacent turn, (b) to another winding or ( c) to the core.
3. A winding has an intermittent short or open, possibly intermittent with vibration or temperature.

You can test for items 1 and 2. Since failures of transformers in general are unusual, the intermittent failures of item 3 are downright rare. Not impossible, but rare.

To test a transformer, you need
- an application schematic, showing how it's connected, particularly what transformer leads are connected to internal windings
- a multimeter that will measure AC volts and ohms
- a source of low voltage AC, like a filament transformer output
- a neon bulb, like an NE-2
- a battery; the 6V lantern/flashlight battery is ideal.

Test 1: Winding shorts and opens
- Disconnect the transformer from the circuit so the circuit does not give you false readings.
- Set your multimeter to ohms and measure the resistance of each winding. You're looking not for a specific value of ohms, but for the difference between a possibly long length of copper wire and an open circuit. Windings may be a fraction of an ohm, or may be a few hundred ohms. But they are NOT 1M or over. So it's easy to tell.

Each lead shown on the schematic as being connected to a continuous winding must show *not* open circuit to all other leads on that winding. If one or more leads on a winding are open to the others, the transformer has failed. Then measure resistance between windings; it doesn't matter which of the leads on the windings you pick, as the resistance between any two leads on different windings *MUST* be an open circuit.

If there is a resistance, or worse yet a very low ohms value, and the schematic does not show that they are connected, there is an internal short. Notice that you MUST open up any external connections that may cause an external connection of the two. Two unused leads accidentally touching each other or the chassis can cause a false positive.

- Make sure *all* windings are open. Connect the neon bulb across any winding. Doesn't matter which one. Pick a winding, doesn't matter which one, and temporarily connect it across the lantern battery. If the neon bulb flashes when you remove the connection to the battery, there is no internal short inside a winding. Notice that any external loading or connections can show a false failure on this one.

- If it passes all that, it's almost certainly good. To be extra sure, again make sure that all leads are open, and then connect the low voltage AC source to one winding. Low voltage windings are preferable but not critical. Measure the AC voltage on each winding section. There should be an AC voltage proportional to the winding's ratio to the winding being driven.

That's it. If it passes all of those, the only possible flaws left inside are latent ones, things that only short or open under high voltage stress or high temperatures. As noted, these do occur, but are quite rare.

Questions?

[R.G.]

Thermal Fuses

A power transformer that fails Test 1 with the primary or one of the primaries open may have a thermal fuse inside that has overheated and opened.

What are thermal fuses?
A thermal fuse is what the name says - a fuse in series with a winding that opens not on overcurrents, but because it got too hot.

Why are thermal fuses?
In common with AC power fuses it is not there to protect the transformer or any of the circuits inside the amplifier. It is there to prevent a failure inside the amp chassis from starting a fire.

There are specific safety regulations and standards in most countries. Among these is a requirement that any possible single fault, including especially the logically worst-possible one, must not cause flaming debris to get outside the chassis to start a fire, nor cause skin-burning temperatures on the outside of the chassis, nor cause electrocution hazards. A power-line transformer can cause any of these if some condition inside or outside it causes it to overheat internally, short the AC power line in a way that does not blow the AC power fuse. The thermal fuse inside the transformer is one way that has been developed to prevent this from causing a safety problem. When the transformer overheats, the thermal fuse opens and stops the AC power from continuing to heat the transformer.

Well, cool! I'll just replace the thermal fuse if that's the only problem!
Good idea. But impractical.

Here's the problem. To properly protect against the situations that cause transformers to go into thermal runaway, the thermal fuse has to be buried at least a little way inside the transformer. So right away, to replace it you have to go commit surgery on the transformer. In addition, the conductors the thermal fuse connects to are the ones which carry the AC power line currents, so there are all of the safety issues in AC power line insulation that apply to that, and in addition the requirements to make it work properly *thermally* inside the transformer so it will protect it again. That may be hard to do, and certainly requires knowing how to replace it properly so it still offers protection.

About now, someone reading this is thinking "Well, no big deal, I'll just short the two wires where the fuse was." That has exactly the same effect that bridging around an overcurrent power line fuse has - it permanently removes the possibility of the fuse doing the job it was supposed to do - keep component failures from killing people by electrocution or starting fires. And because of the surgery required, it's possible to invalidate some other of the insulation methods that were put there to keep the transformer safe in ways unrelated to the thermal fuse, creating a new shock hazard. The hazards may not surface for a long time, so having it work when you get done is not necessarily a successful repair, only a not-yet-failed-again one. Like brain surgery, this isn't the kind of thing you want do learn by trying for yourself the first time.

And in common with AC power line current fuses, the thermal fuse blew for some reason. Fuses can and do fail simply because the fuse was faulty, but it's rare. It's much more likely that something the fuse was supposed to protect against happened, and the fuse did its job right. So just replacing (or shorting out) a thermal fuse does nothing to cure the problem that caused it to blow in the first place. It may be a step in the debugging process to find out what else made it blow, but it's not a complete cure.

Like heart or brain surgery, opening up a power transformer and "fixing" a thermal fuse may or may not be a success depending on the process used to close up after the repairs, and simply eliminating the blown part may have consequences far beyond applying the patch. For myself, I believe that music isn't worth dying for, at least not for something that I can get replacement parts for. So at best, bridging or replacing a thermal fuse is something I consider a field expedient, something that may get me limping along until I can get the proper fix applied, but not something I'd want to live with.

[trobbins]

RG, you may want to add a caution when applying a filament (5V or 6.3V) supply to an OT secondary, or a similar PT filament winding. The resulting primary winding(s) voltage may be very high and hazardous. At the other end of the spectrum, only apply this level of voltage to nominally 5W or higher rated OT's, or the same filament voltage winding on a PT.

[R.G.]

Thank you.

Yes, what he said. Don't electrocute yourself on a transformer you're testing (not least because it would invalidate the testing... ) and don't do this to transfomers that are too small.

In that vein, you can use half of a 6.3V filament winding instead of the full 6.3V.

[Chuck H]

I actually use my signal generator. I'm sure it's possible that the reatively huge transformers inductance can skew the test but I find that if I measure the input voltage with the unit under test that results are pretty accurate. It might help to include a link to info for testing relative ratios for the purpose of detecting shorted turns. The math is simple enough but my explaination may suck. If I find a link I'll tack it on.

[R.G.]

A signal generator is good if it has the output voltage to do the job. I actually started this as an aid to the semi-infinite number of questions that start "My amp quit. It's the output transformer, isn't it? Give it to me straight, Doc. I can take it."

I pretty much assume that the condition of having a signal generator means that a person has enough experience to not ask the question that way. Someone with a signal generator is much more likely to say "How do I test transformers?"

A high inductance transformer with all windings open circuit is actually easier on the signal generator than a low inductance one. Higher load impedance.

[nosaj]

Here's an example.
primary brown and blue Red center(HV?)
brown-Blue 144 ohms
Blue-red 54ohms
Brown-red 60.3 ohms

Secondaries
yellow Green black yellow/Green(was alway heatshrinked no connection)

yellow- green .6 ohm
yellow-black .3 ohm
black-green .3 ohm

Also on the schematic d301 and d302 diode test fails. I do not have a lantern battery, but I do have variac or a nine volt. Output transformer is completely disconnected and diode test tubes were pulled.


I'm confused by this statement below so I included the schematic of the tranny as I'm not sure what my numbers reveal
If there is a resistance, or worse yet a very low ohms value, and the schematic does not show that they are connected, there is an internal short.

First, it's probably not dead.

It could be, but it's not the first thing to guess. Transformers are in general quite durable. They fail two ways: being overheated and being punctured by too-high voltage.

When they fail, there are three kinds of failures possible. These are:
1. A winding burns open and will not conduct at all.
2. A winding shorts, possibly (a) to an adjacent turn, (b) to another winding or ( c) to the core.
3. A winding has an intermittent short or open, possibly intermittent with vibration or temperature.

You can test for items 1 and 2. Since failures of transformers in general are unusual, the intermittent failures of item 3 are downright rare. Not impossible, but rare.

To test a transformer, you need
- an application schematic, showing how it's connected, particularly what transformer leads are connected to internal windings
- a multimeter that will measure AC volts and ohms
- a source of low voltage AC, like a filament transformer output
- a neon bulb, like an NE-2
- a battery; the 6V lantern/flashlight battery is ideal.

Test 1: Winding shorts and opens
- Disconnect the transformer from the circuit so the circuit does not give you false readings.
- Set your multimeter to ohms and measure the resistance of each winding. You're looking not for a specific value of ohms, but for the difference between a possibly long length of copper wire and an open circuit. Windings may be a fraction of an ohm, or may be a few hundred ohms. But they are NOT 1M or over. So it's easy to tell.

Each lead shown on the schematic as being connected to a continuous winding must show *not* open circuit to all other leads on that winding. If one or more leads on a winding are open to the others, the transformer has failed. Then measure resistance between windings; it doesn't matter which of the leads on the windings you pick, as the resistance between any two leads on different windings *MUST* be an open circuit.

If there is a resistance, or worse yet a very low ohms value, and the schematic does not show that they are connected, there is an internal short. Notice that you MUST open up any external connections that may cause an external connection of the two. Two unused leads accidentally touching each other or the chassis can cause a false positive.

- Make sure *all* windings are open. Connect the neon bulb across any winding. Doesn't matter which one. Pick a winding, doesn't matter which one, and temporarily connect it across the lantern battery. If the neon bulb flashes when you remove the connection to the battery, there is no internal short inside a winding. Notice that any external loading or connections can show a false failure on this one.

- If it passes all that, it's almost certainly good. To be extra sure, again make sure that all leads are open, and then connect the low voltage AC source to one winding. Low voltage windings are preferable but not critical. Measure the AC voltage on each winding section. There should be an AC voltage proportional to the winding's ratio to the winding being driven.

That's it. If it passes all of those, the only possible flaws left inside are latent ones, things that only short or open under high voltage stress or high temperatures. As noted, these do occur, but are quite rare.

Questions?

[pdf64]

primary brown and blue Red center(HV?)
brown-Blue 144 ohms
Blue-red 54ohms
Brown-red 60.3 ohms

That indicates a problem; R brown-blue should be 54+60.3=114.
Was 144 a typo, and it is really 114 ohms?

[nosaj]

Yes a typo it is 144 ohms just verified. So what do you Transformer good or bad and whatabout the diodes on the output tubes being bad(could those be volume suckers?

Thasnk,
Jason

[R.G.]

I'm confused by this statement below so I included the schematic of the tranny as I'm not sure what my numbers reveal
If there is a resistance, or worse yet a very low ohms value, and the schematic does not show that they are connected, there is an internal short.

Well, I have to admit, it's not exactly a marvel of clarity. I'll try again.

The schematic should show which leads from the transformer are supposed to be connected to a given winding. In your example, the primary is brown, red, and blue. The secondary has leads yellow, green, yellow/green, and black. For a given winding (e.g. primary) the leads should all show resistances between them, generally less than 1K.

What this test tells you is whether each wire lead that's supposed to connect to the internal windings does. In your case, it succeeds, and no lead is open. So far, so good.

What the sentence is trying to say is that you now need to use your meter with one probe on any primary lead, and the other probe on any secondary lead, and see if that is an open circuit. It should be if the transformer is not internally damaged and there is no external connection between the windings. In other words, the transformer should completely isolate primary from secondary in your example. Your data does not show that. You need to test for resistance between any primary lead and any secondary lead. This should show open circuit.

It is true that the resistances between each lead ought to add up to the sum of the resistance across the winding. However, I intentionally did not include that or say anything about it, because it is an unreliable test unless the resistance through a perfectly-working winding is over about 50-100 ohms, and even then I suspect it. This is because (a) it is difficult to measure low resistances accurately without special equipment and (b) even if you can, the lead resistances get in there and mess things up. If the transformer ever worked right in the first place, then it is unlikely that trying to squint small differences in winding resistance will tell you anything useful. So this is not a particularly useful test.

What people who do the sum-and-difference-of-resistances test are trying to do is to tell if one or more turns are shorted internally, reasoning correctly that this will lower the total resistance by the number of turns shorted out. The problem is that it's not likely that you can make this measurement with enough accuracy for it to tell you anything at all.

That's what the flashing-neon test is for. If the bulb does not flash, then there is a short. It can't tell you which winding or how many turns, but you don't need to know that. The transformer is dead without rewinding. It's not a subtle test; it says yes or no, dead or alive. Much better than squinting milliohms.

I do not have a lantern battery, but I do have variac or a nine volt. Output transformer is completely disconnected and diode test tubes were pulled.

The reason that the neon bulb test works is that the battery charges up the inductance of a winding with DC current. When you open the circuit, the magnetic energy in the core from the DC current causes an inductive kickback. However, this can come out on any winding on the core. All of the windings see the sudden DC voltage spike appear. So whichever winding conducts first "eats" the magnetic energy out of the core. If all windings are otherwise open circuited, this will be the winding with the neon bulb on it. The NE2 fires at about 90V, low enough to be sure that the wire insulation inside the transformer is not punctured. However, if there is an internal shorted turn of any kind, the shorted turn first prevents the DC loading of the core from happening, and then "eats" any kickback that might happen from any stray field that does get formed. So any short keeps the neon from flashing, no matter which winding it's on.

It is possible that a 9V battery would provide enough DC current to make a flash. I have not tried this. Try it on a known-good transformer and see. AC voltages *will not work* for the shorts test as stated.

So you have two tests to do yet. First measure the resistance between primary and secondary. It should be open circuit. Then run the neon bulb test and see if there's an internal short.

Also on the schematic d301 and d302 diode test fails.

Those diodes are there to protect against transient spikes. If they fail, that all by itself is enough to keep proper sound from coming out. Replace them. It may be the transformer is fine. It may be that the transient that killed the diodes also killed the transformer. Can't tell without some testing.

[nosaj]

Well pri/sec test passed, NE2 flashed on secondaries.
And yes a 9v works.
Using a variac set to 6.7vac(that's the closest I could dial it.) Feeding yellow wire and green wire.
I get on Brown-Blue 294 VAC ratio of 43:1
Red-Brown 159 VAC Ratio of 23:1
Red -Blue 162 VAC. Ratio of 24:1

So I would safely say the output transformer is not the issue and more likely the Diodes.

I could cut them out just for a volume test, correct?
Don't know if it matters or not this is a fender prosonic OT b-049009 EIA 606-601.

Thasnk,
Jason

[Prairie Dawg]

RG, I've got something I'm working on that has me stumped in this line and maybe you can help. I have a Champion 600 that I'm repurposing as a 5C1, and the objective was to reuse the original Ningbo Chaobo power transformer with a 6v rectifier tube-a 6X5-and do this at the lowest cost, so diodes are out. I want rectifier sag.

The original power transformer has no center tap. The original power supply looks like this:



I found a hybrid power supply circuit on diy audio that looks like this:



I added in the diodes in the Ciuffoli layout and I am now getting 266v dc on pin 8 of the 6X5-not enough, I want about 350v or thereabouts. I do not yet have any load connected like the filter capacitors and dropping resistors. Would that make a difference?

Is there a way to do this aside from using the original iron as a paperweight and going shopping? I mean, if I had another lifetime after this one I earnestly promise I would get an EE and learn everything there is to know before tackling stuff like this but absent reincarnation I'm forced to depend on common sense, standard torques, and the kindness of friends.

As always your help is appreciated.

[R.G.]

Connect the first filter capacitor and put a 100K or so bleeder resistor across it so it will run down. Yes, not having any filter capacitor on it will make the metered voltage lower. The filter capacitor charges up to and holds the peak of the rectified waveform.

[Prairie Dawg]

RG, it worked like a Champ if I may be permitted a small pun. Right now with tubes installed I have a solid 330v-316v-275v of DC on the B+ string and 6.4v on the filament string, plus I think I saved a lot of transformers from landfills or duty as paperweights.

Folks, you can now build a tube rectified Champ clone without changing your Champion 600 power transformer.

[elushi]

connect the low voltage AC source to one winding. Low voltage windings are preferable but not critical. Measure the AC voltage on each winding section. There should be an AC voltage proportional to the winding's ratio to the winding being driven

This method would be helpful too if I want to identify the secondary windings right?
The secondary winding is just wired to two 8-ohm speakers in parallel. So I guess they are connected to a 4-ohm tap. I want to know if the transformer has other secondary taps (like 8 and 16 ohms), how many of them and which one is each one, since it's from a vintage amp and doesn't have any mark or legend anywhere. Just the connections.
So if I connect the filament winding to the primary and read the voltages on the secondary windings I would be identifying the taps right?
-from one of this connections (not plugged to any speaker) is where the negative feedback is taken to the phase inverter stage so I'm sure that there is actually another tap but I don't know which one is.-

I don't have a variac nor a signal generator. and don't have plans to buy one of them neither