Testing output transformers

[Plimpington2]

I do not have a signal generator, so I gather my testing of an output transformer is not ideal. That being said, I’ve done some basic things to check for shorts and continuity, and have a question that i don’t quite understand:

On the primary, there is continuity between each leg and the center tap (my continuity meter gives a sustained beep). However, the R between one of the legs and the center tap is 104 Ohms and on the other it is 126 Ohms. Is it strictly normal to see this vary between these two readings? Does this mean the center tap isn’t *quite* centered?

Moreover, I cannot get a resistance reading between the two legs of the primary. My multi tester just flashes as if *trying* to find a reading, but never does. Also, on the continuity tester setting, it beeps once and then stops.

I note these findings hold for both OT’s I am testing (these happen to be a set of monoblocks. I would doubt they are both bad.

Thanks
Justin

 

[trainbuftony]

Sounds about right. The primary halves dont look like perfect halves to dc but that doesnt indicate a problem. I have measured similar results on many transformers which work just fine.

 

[primosounds]

I do not have a signal generator, so I gather my testing of an output transformer is not ideal. That being said, I’ve done some basic things to check for shorts and continuity, and have a question that i don’t quite understand:

On the primary, there is continuity between each leg and the center tap (my continuity meter gives a sustained beep). However, the R between one of the legs and the center tap is 104 Ohms and on the other it is 126 Ohms. Is it strictly normal to see this vary between these two readings? Does this mean the center tap isn’t *quite* centered?

Moreover, I cannot get a resistance reading between the two legs of the primary. My multi tester just flashes as if *trying* to find a reading, but never does. Also, on the continuity tester setting, it beeps once and then stops.

I note these findings hold for both OT’s I am testing (these happen to be a set of monoblocks. I would doubt they are both bad.

Thanks
Justin

If your trannys are still hooked up the connection to the PS will mess up your readings since the PS caps will be affecting your readings with their residual voltage. There is some difference between halves of a PP OPT, but, the resistance reading should be fairly close. I am not sure if your readings indicate an issue especially if both of your OPTs read very similar in terms of resistance.

 

[century tek]

I don't think that I have ever found a single push/pull output transformer that measured the same from the center to the outer plate wires. I do remember when I was a teenager I thought an output transformer was bad from an old tube amp just because it didn't measure right from end to end. I threw it away. Man! If I only knew then what I know now..........that was a damn good 20 watt 6L6 amplifier. I remember it well!

If you are getting continuity from both plate wires and center tap, then it's good. I mentioned not to worry about them in your other thread.

 

[Tom Bavis]

Inductors like transformer windings will often fool auto-ranging ohmmeters. When it changes ranges, the current changes, which makes a voltage "kick" that sends it back to the previous range. Over and over... Try a different ohmmeter, preferable with manual range setting. You may be able to set range manually.

BTW - a transformer is often wound as half the primary, then secondary over that, other half of primary last. The last winding is longer for the same number of turns, so has a higher resistance.

 

[gadget73]

Dumb test gear sometimes works better for this sort of thing. I honestly have never been a huge fan of auto-range meters. I find them to be overly slow between the time it takes for them to range and then me to figure out what range its even on to work out the value.

 

[Recycleideas]

I do not have a signal generator, so I gather my testing of an output transformer is not ideal.

Yes you do, if you are working with tubes. It is call the filament output of your power transformer. Its output its just fixed to 60 or 50Hz. It is still a usable audio range for output transformer. If you worry about it being short out during test, you can put a 1K or so resistor in series. For example, if you put a 6.3V signal into a 5K primary taps, you should expect 0.25V at the 8R secondary taps.

Just don't use your AC power outlet or the High voltage taps (make sure they are taped up). Those are suicide parts and not a signal generator.

 

[Ken Boyd]

I know an old thread, but I have not seen any post regarding megging the insolation to test for condition of the insulation on the windings. Knowing if there is a break in the conductor it will be dead in the water but what about impaired insulation. Many times, I am required to meg cables prior to installing and then after installing to make sure no damage was done to the insulation while pulling and termination of conductors. Has anyone had any info on if this would be a reliable test method to find the condition of the insulation on the windings of the transformers varnish.

 

[gadget73]

It should, I have seen "hi-pot" markings on output transformers, so fair bet they got that treatment before they left the factory. Primary to core and primary to secondary tests wouldn't be a waste of time.

 

[AlexHempel]

Yeah, should be able to simply DC hipot as Gadget described; apply DCV between primary and secondary and primary and core. A more difficult one is failing insulation across the primary, which would require an AC hipot test to uncover. I have an AC/DC hipot unit but so far have only used the DC function. I believe that shorts across a winding are far more common than shorts between windings or winding to core, because most transformers have a thick insulator between windings.

 

[gadget73]

yeah the DC test won't give you leakage between turns. Shorts are easy, put it in circuit and it will make garbage power and the tubes will hog lots of current doing it. Leakage that only happens at higher signal levels would be harder, though if the insulation is compromised enough to do that, it may also show up as leakage to the frame or secondary.