how to test a SE output transformer

[mark enger]

Is there a way to test a single ended output tester with a meter? mark

[Phil_S]

What do you want to find out?

[mark enger]

Just that its not blown i know the specks

[Phil_S]

Primary DCR should meter maybe 100 ohms, but it could be just about anything from 10-1000. Secondary is often very low, sometimes less than 1 ohm and hard to check without a very good meter, like a Fluke. The easiest thing to do is run 6.3VAC or 5VAC (filament winding on a PT) into the secondary and see what voltage show on the primary.

Let's say the primary is 5K and the secondary is 8R. The impedance ratio is 625:1. The turns ratio is the square root of that or 25:1. If you input 5V to the secondary, expect 25 x 5 or 125VAC on the primary. That will tell you what you want to know.

[pdf64]

Shorted turns are difficult to diagnose but see http://www.geofex.com/FX_images/xform_test.gif
Pete

[alvarezh]

Phil, I used your posted test method, it works. So this means that both primary and secondary windings are not shorted (nor opened), therefore the OT is good?

Thanks in advance.

[Phil_S]

Phil, I used your posted test method, it works. So this means that both primary and secondary windings are not shorted (nor opened), therefore the OT is good?

Thanks in advance.

In general, I think it is reliable. Voltages are high enough, IMHO, to jump over weak insulation. I'm not going to say it is 100% reliable, but the numbers don't lie. If the static turns ratio data appear reasonable, I'd certainly be willing to say it's OK. You might find out later that it isn't OK.

We don't know the voltage that will be applied to the primary. I'm not sure I fully understand how the OT works. Plate voltage (B+ @ DCV) is likely to be quite a bit higher. But the signal from the power tube plate is going to be VAC and I don't know what that should be. We need someone with real technical expertise to comment and educate us.

I am guessing, if there is any reason to be concerned about shorted turns, further testing is probably appropriate. I can't say what that testing is. Pete posted a valuable link to an R G Keen article that looks right on-point. As we all know, the internal insulation between turns is an enamel coating on the wire. I can visualize the coating having melted and exposed a bit of naked wire, but the remaining insulation keeping that bit of wire from making a physical short. At high voltage, I'm guessing it is nothing for the electricity to jump across that very tiny gap. I keep thinking if it stands up to the turns ratio test, it certainly diminishes the possibility it will arc internally like that.

[alvarezh]

Thanks Phil, clear enough, hopefully someone else will post any additional knowledge on this subject.

I have used Pete's posted method also. If I am not mistaken, the neon method is used because sometimes shorted windings don't have enough of a resistance value to be easily measured with a common DVM, it seems the neon method is much more sensitive in detecting shorts, therefore, yielding a fairly accurate result.

Again, gracias!

[katopan]

Once you've done the tests already mentioned, put main voltage on the primary. Make sure connections are covered/taped and safe from kids, pets, yourself. Run it for a while and it should stay cold. If there's an internal fault like a shorted turn it would get pretty hot and probably hum excessively.

[alvarezh]

Once you've done the tests already mentioned, put main voltage on the primary. Make sure connections are covered/taped and safe from kids, pets, yourself. Run it for a while and it should stay cold. If there's an internal fault like a shorted turn it would get pretty hot and probably hum excessively.

Thank you for the tip.

One question, we were originally discussing about an OT test, this main voltage test you are recommending, applies to PTs, OTs, or both?

All the best.

[Phil_S]

I don't see why it couldn't apply to both PT and OT. I wonder if Katopan is thinking AU mains @ 240. In the US, we only have half that.

[katopan]

An internal short will still get hot, even with your woosy 120V supply.

The other thing I forgot to mention is check for continuity with your multimeter from each winding to the frame/lams of the transformer. It's not as good as insulation resistance testing at a proper voltage, but I've found blown transformers shorting primary to frame with just a standard meter. This covers the situation where on the bench the transformer seems to work OK, but actually you've got leakage to the frame (which isn't earthed just sitting on the bench). In an amp that seemingly OK transformer will then get hot or blow a fuse when mounted onto an earthed chassis.