Best way to test a power transformer?

[voxnut]

Got my brother's E 120 working nicely. Lasted for a week or so. He left a message to let me know that the box tried to select a record and then he heard a "pop" and then it died. By "died" he means that he can't select any records and the carriage doesn't receive any power. I had him check the fuses in the junction box (I live a few towns away) and he said they were good. So I thought perhaps the selenium rectifier shorted, and it's my understanding that they short open. I had him order a 400V 25A bridge rectifier and said that I'd come down and have a look when that came in.

Long story short, I check the fuses with a continuity tester and they check out, I remove the junction box from the side wall of the box and I'm surprised to find that the previous owner had replaced the original rectifier with a bridge rectifier, although I couldn't find a value on it. I also notice that it stinks to high heaven in there - the smell of electrical death. I had read that with a bridge rectifier installation you should add a fuse one of the legs in the AC circuit, a 2A slo-blo, because when bridge rectifiers short, they short closed and then can cause the transformer to overheat. This one did not have a fuse added. The sniff test leads the stink to the transformer itself. None of the wiring inside the junction box appears to be fried. 


What is the best way to test the transformer? I would imagine given the olfactory clues the transformer is cooked, but I'd feel foolish if that's not quite the case. Also does anyone know what the specs are on the E-120 power supply transformer? It's right to assume 120V in, and then I've seen 28v to 32v DC output, but that's after the rectifier. What would the AC output need to be?


Also, if it is in fact fried and useless, is it only a matter of searching for a used E-40/80/120 output transformer, a used one from a slightly later AMI model that would work, or is it possible to find specs for a modern replacement?    

Thanks!
Dean

[jukenorman]

Hi Dean,
The original selenium rectifier was a bridge rectifier; I assume you mean that the selenium rectifier was replaced by a silicon bridge rectifier? In which case, you are correct that the transformer secondary should have been fused because silicon diodes do tend to fail short circuit. You haven't said whether you tested the rectifier and it has failed. The transformer could have failed for other reasons - for example if it had been contaminated by moisture.

The transformer seems to have only a single secondary winding so you could simply check whether that is open circuit on ohms (most likely). A replacement transformer with a secondary winding of 22V approx at 3A should be more than fine. A 25A rated bridge rectifier is overkill.

Norman.

[voxnut]

Thanks for taking the time to reply, Norman. I appreciate it. You are correct, I did mean a silicon bridge rectifier. I will check out the rectifier and the transformer. I appreciate the tip on a replacement as well.

Dean

[voxnut]

This was the closest I could find to 22V secondary with 3A. A suitable replacement? www.digikey.com/en/products/detail/hammond-manufacturing/166M24/2358121?s=N4IgjCBcpmBMAGKoDGUBmBDANgZwKYA0IA9lANoiJgCsAnACwgC6xADgC5QgDKHATgEsAdgHMQAX2JgAbGADsyEGkhY8RUhXANGAZibSaNMAA5dLdl0i8BI8RIdA

[jukenorman]

Hi Dean,
Yes you would probably be OK with that transformer although I see that you have stated 120V in and the Hammond transformer states 115V input. The output would be of the order of 25V, so you're pushing the voltage higher. What about the Hammond 167M20 with 20V secondary but with 120V applied would be close to 21V - it's also an enclosed transformer?

Norman.

[Ron Rich]

Hi Norman/Dean,
It's "common" for a transformer maker to state a "115 vac" primary-- but that's usually a "nominal" voltage-- usually anything from 109 to 125 vac will produce a real close secondary voltage. On that transformer-- I doubt it's critical-- I have replaced one of them--years ago, with an old, Seeburg wallbox power transformer-- as far as I know--it's still OK ! Ron Rich

[voxnut]

Thanks Norman and Ron for taking the time to reply! Norman, I see that you are in the UK - here in the US mains voltage is stated at 120V +/- 5%, which means on different grids you'll get anywhere from 114V to 124V at the wall socket, so it's hard to be precise. Good info, Ron! Thanks!

[Ron Rich]

"Maximum allowable" voltage, here in California, is 123.5 vac-- I have caught our power company supplicating above that limit-- they are very quick to show up at that type call-- they went to the power pole with the transformer on it, and fixed it--quickly ! LOL ( You do know-- the higher the voltage the faster it "spins" the meter !) Ron Rich

[jukenorman]

Hi Ron and Dean,
Now I come to think about it, AMi's do seem to be quite tolerant on voltage. In the 1980's a lot of ex US used AMi's that had been exported to Greece came to mainland Europe as restoration projects. You've never seen an AMi move as fast as a Greek one!

Norman.

[Ron Rich]

Hi Norman,
I'm not quite sure how to take, THAT, comment --- LOL-- Ron Rich