Very hot transformer at 40V - Videokarma.org TV - Video

ldatlof · #1 07-28-2006, 03:46 PM

At 40V on my variac the power transformer of a Philco 39-36 radio was way too hot to touch. There is also a funky smell now - this unit does not have selinium rectifier that I can see. Did I burn out the transformer? The pilot light was lit so the PT is still making some power.

If the PT is not ruined, what would be my next step to get this to work properly?

Thanks.

blue_lateral · #2 07-28-2006, 04:15 PM

For now lets hope the transformer is ok. I'm not familiar with that model, but think the rectifier tube or one fo the filter caps is shorted. You will need to limit current to test. I would set up a dim bulb tester (bulb in series with the radio). You can then see if pulling the rectifier tube helps (this disconnects the b+ winding from everything else). If that doesnt do it, try unsoldering the filament winding.

John

blue_lateral · #3 07-28-2006, 04:23 PM

Ok, I found a schematic here:

http://www.nostalgiaair.org/Resources/295/M0013295.htm

It looks like this set has a rectifier with a cathode, that runs off the same heater winding as the rest of the tubes. There is b+ on the cathode, and the heater has one side grounded. A setup like this is a short looking for a place to happen. I'll bet it's the rectifier tube (84).

At some point fuse the power transformer. It could happen again.

All the best,

John

Edit: The electrolytics could be shorted, also. Now would be a good time to change them if you havent. 17,18,and 43 on the schematics being likely suspects.

Any paper caps that are still there will likely cause grief, too, sooner or later. Probably sooner. I'd change them out. In some old Philcos they are in bakelite blocks.

ldatlof · #4 07-28-2006, 07:52 PM

Thanks for the diagnosis. My tube testers can't test the 84 tube so I will have to get a new one. I am going to recap the whole thing before I plug it in again. What size fuse would you suggest I use for the tranny? Blue_lateral thanks for the help.

Chad Hauris · #5 07-28-2006, 07:56 PM

I would replace all caps before testing...these old electrolytics have are almost always bad. Yes, the fuse in the power line is a very good idea (1 amp should do).
Also check for bad insulation on the power transformer wiring.

ldatlof · #6 07-28-2006, 08:40 PM

Thanks Chad. Upon further inspection the power tranny has 3 wires coming out of it that go nowhere. Looks like they were cut and have what passes for electrical tape wrapped around their ends. 7 other wires are being used though. Is this normal? Safe? Also the part number on the tranny doesn't match the schematic. Same for two of the electrolytic caps. Maybe this radio has been serviced before? It's 67 years old.

Chad Hauris · #7 07-28-2006, 10:24 PM

The 7 wires that are being used should be: (just assigning some arbitrary numbers)
1,2: 110 volt ac input
3,4: 6.3 volt filament
5,6,7 high voltage secondary and center tap.

The extra wires on the replacement trans.may be for a 5 volt rectifier filament with a center tap, but you will need to check with voltmeter to be sure.

Tom Bavis · #8 07-29-2006, 10:46 AM

If the transformer gets hot with all the tubes pulled, it's shorted. Most likely primary or HV secondary would have some shorted turns.

The 84 is the most likely culprit if the transformer is OK - they are prone to shorting, and it's probably the reason the transformer was previously replaced. A fuse is a REALLY good idea.

ldatlof · #9 07-31-2006, 01:19 AM

Quote:

Originally Posted by Tom Bavis

If the transformer gets hot with all the tubes pulled, it's shorted. Most likely primary or HV secondary would have some shorted turns.

The 84 is the most likely culprit if the transformer is OK - they are prone to shorting, and it's probably the reason the transformer was previously replaced. A fuse is a REALLY good idea.

I have had the transformer on at 40V for an hour and a half and its not hot (barely warm) so I tuned up the varaic to 50V. All the tubes are pulled. Can I just go to 120V if the tubes are pulled or do I have to do it gradually? Thanks to everyone for all your help.

blue_lateral · #10 07-31-2006, 02:13 AM

You probably dont have to do it all that slowly. The "slowly" thing is done to possibly "re-form" the electrolytics, and possibly save them. Opinions vary whether that works or not.

It doesnt matter right now, because witht the 84 pulled, the electrolytic caps are not in the circuit.

A little caution wouldnt hurt. You might turn it up a little at a time, making sure it isnt pulling too much current or getting hot. You dont have to take hours like if you were re-forming the electrolytics.

ldatlof · #11 07-31-2006, 02:44 AM

Thanks blue_lateral, that was the answer I was looking for. I try it out in the morning and see how it goes.

ldatlof · #12 07-31-2006, 02:42 PM

At 100V the transformer really heat up and made a foul smell. Also some black goo leaked out from the bottom on to some wires and the table top. Looks like the transformer is shot.