[Winky Dink]

To avoid confusion, since I have assumed a new avatar, I was previously represented as:

[Image: https://c1.staticflickr.com/5/4155/33598...220e_s.jpg]

My question today:

I’m currently engaged in the electronic restoration of  a Model 60.  My first step in any project to check continuity through all coils.  I’ve been very lucky with power transformers, but this one concerns me because my secondary coil resistances show more variance from the original specs than I’m used to…

[Image: https://c1.staticflickr.com/5/4177/34280...1186_z.jpg]

…and the set has been operated with a supplemental 8 mfd cardboard electrolytic condenser, and I don’t know if that has the potential to damage the transformer.  The primary coil checks out at 11 ohms.

So, Philconians, should assume the transformer is OK, or should I test it out before putting a lot more time into the project?  I’ll be most appreciative for any advice.

Thanks,
Henry

(FYI, the new avatar comes from this 1928 cartoon  https://flic.kr/p/TbVbVs  which I first saw about 60 years ago.)

[Radioroslyn]

Hi Henry,
Methinks your in good shape on your power transformer. If you measured the resistances of the heater windings w/the tubes plugged in then that will give you a significantly lower reading as the the tube's heaters are in parallel the winding. The additional 8mfd cap is more or less invisible to the transformer. It's being seen though the resistance of the field coil at the most critical time, startup.

[morzh]

Ditto.

And, testing never hurts anyway: I always test mine.

[winkydink]

Hi Winky Dink,

Nothing about the radio.  Sorry about the name conflict and Avatar similarity.

I have been on the Phorum since 2012, but have been inactive for a few years.  I came back because of a question I recently had on one of my radios (a 1948 table-top).  I saw that my avatar was gone and put up a new one.  I have replaced my avatar with something slightly different.  I didn't mean to be too similar to your avatar picture.  So sorry about that.

[Winky Dink]

winkydink,
I changed my avatar before I was aware of yours. I decided to do this several weeks ago, and it's only coincidental that I saw your post last night. I only mentioned the change so other members would recognize me as the "doesn't-know-much" guy.

However, when I first posted on VideoKarma about five years ago, someone did mistake me for you.

[Winky Dink]

Terry, Morzh,
Thanks. Can you suggest how much load to put on the high-voltage secondary for testing? At least I know enough to not run the transformer without a load.

[morzh]

No, you can run it without load with no ill effects. You might be thinking of audio output transformer: when running a radio, you do want a load there. But the power one is fine. No load necessary: just bring the primary to the nominal voltage and watch the secondaries give you a proper output: I usually limit myself to watching the filament one (as I know what should be there) and then simply watch the high voltage go into the general vicinity of the expected value (the centertapped high voltage winding will give you something on the order 500-600V unloaded).
Keep it for a few minutes to see if anything heats up; that is usually sufficient.

[Radioroslyn]

Or you can remove the 80 tube. Use your AC voltmeter connect one lead to the chassis. Use the other lead to measure the high voltage at each of the two smaller pins at the 80 tube socket separately. Should see about 300-350vac on each. This also tells you that the bias resistor is ok (the big one connected the the center tap of the hv winding).

I rarely measure power transformers unless it has signs of significant overheating (wax or tar on the wood under the transformer or has a burn smell). Output transformers if there is no audio at all. Ant and osc coils on pre 1937 Philcos or if there is a problem.

[morzh]

Goes without saying, the xfmr test is performed without the rectifier tube. And best remove all tubes first.

I also agree with Terry, if I see the xfmr does not seem abused, I will test it only once before I put in the rectifier after the recap. After that I just put in tubes and the load and test the whole thing.
Transformers do go bad, but 1) rarely, 2) it is often quite visible if they are damaged.

[Winky Dink]

Well, it took me a while to process everything y'all said. Regarding my question about loading the transformer--I was, indeed, confusing it with something I read about not powering the radio without connecting the speaker.

I was concerned that wax under the transformer is a bad sign. I had unbolted the transformer a few days ago to find the part number, and there was a thick layer of wax underneath.

The good news is that I ran the thing up to 120VAC and got 382V on each side of the center tap. The data says 680V at the plates, so that's pretty close. My meter only reads up to 600V, so I checked the output voltages with 60VAC and 90VAC, and the results were in appropriate proportions.

Now I have to resolve an open coil in the antenna transformer. That should be the last obstacle to completing the restoration.

Thanks again,
Henry

[Arran]

It's been my experience that it's usually the high voltage winding that fails on a power transformer, but you can usually find this out by measuring the resistance between each leg and the center tap of the HV winding, they should be pretty close to equal, like within 20 Ohms. If it's more then that then the HV winding may have shorted turns, which will cause the transformer to get hot even without a load.
Regards
Arran

[Winky Dink]

Thanks, Arran.The resistances on either side of the high voltage center tap were 169/151 ohms but, as noted, the voltages were identical.  The transformer was stone cold after 10 or 15 minutes of operation.

I spent some time preparing questions about the apparently open coil before I pulled the antenna transformer, but I decided to try to work it out myself.  And I did.  

With the transformer on the bench I still couldn’t physically identify the offending coil until I had some photos.  Then it was a matter of excavating wax to follow the wires (I‘m sure you‘ve all been there).

[Image: https://c1.staticflickr.com/5/4184/34454...96d3_z.jpg]

When I found the coil, it appeared to be intact before removing the wax.  It’s puzzling how the wire could break at this point--there was no oxidation and the wax appeared to be unmolested.

[Image: https://c1.staticflickr.com/5/4168/33685...f330_z.jpg]

But it was broke, and I did fix it.  

Here’s my lesson of the week:  The transformer covers are riveted to the chassis.  Two days ago I cut the rivets to remove the cover so I could remove the transformer.  Just a few minutes ago I realized that I didn’t need to remove the covers at all--I could have pulled the transformer through the chassis instead.  Duh!

- Henry