[w4usf]

Curious to know if transformers often also go bad on these old radios and if/how you test them, too (vom?).  I am soon to pick up a 40-195 and will replace electroylitics and paper caps as well as power cord and out of tolerance resistors. Dial glass and push button knobs also will be needed (Radio Daze), but I suspect if the transformer is gone I may have to think twice before these purchases. Not buying this for an investment but more for sentimental reasons, as my grandfather had this radio years ago (in the 50s for me). Also, enjoying Ron's 1928-42 Philco book! Best - John

(Edit-this post was split from a separate thread to collect responses under the new title)

[klondike98]

Hi John, welcome to the Phorum!!!  You ask a great question but its kind of lost in the thread you originally posted in.  I'm going to split it out with a new title.  Hope that's OK....

How much testing you can do on yhe power transformer before you buy is highly dependant on the seller but most probably are not too interested in having the chassis pulled for a long inspection, and you can't do that if its an Ebay sale.  If the windings give about the right resistance reading, its probably good.  I've only restored around 15 radios so those on this Phorum with much more experience will surely chime in with their thoughts on how they evaluate for purchase.  I like to hear the seller say the tubes light up and the speaker has static.

[morzh]

How you check a power xfmr depends on whether you have bought the radio, or you are asking a seller to do it, or the seller has done it before you asked.

The basic way is what Bob just said, just measure the windings with an Ohmmeter against the known values.
The best way is applying voltage and measuring outputs without rectifier and other tubes plugged in.
This is best done using a variac, so if the xfmr is shorted you do not smoke it.

If, as Bob noted, the seller volunteered plugging it in and reports lights and static, it is good, though thos is usually done against the accepted practice of not powering up a unrecapped and orherwise unchecked radio.

[Jayce]

One way my late friend used to test power transformers was to pull the rectifier tube and turn the radio on. If the tubes light, that is a a good start. Then, he would watch for any smoke and odd smells and if the set could be on 15 minutes or more without the transformer getting super hot, there is a good chance it is ok. I've used that method myself and it seems to work for an initial check. One other way to check is to pull the chassis and look for melted tar. If you see a good bit of that, beware!

[klondike98]

morzh wrote:
....plugging it in and reports lights and static, it is good, though thos is usually done against the accepted practice of not powering up a unrecapped and orherwise unchecked radio.

LOL...yes but you almost always hear them say they plugged it in  

[morzh]

Yep, their loss if goes awry. More power to them (pun intended).

[Arran]

 Before you test anything you should get a copy of the schematic to tell you what is what with regard to the power supply and it's components, particularly if there ares any resistors connected in series with the center tap of the H.V winding. With most radios you don't need to pull the chassis to check the power transformer, you can unplug the rectifier tube from it's socket and measure the H.V winding across the two plate pins, by poking the probes into the pin holes, and from each plate pin to the center tap which is either connected directly to the chassis, or through a resistor or two to the chassis.  You can also test the filament winding for the rectifier tube whilst the tube is pulled, but for checking the filament winding for the other tubes you need to pull all the other tubes out of their sockets and measure across the heater pins. The other winding to check is the primary winding, which you can do across the two pins of the power plug if the cord is good and the power switch is still good. It's not really necessary to test the filament/heater windings, they are probably the last things that could fail on a power transformer, and if someone shorts across one it's the primary that will burn up. 
Regards
Arran

[codefox1]

Agree with all. If the secondary (B+ to be) winding is center tapped, the AC voltage for each half should be very close to each other. The resistance of the windings will be off some because one half of the winding is on the inside of the core, the other on the outside, but the number of turns are equal.

A transformer with no load, (all tubes removed) should not become more than a little warm if left connected to power for an hour. If it gets hot, there is a short somewhere.

[w4usf]

Thanks for all your help here. Just got the radio and will check it out. Best - John

[Old Buzzard]

Thanks for all your help here. Just got the radio and will check it out. Best - John

As stated above, Make sure you remove the rectifier tube if you're going to test the transformer that way!!!!!!

[palegreenthumb]

You can check most of the windings with a couple simple tests at the seller's house.

1. Unplug the radio, turn the power switch on, measure resistance through the primary across the pins of the power plug.  Usually 10s of ohms.

2. Pull the rectifier, measure the 5V winding resistance across the heater pins of the rectifier socket.  Should be a near-short (tenths of an ohm).

3. Measure the high-voltage winding resistance across the plate pins of the rectifier socket.  Should be several hundred ohms.

4. Then, as stated previously, leave the rectifier out, plug the radio in, and see if the heaters of the other tubes light up.

The most common way this fails is that the power switch is failed open,  which means you can't do checks 1 and 4.

[morzh]

#1 pften does not work: almost in half of cases the switch itself is bad, so you may see open across the plug with a perfectly good xfmr.

[palegreenthumb]

Agreed.

Just had an instance of that a month or two ago.  Went to a seller's house to look at a 5-S-29, and got a bad feeling 'cause the primary seemed to be open.  Seller wouldn't budge on price though because he knew what the radio was worth.  So I brought it home at his price--still not a bad price, just couldn't talk him down lower.

First thing I did when I got home was pull the the chassis and check the transformer proper.  It's fine.  It's the switch that's bad.

[morzh]

Most Philco 20 suffer from that.
Worst yet, those switches that are dedicated switches (not combined with volume pots) are not easy to disassemble, unlike those in volume regulators.
And, as I said, about half the radios I've done with volume pot switches had dirty contacts that resulted in full open. Simple alcohol washing did the trick.