Is this transformer toast? - Printable Version
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Is this transformer toast? - pokey - 06-06-2020
This is from a 46-1209 in very bad shape I picked up as a learning project. Before I attempt to replace the capacitors (which I understand is a good place to start if you're a total novice), I'm familiarizing myself with the chassis and schematic.
Anyway, does the power transformer look like a goner? I don't have the skills to test it -- just curious whether one can give a rough diagnosis based on its rusty looks. Thank you!
RE: Is this transformer toast? - rfeenstra - 06-06-2020
External rust is not an indication that the transformer is a goner. You have to check with a meter to see if there is an open winding, particularly in the hi voltage winding. Filament windings will register a very low resistance. The primary, according to the service info, should be about 4 ohms. From either Yellow to Green should be about 260 ohms. If these measurements check out, you may be ok. Final test is to power up the transformer, preferably through a current limiting light bulb (in case there is a short in one of the windings), then check the output voltages on each winding; filament and HV windings. Careful, there is several hundred volts on the HV winding! You say you don't have a way to test. does that mean you don't have a meter? If not, buy one! It is indispensable.
RE: Is this transformer toast? - Phlogiston - 06-06-2020
It is mostly iron and copper. - - - yum, toast!
RE: Is this transformer toast? - pokey - 06-06-2020
Thank you, rfeenstra. By not being able to test it, I meant I don't know what I'm doing. Today I used a multimeter for the first time ever (to see which tubes might be okay).
Would you mind walking me through how I take the measurements you mentioned with the meter? I see in the schematic yellow and green going from the secondary to the rectifier, but when I look under the chassis I don't know where to apply the probes. Please assume near-zero experience on my part, so if you're up to it, the "for dummies" version will be immensely appreciated!
RE: Is this transformer toast? - rfeenstra - 06-06-2020
Okay. You are starting from the beginning, and we all did at one time!! There books out there that deal with the restoration of antique radios that are mentioned in other threads on this phorum. I would suggest your search and educate yourself as much as possible. It is a very rewarding experience. Take this project slowly.
Unfortunately, time has eroded the clear delineation of colors in these old radios. Many wires that were once various colors, now look similar. Let's start with the transformer primary. The service info says both wires coming from the primary were once white. These 2 wires will go to the power cord. One directly, and the other through the power switch. Put your meter probes on these 2 connections coming from the power transformer and measure the resistance. It should be about 4 ohms.
I can't tell much from your pic. Not sure what I'M looking at since I've never worked on this model. The 2 yellow wires from the power transformer should go to pins 4 and 6 of the 5Y3 rectifier tube. The resistance between those 2 pins should be about 520 ohms. there will be a little variation. Let's start there and see what we've got.
Rob
RE: Is this transformer toast? - pokey - 06-07-2020
Thank you! Yes, I've been reading as many sources as I can get my hands on. So far, the Navy's 1946 "Introduction to Radio Equipment" has delivered the most a-ha! moments. The whole experience is indeed very rewarding. Addictive, even.
Ok, found the connections. For the two white ones (one of which shares a joint with one power cord black), the meter jumps all over the place. But when it settles, it reads 0.4, which I guess = 4 on my meter's lowest possible setting of 200. That's good, right? For the two yellows on pins 4 and 6 of the rectifier, I'm getting 185.
I appreciate the guidance!
RE: Is this transformer toast? - rfeenstra - 06-07-2020
What does your meter read when you short the two probes together? Just curious. .4 seems low. If the schematic is at all correct, it should read about 4. It's possible that the capacitors in the Bakelite block are causing a reading error. You might remove one of the white wires from any connection and measure again. And the 185 seems low. Is that between the two yellows or from each yellow to the green?
RE: Is this transformer toast? - pokey - 06-07-2020
Aah, the meter does read .4 when the two probes touch, the same value I'm getting between the two white wires. I removed one of the white wires like you suggested and there was no change.
Now I know what a Bakelite block is! And that it has two capacitors in it. If those need replacing, I'm doing them last :-)
For the 185 reading, I'm measuring between the two yellows that go to pins 4 and 6. The tube doesn't have to be in, right? I would try yellow to green but I can't find the green (I do see it in the schematic).
RE: Is this transformer toast? - klondike98 - 06-07-2020
Those bakelite blocks are actually pretty easy once you have done one or two. Here's a tutorial from our library section. I usually take them out of the chassis completely but here Mike (morzh) does it without desoldering:
https://philcoradio.com/library/index.php/shop-talk/techniques/bakelite-capacitor-rebuild-without-desoldering/
Service Bulletin #289 is also useful:
https://philcoradio.com/library/index.php/shop-talk/general/bakelite-block-data/
RE: Is this transformer toast? - rfeenstra - 06-07-2020
Hmmm. That would suggest a short circuit in the primary You may have to remove the transformer and open it up to see if there is something obvious causing the low resistance reading. Don't worry about the yellow wires until you find out why the white wires measure a dead short. If you wish, remove the transformer and send it to me and I'll do the best I can to test it for you. PM me if you are interested.
RE: Is this transformer toast? - pokey - 06-08-2020
Thank you for the links, Bob. Really useful. (The second one doesn't work but I'm sure I can track down #289 with a little internet digging.)
And thank you Rob for the kind offer. I'll open up the transformer and if I have no luck spotting the issue I'll PM you.
RE: Is this transformer toast? - pokey - 09-11-2020
Ok, giving this 46-1209 transformer one more attempt. Since my last post, I've replaced the caps (including Bakelite block and electrolytics) and power cord.
The 40-watt bulb on the dim bulb tester—I made one!—glows faintly, which I understand is a good sign. I also opened up the transformer and, while it ain't pretty, nothing appears obviously damaged.
Primary white to white: 0.4 ohms, like last time. (Stupid decimal point.) Am I checking at the right connection points? Please see orange arrows in photo.
Rectifier yellow to yellow (pins 4 & 6, tube taken out): 184 ohms, like last time.
Power cord prong to prong (radio switched on but unplugged): 50 ohms. Don't know how to interpret that.
Are there other basic checks I can try before I give up and ship this thing off to more able hands?
RE: Is this transformer toast? - Radioroslyn - 09-12-2020
The measurements seem reasonable. Not sure about the resistance across the cord but in may just be an anomaly do to the digital meter reacting to the inductance of the primary and the line caps. Wouldn't give it too much thought.
Next steps in my mind would be to put the transformer back together and connect your dc volt meter across C-49 noting the polarity. Plug in all of the tubes and the spkr. Plug in the cord and turn it on while watching the meter. DC voltage should start to rise after a few seconds. Will peak @ abt 370v and start to drop to abt 320v as all of the tube start to conduct. If you don't see several hundred volts after it's been on for 10 seconds or so turn it off. Else bad things could happen.
Have Fun!!
RE: Is this transformer toast? - pokey - 09-13-2020
Thank you, Terry! As soon as the replacement tubes arrive, I'll go ahead with your instructions.
Should I worry about possibly ruining the new tubes *before* the 10-second cutoff?
RE: Is this transformer toast? - Radioroslyn - 09-13-2020
>Should I worry about possibly ruining the new tubes *before* the 10-second cutoff?
Should be OK. The tube that will take a beating if left on is the 5Y3. If you notice any resemblance in the coloring of your 5Y3 and this 280 turn it off turn it off turn it off!