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Help with 32-8274 output transformer - Printable Version

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Help with 32-8274 output transformer - Bigreddog42 - 04-06-2021

Good evening. I am a new member restoring a 48-1270 and have been assessing all the components and planning  to recap before applying any power. I believe the 32-8274 output transformer is defective. According to the schematic (attached) I should see 260 ohms from white lead to orange CT and 300 ohms from blue lead to CT; however, I get 146 ohms in the first case and as about 2000 Ohms in the latter. I took the capacitor out of circuit to make sure no interference from that. The tubes are unplugged as well. So, my conclusion (or SWAG) is that the transformer is defective. I would appreciate some feedback on that. Oh, btw I have a donor set that has similar readings so that has led to some of my uncertainty.

The other big question I have is choosing the replacement. According to some other posts I have read the 32-8274 is good for 9000 ohms impedance and rated 15 watts according to Philco catalog, but the 15 watts may be in error as I have another unathoritative source that says 12. Comparing size to replacements I would have guessed it to be about 10 watts. Anyway, I am planning in replacing it with a Hammond universal 125D or 125E (10 W and 15 W) respectively. I would prefer to use the 125D (10 W) due to size constraints but I am wondering what the pros and cons are in choosing between these sizes. 

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RE: Help with 32-8274 output transformer - Radioroslyn - 04-07-2021

RCA sez P/P 6V6's w/250v on the plates max output is 10 watts. That make the decision easy.

RE: Help with 32-8274 output transformer - Bigreddog42 - 04-07-2021

Appreciate the feedback. I’m gonna do the 10 W version. Do you agree the old one is probably toast?

RE: Help with 32-8274 output transformer - Radioroslyn - 04-07-2021

The primary winding on each side of the tap should close in resistance. Since it's all about ratio and the winding at the start to the tap is going to be smaller (shorter lower resistance) than the tap to the finish it's large and longer (higher resistance). It's wound over the first part of the primary. So the resistances are going to be a little different like 100 ohms or so. Yours is way out. It is odd that you have two w/the same failure.

RE: Help with 32-8274 output transformer - Bigreddog42 - 04-07-2021

Thanks for the info! Yeah I don’t know how I got so lucky. I bought the second one hoping to get a good transformer. At least it had a full set of knobs, push buttons, and phono control knob, and some more tubes. It was not too bad monetarily but wife sure got mad about more “junk.”

The other quirky thing is they both may have bad power transformer, in both cases I only see 2.3 ohms across the primaries. I’m guessing that is indicative of short?

RE: Help with 32-8274 output transformer - Radioroslyn - 04-07-2021

As for the power transformer where the wires exist the bottom is there a bunch wax or tar? Is the chassis all yellow/brown colored around that part of the chassis? If not just remove the 5U4 FIRST plug the cord in and turn in on. If the lights go off in the room there's a problem. If the remaining tubes light leave it on for 5 or 10 min and see if the power transformer gets hot to the touch. If it does that's not so good. Probably shorted.
Check the hvac from chassis ground to each plate of the 5U4. Voltage should be within a few volts of each other.

RE: Help with 32-8274 output transformer - Bigreddog42 - 04-07-2021

Chassis is remarkably clean. Nothing obvious. Appears to be all original. Sounds like a good test plan. I’ll be setting up with my isolation transformer and dim bulb, but I have to get a new power cord so it’ll be a couple of days before I can get back to this. The original is badly deteriorated and probably shorted. I really appreciate your help. This is second radio I’ve worked on and it has plenty of new challenges. 

I’ll post outcome...

RE: Help with 32-8274 output transformer - Bigreddog42 - 07-05-2021

Update on transformers:

I replaced the output transformer with Hammond 125D. It works fine. Here's something to consider when using universal transformer. Initially, I missed the inverse feedback circuit implemented off the transformer secondary and missed the need to wire the secondary leads with regard to polarity. Of course, I got it wrong and had some pretty good high pitched oscillations when treble was high and volume low. I switched the leads so that the high side connects to the feedback circuit and that solved the problem. The oem transformer has one lead soldered to the transformer frame to prevent this mistake.

Regarding the power transformer, I purchased a tested replacement, but found that the DC resistance on its primary was about 2.8 ohms (just like the one in my radio). I bench tested it and all secondary voltages were pretty much on spec. So, that was encouraging. After recapping the radio, I powered up the radio with its original transformer slowly increasing voltage with variac and found no problems. The transformer output on all secondaries is pretty much per spec. So, it seems that the schematic showing the primary DC resistance at 28 ohms is incorrect. 

The radio works okay on SW, AM, and FM. I will do the alignment in the next couple of days and then put it all back together. I really appreciate your help and all the other Phorum members - I've gleaned a lot of good information from a lot of previous posts.

I do have a question about alignment but will put that in a new post. Also, first chance I get I'll post pictures and some more restoration detail.