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 Post subject: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 22, 2017 5:11 am 
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Location: Centennial, CO
Hi everyone,

I'm working on a Philco 41-250, and it seems to have a bad output transformer (32-8120: connection from either end to the center tap is open). It looks like a Hammond 125B should be a pretty similar equivalent, and would have no problem with the 2 watts of audio output. Here's a link to that: http://www.hammondmfg.com/125a.htm

Just want to check, first off, am I correct that this would be a good replacement? Or is there a better option? Next, how will I hook this up? The original has 3 wires: black, white, and black white (center). The hook-up data shows blue, brown, and red (center) wires on one side, but also a bunch of other hookups that get used in different scenarios. Do I need to do anything with that? Or can I just use those 3 wires?

Schematic of the radio: http://www.nostalgiaair.org/pagesbymode ... 013407.pdf

Thanks!


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 22, 2017 11:43 am 
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Location: Annapolis, MD
You have a push-pull output stage, so there are three primary connections. Then there are two secondary connections ( to the speaker ). You choose the secondary connections to get the right impedance match.
If the speaker impedance is not stated somewhere, start by trying 8 ohms.**

To get the primary impedance, look up the tubes. Look at the "typical operation" section, and there will be an entry for recommended load.**

Those universal transformers come with a data sheet that shows how to wire for different impedance ratios.**



**what counts for an output transformer is the impedance ratio, which is the square of the turns ratio. Note that impedance is not the same as DC resistance.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 22, 2017 2:29 pm 
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Joined: Nov Wed 07, 2007 12:44 am
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Location: Hawthorne, Ca
I would think that the Hammond PT125C would be a good replacement for the original output transformer as the 41-250 uses two type 41 tubes in push pull which should be good for around 5 or 6 watts output. The PT125C is also a universal replacement transformer which makes it easier to match the specs of the original part. Harry


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 22, 2017 2:41 pm 
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125B is also universal...

One crude way to get the right transformer---get the same core size.

With respect to power rating: RM lists that set as having 2 Watts audio output, which does indeed seem low for P-P 41s. Of course, there is that 6" speaker.......

41 data sheet:
https://frank.pocnet.net/sheets/127/4/41.pdf

recommended load (plate to plate) 12Kohms
power output: ~10 watts

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-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sat 23, 2017 2:45 pm 
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Joined: Nov Wed 07, 2007 12:44 am
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Location: Hawthorne, Ca
The Hammond PT 125C transformer is universal and will work well in most radios without breaking the bank looking for an exact fit replacement transformer. The type 41 tubes in a push pull circuit want a plate load of 12,000 ohms resistance for maximum performance. The 41 tube specs are the same as a type 6K6 tube. I would also think that the speaker impedance is 3.2 ohms. Harry


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 29, 2017 4:13 am 
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Joined: Aug Fri 28, 2015 3:22 pm
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Location: Centennial, CO
Ok, I got a 125C in the mail today. Still a bit confused. From my Dejaview schematic (which is largely the same, but a little clearer and has a parts list) the final audio/speaker section looks like this:

Image

If I assume 3.2 ohm voice coil impedance and that 11,500 ohms is close enough to the 12,000 ohms Harry mentioned, that would mean I should use secondary lugs 1 and 3, right? Going by the table at the end of this: http://www.hammondmfg.com/125.htm

So... what exactly do I hook up to those lugs? I assume the blue, red, and brown wires on the new one are the black, black white, and white wires on the schematic, correct me if I'm wrong. On the speaker on the schematic, one side obviously goes to ground, and then the other goes to what appears to be an unused terminal by the antenna screws.

Apologies for the dumb question - first time replacing a transformer, and I only kinda-sorta get what they do!


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 29, 2017 2:41 pm 
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Location: Hawthorne, Ca
The red lead goes to the B+ which is your black/white lead on the original transformer. The blue and black leads go to the plates of the 41 tubes and pick the taps on the secondary side of the 125C which will give you a plate resistance of 11,500 ohms with a 3.2 ohm voice coil. Follow the instructions that come with the transformer and you will be in good shape. Harry


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Fri 29, 2017 7:36 pm 
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Location: Centennial, CO
Thanks Harry. I had the output tube plates and B+ parts figured out ok, it was the voice coil I needed to hear - I later realized that was probably it, but wasn't sure. This is also for my dad's friend, and has some sentimental value, so I really don't want to blow anything on it!

Is the 3.2 ohm measurement referring to AC or DC resistance? I'd like to double-check, but I don't have an easy way of measuring AC resistance. If not, and you're reasonably sure it's a 3.2 ohm speaker, I'll go with that.

Thanks!


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sat 30, 2017 6:33 am 
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Joined: Aug Fri 28, 2015 3:22 pm
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Location: Centennial, CO
Aha, got it! Thanks for your help, everyone! I think I was mostly tripped up by the fact that the secondary connections were a little hard to see on the original. One part to the voice coil was a pretty small, somewhat covered wire. The other side, going to ground, was just a small thing that went to the body of the transformer (which was of course attached to the chassis).

I dremeled the rivets holding the old output transformer out, drilled a new hole so I could mount the new one, and wired it up as we talked about. Fired it up, crossed my fingers (even though I had every reason to believe the old one was bad, you can't help but to get nervous when you recommend the owner buy a $40 part you've never replaced before) and it worked beautifully!

Here she is, singing her first words! https://www.youtube.com/watch?v=gizSZY1T53U


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sat 30, 2017 2:58 pm 
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Location: Hawthorne, Ca
Congratulations on getting the old Philco back into working condition. I keep a 125C on the shelf as I've run into bad output transformers in the past and found that the 125C is a good replacement transformer for most radios with push pull output sections. Harry


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sat 30, 2017 4:13 pm 
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Following comments are possibly irrelevant---given that the set is working.

The set has a "hum-bucking" coil in series with the voice coil. I have never looked at how that affects the net impedance seen at the primary.

Digression--and answer to an earlier question: Impedance is the general term for the relationship between voltage and current (for both AC and DC). DC resistance is a special case of impedance.
The transformer changes the impedance based on the turns ratio (impedance ration is the square of the turns ratio), but it acts only on the AC component.

The impedance of a speaker is often specified at about 400Hz, and includes the mechanical properties of the cone.

If the actual speaker is --eg--4 ohms, then the addition of the hum-buck coil does something, but I'm uncertain how to calculate it. An interesting experiment would be to change the secondary taps while monitoring the output with a distortion analyzer.

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sun 31, 2017 6:58 am 
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Joined: Aug Fri 28, 2015 3:22 pm
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Location: Centennial, CO
Hmmm, now that I have the volume pot cleaned up, I noticed that the volume runs pretty high. At its lowest setting, it sounds like someone talking in a low voice. Not the end of the world, in fact it may be useful to remind someone that the radio is still on. But I only have to turn it a little bit before it gets so loud that distortion is an issue, and it can get quite a bit louder beyond that! I don't dare risk another output transformer to see just how loud it can get, even if I do have a bigger one :roll:

Is this something that can be adjusted from the output transformer terminals used? Don't get me wrong, it's not terrible as-is, but if I can adjust it easily, I'd like to.


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sun 31, 2017 5:43 pm 
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Yes!! This chart--linked earlier--shows what taps do what.
http://www.hammondmfg.com/125.htm

If you used Big Harry's suggestion, then you hooked up taps 1 & 3. 11,5 Kohm to 3.2 ohm, impedance ratio 3600:1

Suppose the effective impedance, including the humbuck coil, is more like 6 ohms. This means the tubes would be seeing an impedance of 22Kohms. This would lower the power, and increase the distortion.

We could wallow in the theory forever, but it's quite OK to just pick the tap that sounds the best....:)

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Dec Sun 31, 2017 6:49 pm 
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pixellany wrote:
it's quite OK to just pick the tap that sounds the best....:)


THAT is exactly what I needed to hear! :)


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Jan Mon 01, 2018 2:59 pm 
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Location: Hawthorne, Ca
It's been my experience that many older radios exhibit the same symptoms yours has with the volume not turning all the way down. It's my theory that there are more radio stations on the air than when your radio was built so the signal strength the radio sees is higher than what was available in the early 40's when the radios were farther away from the transmitters then they are now. If you think the volume control is a problem, you can contact Mark Oppat at **** and he can fix you up with a new one. Like pixellany says, use the transformer taps that sound the best to you. The radio won't be damaged by trying different taps on the transformer. Harry


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Feb Thu 22, 2018 9:46 pm 
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Having just picked up a 41-250 yesterday, I was wondering if you could either provide a copy of the clearer schematic and parts list you mentioned or at least let me know where to find it. The ones in the Rider's manuals and at RM are pretty low-res.

Also, did you encounter any problems with the power switch? I notice on this one that it goes in, but you have to pull it back out. From the posts I've seen I take it that's rather common. Not sure if there's any way to fix it, but would be open to suggestions.

Thanks,
Brian

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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Feb Fri 23, 2018 12:31 am 
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Joined: Aug Fri 28, 2015 3:22 pm
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Hello fellow guy named Brian!

Here's where I found the clearer schematic. Note that the file is NOT a PDF, you'll want to use something like windjview (https://windjview.sourceforge.io/) to view it. Don't worry, it's a safe, low impact program.

https://windjview.sourceforge.io/

Fortunately (for me, at least) the power button worked fine, so unfortunately I can't help you with that.

Good luck! This radio was a great performer by the time I was done with it, at least on AM. KRLD (Dallas) came in really well from my garage in Denver, and KOAI out of San Antonio was also pretty solid. https://www.youtube.com/watch?v=EoyrBjRjkbs


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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Feb Fri 23, 2018 1:31 am 
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Thanks, Brian!

At this point I haven't tested the radio yet, so I'm only guessing about the power switch. I've recapped it, but am waiting for the one tube I didn't have in stock. What's amazing is the overall condition. The pushbuttons, the knobs, and the case are all great. The front panel isn't perfect, but I'd rather stick with it than buy a reproduction. And, to top it off, even the original Philco decal is in great shape.

My wife likes the looks and even suggested it might replace the Metz 308 3D in the living room, but when I pointed out that the Philco couldn't get FM, she changed her mind. Anyway, we'll just keep having fun with these old radios and maybe bring them to a few new people.

Brian

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 Post subject: Re: Philco 41-250 Output Transformer Replacement
PostPosted: Feb Fri 23, 2018 2:20 am 
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Joined: Aug Fri 28, 2015 3:22 pm
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Location: Centennial, CO
Yeah, the one I worked on came from my dad's friend. I told him I would fix it for $50 plus parts, or if he didn't want it I'd give him $20 for it as-is. Apparently it was his wife's uncle's radio, retrieved from a barn, so he wanted it restored, at least electrically. The outside wasn't in bad shape, all things considered! I think my dad got a new faceplate, though.

Now I just need to get it back to its owner. I live in Colorado, and my dad is in Minnesota. I've got some guests in town that will be diving back to Wisconsin (2 hours away) soon, so at least they can get it close!


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