Philco 77
 

I've got a Philco 77 in which someone replaced the speaker. It appears as if it's missing the field coil and is missing the center tap on the input of the output transformer. They somehow connected the wires weirdly, they hooked the outputs of the output transformer and the voice coil in series and then hooked that up to where the Field coil should be. Would this have worked?

The resistances of the speaker are as follows
~Voice Coil 5 ohms
~Output of transformer 0.5 ohms
~Input of transformer 855 ohms

Here's a picture if you're as confused as I am
Attachment 197796
The blue and green go to the input of the trans
The yellow and black should go to the field coil

In case of a non-field coil speaker being used...I would replace the coil with a generic choke...and try to get a voice coil whose impedance closely matches the amp's output trannie.

Are you sure that speaker does not have a field coil (it looks like there is cardboard in the magnet and some wires going to it)? Some field coil speakers had an extra winding on the field coil that was placed in series with the speaker...This extra winding picked up any hum from the field coil and fed it to the voice coil with reverse polarity to cancel the hum from the field coil.

If you need to sub a perm magnet speaker for a field coil type you can place a power resistor of comparable resistance to the old field coil in place of the field coil too...A choke will reduce hum more, but a resistor will work too if on a budget or in a pinch.

Well, I mixed up my terminology. I think I've confused the output transformer and the field coil. (Sorry about that still new to the hobby) What can I do about a missing output transformer?


This appears to be the speaker that I have.
Philco 36-1450

Look for a transformer with the correct primary topology (push pull or single ended depending on how many audio output tubes there are), Primary impedance (should roughly match plate resistance in output tube data sheet), secondary impedance (should match speaker voice coil impedance which is usually the same as voice coil resistance), and should be rated for the max audio power the radio is rated for (or the tubes are rated for if you can't find an audio power spec).

Note: not all radios regardless of speaker type (PM or field) mounted the audio output transformer on the speaker, some mounted it on the chassis instead. If the speaker was subbed and they were smart enough to wire the field coil correctly then they probably made sure an output transformer exists within the set.

Here's the schematic..

http://www.nostalgiaair.org/Resource...=\M0013838.pdf

That's the one I've been using as well as this one, which is nearly identical and has most of the values. http://www.philcoradio.com/images/phorum/schem/76.jpg

No output transformer that I can see, the 45's plates go directly to the speaker. Not sure if it ever worked, either way, can someone help me figure out the maximum wattage I'll need? The datasheet calls for like 18 watts max watts in a push pull configuration.

http://www.hammondmfg.com/125H&J.htm
The Hammond 125 series seems to suit the resistance needs fairly well

This is the datasheet that I've been using
https://frank.pocnet.net/sheets/021/4/45.pdf

Your schematic definitely shows the output xfmr, designated #24. The field coil is #26, and the power supply choke #35. Can you post pics of the top and underside of the chassis? The output xfmr has gotta be there somewhere unless it's been physically removed.

(This design uses a dedicated field coil, whereas later designs made the field coil double as the choke).

(Edit.) In your photo of the speaker, it looks like the speaker has been replaced with the later type that uses the field coil as choke. Notice the two black wires coming out of the field frame. Those go to a 'hum bucking' coil which is in series with the voice coil. It serves to null out the field's ripple. Your radio doesn't need hum bucking, since the field coil does not double as choke. So just take the HB coil out of circuit, and run the voice coil direct.

Heres pictures of my chassis.
Attachment 197798
Attachment 197799
The plates of the 45's go directly to the speaker plug. My understanding was that the output transformer was mounted on the speaker frame however, when they replaced the speaker, they bypassed any sort of output transformer and hooked it directly to the speaker. If you look at the schematic for the 77, you can see the plugs off of the 45 tubes and off of the field coil

Push-pull amplifier. Requires tapped output transformer to speaker and field coil.

Yep you're right. Originally the p-p output xfmr was mounted on the speaker.

My only other question is how do I determine the wattage I'll need?

This line seems to suit the demands given
http://www.hammondmfg.com/125a.htm

A 10 watt p-p should be just fine for your Philco.:yes:
Ed

Well, I'm happy to report that I received the transformer today and promptly installed it. I'm also happy to report that I'm now receiving stations, though slightly garbled and with a high pitched whine. I'm assuming that aligning each section should fix the whine, though I'm not sure about the garble. (Weak output tubes maybe?)
Thanks for everyone's help,
Zach

You seem to be missing the large tube shield. This very well might be the cause of your issues. Also be advised that the input filter cap (inside the box from connections 2 to 6) should be a non polarized mylar 2mfd/630v. If you use an electrolytic cap it will short after a while do to the high ripple current at that point in the p/s.
Depending the style of the cabinet the original speaker would have looked like this one. https://www.ebay.com/itm/vintage-PHI...gji:rk:22:pf:0
GL
Terry

Shielding is indeed very important, especially in TRF sets. Any feedback that can happen will happen and will produce distortion and funny noises. <--Murphy's Law of TRF. :D
Shielding prevents feedback and interference.

If you get it shielded up and there is still distortion pull the detector tube and inject audio downstream of it...Remember HiFi wasn't a thing until the late 30's (and then was only a thing on high-end sets). If speech is reasonably understandable it is probably about right. That Philco dates from a time where having a paper cone loudspeaker was a new technology just being grasped by manufacturers.

OBTW it's not uncommon for the primary winding on the rf coils to be open. This could but the cause of the garbled sound via the ant coil. Easy to repair, abt 50T of 34-38G magnet wire will do the trick. It's not too fussy as it's not part of the tuned circuit.

Terry

Sorry, forgot to mention that I had the shields once I started getting a hum in the speaker. I had almost completely forgotten about them, as this set has been sitting for awhile now.

As to the audio quality, The one's I had seen online have better quality that what I'm getting. I'll make sure to replace the other filter with Mylar caps and I'll check the coil as well. Haven't really gotten the opportunity to troubleshoot much and I don't think I'll have much time this weekend.

Well, finally got back to the bench during daylight hours. Replaced the 2uF electrolytic with mylar caps and was able to isolate the garbling/noise issue down to a cold solder joint on the speaker terminal. Reflowed it and was able to get a much cleaner sound. Proceeded with an alignment, and now the set works like a beauty. It's amazing what a 4ft antenna does for this set, I've been listening to it for the past hour picking up a station about 50mi that broadcasts classic music.

Workbench lights don't help much with RF interference. Seems to affect this set more than others I've had.

Thanks all,
Zach

Old radios are a beautiful thing. It's sad that two radios are in the shop and one radio is dead. I need a fix!