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Philco Lowboy 20 - restoring speaker - Printable Version

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Philco Lowboy 20 - restoring speaker - Guest - 04-16-2009

Hi. I am new here.

I have just bought myself a project - a Philco 20 Lowboy. Since sometimes there are different understandings of what it is - here s a link to it.
http://www.tuberadioland.com/philco20_consolette_main.html
or our own gallery
http://www.philcoradio.com/gallery/1931a.htm#b , the picture of BABY GRAND CONSOLE (Model 20)

The speaker inside this radio is like this one:
http://www.radiolaguy.com/images/speakers/PhilcoSpkr.jpg

Well, the speaker happened to miss major part of the cone.
I spoke to Hank Brazeal, and he, having called this speaker "a big mistake by Philco" recommended first to check the field coil etc.
It does seem like the field coil exhibits 3050 Ohms of DC resistance, when check by a meter.
The transformer on the speaker does seem to have one winding showing 300 Ohms. Another one is in parallel to the voice coil, so I do not know whether it is OK, or ot is the voice coil, although I suspect it is OK (secondaries are usually thick wire, so it is not likely to break).

So I am almost ready to send it for re-coning, but here what I do not understand so far: the plug has 4 pins. However only 3 pins will be buzzed, and 4-th wire is unconnected. The there wired go to field coile and the secondary (mentioned 3050 Ohms and 300 Ohms, one pin is shared) and the 4-th one is soldered to a contact on the transformer but does not go anywhere.

This is not in agreement with the schematic that shows all 4 pins connected (field coil in series with transformer's primary centertap, both field pins connected and primary ends connected).

So unless I am mistaken one of the primary's ends is unconnected (broken>)

then I have another diagram from this website

http://www.philcoradio.com/tech/speaker.htm

Which shows the connection that would buzz exactly as what I have, and this is why I thought that maybe what I have is OK. But then it is nto for Philco 20. Or is it?

So, if anyone knows that speaker/xfmr connection - can you tell me whether you think my xfmr is shot or not? And can the speaker be restored after this or not?

Re: Philco Lowboy 20 - restoring speaker - planigan - 04-17-2009

Hi, I checked the schematics for Philco 20, 20A and 20-A on Nostalgia and they were all push-pull outputs which reguire center tapped primary in output transformer. So, there has to be four wires connected between set and Spkr/Field Coil. Each end of primary goes to a plate on an output tube (2 leads), the center tap and one end of Field Coil to B- (1 lead) and the other end of Field Coil to B+ (1 lead), making a total of 4 leads between chassis and speaker. Search around the transformer bottom at the paper edges for a cut/broken lead. If you find a stub you may be able to carefully peel it back enough to piggy tail a lead on to it and you have your third lead. By the way, I believe you said you have 300 ohms on primary of transformer. I did not note ohms of primary from schematic but that could be 1/2 the resistance or full if you are measuring from end to center tap or end to end respectively. PL

Re: Philco Lowboy 20 - restoring speaker - Guest - 04-18-2009

Hi.

thanks for the reply.
yes I also thought so, especially in view of teh output being Class B Push-Pull.

However I called The Speaker Shop, and the guy there who knows Philco 20 speakers well, told me that it is correct that one of the 4 wires is unconnected, and it is there just in case for other versions, and the XFMR is without cenertap.
I am not sure how the Pushpull can handle a XFMR without centertap - it is a standard way of doing this; no centertap usually mean Class A amplifier would be used, not a pushpull.

I am confused. I don't think I'd have a wrong speaker arrangement in there, and I clearly see the chassis being Philco 20.

Re: Philco Lowboy 20 - restoring speaker - Ron Ramirez - 04-18-2009

morzh Wrote:However I called The Speaker Shop, and the guy there who knows Philco 20 speakers well, told me that it is correct that one of the 4 wires is unconnected, and it is there just in case for other versions, and the XFMR is without cenertap.

Wrong, wrong, WRONG! That guy does not know his Philco 20 speakers as well as he thinks! The Philco 20 speaker requires 4 wires and each wire is indeed active. In addition, the output transformer on the 20 speaker is a push-pull transformer.

Re: Philco Lowboy 20 - restoring speaker - Guest - 04-18-2009

That was what I thought.

I think I know however where the mistake comes from.

The same exact 8.5" base speaker is used in Model 70. Sometimes they are listed as "Model 20 70 90" speakers.

But the XFMR installed on that speaker is no-centertap, as Model 70 uses Class A output cascade.

I will probably have to disassemble the XFMR to see if a wire got torn off. I just wonder...seems to be "built-like-a-tank" assembly - very hard to move the wire bundle at all, let alone to flex the cardboard solder contacts for the wire inside to break loose.

Re: Philco Lowboy 20 - restoring speaker - Guest - 04-18-2009

Anyone knows where I can get the transformer for this monstrosity?

Last time I wound a power/output XFMT was close to 30 years ago; since I try to leave that to pros with exception of small prototypes for switchers.