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Philco Model 20 to restore - Printable Version

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RE: Philco Model 20 to restore - StoneyD - 08-07-2025

Well heck, disappointing to find that the speaker is kaput. Given the shape that the case is in probably won't be a functional radio but, at least I can get a practice run at recapping,

RE: Philco Model 20 to restore - morzh - 08-07-2025

You could sub the speaker for a PM one.
The Field coil in this case is not really used as a filter, but as a regular resistive divider.
You will need the transformer either grafted, if it is good, or replaced.

RE: Philco Model 20 to restore - Arran - 08-07-2025

You may find, upon investigation, that the field coil has a break near one end, and is not burned out, hopefully it's that and not corrosion in the copper wire. These F.C speakers in the model 20 are easily rebuildable as the pole piece that keeps the field coil bobbin in place is held in with a large nut on the back of the rear bracket. Of course you may have to remove the speaker cone to get it out, but I'm not certain, the only attachment to the pole piece is a machine screw in the center of the spider. Another issue may be the audio interstage transformer, those sometimes fail, usually in the primary, but they can be bypassed just to test the set.
Regards
Arran

RE: Philco Model 20 to restore - StoneyD - 08-10-2025

Worked on the speaker assembly last night. Clipped my ohmmeter leads to the connector pins and found that with slight movement of the cable I would get some intermittent readings. Found that at the speaker end of the cable there were some very dodgy solder connections so, reterminated the connections and am now getting resistance readings of 265 ohms and 5.1 K. I expect the new resistors and capacitors to arrive this week and am hoping to power this radio using variac and dim-bulb tester after new components go in. Wondering if I leave the speaker assembly as is could that result in a Maintenance-Induced-Failure of a serious nature.

RE: Philco Model 20 to restore - morzh - 08-10-2025

You are supposed to see two readings of around 250 Ohms (one higher than the other, due to the windings geometry) and one reading of 3.1K.
5.1K is wrong and requires further investigation. It is encouraging but needs to be traced down to the reason for it.

RE: Philco Model 20 to restore - StoneyD - 08-10-2025

The pictures tell the story of my misadventure this morning.  The 5100 ohms was my misreading of the DVM.  It seems it was actually 517 ohms.  Safe to say the field coil is open?  I'm going to look into replacing with a PM speaker housed in separate box so that I might at least power this radio after power cord and component replacement.

RE: Philco Model 20 to restore - MrFixr55 - 08-10-2025

Hi StoneyD,

I have put a PM Dynamic speaker into a Philco Model 20 with good results. You can install directly into the cabinet without hacking the cabinet up. You do have to hack the speaker. At the 4 speaker holes, cut a notch into the frame to make the holes into an open U. This and 4 washers will allow you to mount the speaker into the cabinet.

To deal with the field coil, merely place a 3.1K (a 3K will do fine) 5W resistor across the field coil leads. Temporarily transfer the transformer to the new speaker (most speakers have mounting points) or fashion a bracket to mount the transformer onto the speaker. There will be a mismatch as modern speakers are usually 4 Ohm or 8 Ohm and the Model 20 speaker is about 1.5 Ohm. However, for testing purposes, you should be OK. I used a Hammond 125D (I believe) which had 5KOhm Plate to Plate impedance and this seemed to work just fine.