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Philco Model 20 Lowboy - Printable Version

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RE: Philco Model 20 Lowboy - morzh - 03-24-2017

Most resistors could be measured in-circuit as often times they end up driving a tube's electrode and have no DC path to complete the circuit. If a resistor measures slightly below the declared value it is most likely OK and can be left in place; if it settles above the value outside 10-20% it then has surely drifted by that much. If over 20% it should be replaced, especially if it is a part of a divider where precision matter more than in, say, being a grid stopper.

RE: Philco Model 20 Lowboy - WallaceRoger - 04-16-2017

I'm making slow but sure progress, mostly waiting for the capacitor and resistor assortments I bought on ebay to arrive. I rebuilt a couple of the easiest capacitors and now I've pulled out part #8 bakelite capacitor of .05mfd , which the parts list notes as also containing a 250ohm resistor. I haven't cleaned the old tarry gunk out yet. Is there really a resistor inside here? I'm assuming a regular old 1 watt metal film resistor is fine?

EDIT: Yep, I found it. Heating up the bakelite block with a heat gun and pushing through a rivet hold with an old 1/16 drill bit got it out nice and easy. I kind of messed up and now trying to figure out what the other end of the resistor attached to...

RE: Philco Model 20 Lowboy - GarySP - 04-16-2017

Hello WallaceRoger.  The particular bakelite block you are referring to doesn't have a resistor, but a small bundle of resistor wire approximating 250 ohms.  I replaced the resistor wire with a 1/2 watt 250 ohm resistor when I rebuilt mine.  Take care, Gary

RE: Philco Model 20 Lowboy - WallaceRoger - 04-16-2017

(04-16-2017, 08:34 PM)GarySP Wrote:  Hello WallaceRoger.  The particular bakelite block you are referring to doesn't have a resistor, but a small bundle of resistor wire approximating 250 ohms.  I replaced the resistor wire with a 1/2 watt 250 ohm resistor when I rebuilt mine.  Take care, Gary

Thanks, Gary. I have a huge batch of resistors and have some 270ohm, but no 250ohm. I know it's considered ok to replace old capacitor values (like .05) with modern common ones (like .047). Is the same true of resistors? If not I can just find the 250, but if I can use 270 I'd like to. Thanks again, I'm learning a lot as I go.

RE: Philco Model 20 Lowboy - morzh - 04-16-2017

I think 270 ohm is fine there.

RE: Philco Model 20 Lowboy - WallaceRoger - 04-18-2017

I paid a guy out in CT to restore a small Trav-Ler clock radio alarm for me several years ago and he installed an inrush limiter. I'd kind of like to do that here, as well as install a fuse at the power to help prevent damage (especially since this is my first project and I might screw something up).

What are your thoughts on this? Are the two together overkill? What fuse amp rating did some of you other guys use on your Model 20?

RE: Philco Model 20 Lowboy - morzh - 04-18-2017

I do not have a fuse on my 20 radios, I might have it on one radio I restored for my dad.
At any event, I would rate a fuse at 1.5x the radio's max current rating, and make it a slo-blo (time-lag) type.
An inrush limiter, in my opinion, is an overkill for this type of application.
I used them in AC/DC switching supplies as there is a very sizeable inrush due to an unprotected discharged capacitance essentially in parallel (disregard the rect bridge) to the AC supply. In a radio with a transformer there is essentially an inductance (which is what an unloaded transformer is) and this does not result in a very strong inrush.
There is one possibility when a switch gets made exactly at the point of the AC voltage crossing zero, that could result in some DC-like behavior and saturation by the transformer, but not for a long time as there is some finite loss resistance there.

This said, the peace of mind is a priceless thing and if it makes you sleep better at nights, by all means the limiter is good.
The fuse is a good idea in any case.

RE: Philco Model 20 Lowboy - WallaceRoger - 04-18-2017

Thanks again, Morzh

RE: Philco Model 20 Lowboy - WallaceRoger - 04-29-2017

I got my last resistors put in, replaced the power cord with a new one from Radio Daze, added a fuse. I'm waiting for my variac to arrive before I try powering it up.

In the meantime, I pulled the speaker out and it needs a recone. What have you guys paid on average for a re-cone? We have a couple places here in Chicago who do speaker work.

RE: Philco Model 20 Lowboy - WallaceRoger - 04-29-2017

I almost forgot the photos

[Image: http://i18.photobucket.com/albums/b145/sebbeng/Radio/IMG_20170429_124319.jpg]

[Image: http://i18.photobucket.com/albums/b145/sebbeng/Radio/IMG_20170429_125407.jpg]

[Image: http://i18.photobucket.com/albums/b145/sebbeng/Radio/IMG_20170429_130714_1.jpg]

RE: Philco Model 20 Lowboy - WallaceRoger - 05-05-2017

Ok! Today I got my variac, so I powered up slowly. The good news is, no smoke and nothing else visually seems to be an issue. The fuse I installed didn't blow.

The bad news is, it produces nothing but a steady hum, the volume knob appears to have no effect on it, and the display lamp doesn't light.

Where would you guys recommend I start checking? I have not replaced anything in this except caps and resistors, so everything else is original (including the long, tubular resistor).

RE: Philco Model 20 Lowboy - Kenneth F. Besso Jr. - 05-05-2017

DID you rebuilt that Big,,square ,,oblong silver can ,,,next to the Black transformer,,,that holds capacitors inside ????

RE: Philco Model 20 Lowboy - WallaceRoger - 05-05-2017

Yep, I heated and slowly worked all the old stuff out, all new caps in there.

RE: Philco Model 20 Lowboy - morzh - 05-05-2017

Tubes tested?

RE: Philco Model 20 Lowboy - Steve Davis - 05-05-2017

Start by checking voltage on all the tube pins. Compare them to the voltage chart on the schematic.

Steve