[Nathan Slingerland]

Thanks guys!

RE: the heat - it's a good question. I haven't had a problem (so far) - but it's possible those have been places where the amount of actual energy dissipation is much less than the rated wattage (also, large values seem to drift much more). I suspect that some heat must be dissipated via the leads (18ga wire) - air + polystyrene seems like it would be a pretty good heat insulator.

According to Wikipedia Polystyrene starts to flow around 100°C.
https://en.wikipedia.org/wiki/Polystyrene

It's probably worth doing some sort of controlled experiment to measure at some point.

[Ron Ramirez]

Amazing, Nathan, just amazing. You, Russ, and Steve Davis really make those things look authentic.

[Nathan Slingerland]

Thanks Ron! Much appreciated.

[TV MAN]

I guess you could do a test by intentionally burning a resistor inside and see what happens  100 ohm works pretty good. Ok I confess, I have burned a few resistors over the years. Allllwright! It was in tech school .... had a German professor, Mr. Schultz. He gave me the nick name "Pyro".  A variable low voltage power supply and lower ohm value resistors work best. I hope the plastic wont ignite. Might be a good test to do for safety.  

[Nathan Slingerland]

Lol - I used to do something like that when I was a kid. Put a low ohm resistor in an switched outlet - then POP!
That was great until I tripped the circuit breaker and got scared. Had to explain it to Dad.  

As for really determining the limit, here's what Ohmite says about their resistors:

The wattage rating of resistors, as established under specified standard
conditions, is defined as the “Free Air Rating” (“Full Rating” or
“Maximum Power Rating”).
[cut]
The relation of the “Free Air Watt Rating” of tubular type, vitreous
enameled resistors to the physical size, is to be set at such a
figure that when operated at their rated watts, the temperature rise
of the hottest spot shall not exceed 300°C (540°F) as measured by
a thermocouple when the temperature of the surrounding air does
not exceed 40°C (104°F). The temperature is to be measured at
the hottest point of a two-terminal resistor suspended in free still air
space with at least one foot of clearance to the nearest object, and
with unrestricted circulation of air.

Yikes that's hot!

I guess there are really two questions:

1. How much power can a repro resistor dissipate before it hits 100°C (under the same conditions as Ohmite lists)?
2. How much power does a given resistor actually need to dissipate?

The easy answer to #2 would be whatever the rating of the original is - but I'd hazard a guess that many (especially small wattage ones) were just selected based on availability. I did a quick search for several 1/4W Philco part numbers in the service bulletins but didn't see them show up until around 1934 or so. It's just a rough guess, but maybe Philco was just using 1/2W resistors by default in 1931 when this model 50 was made.

Of course we could get a real-life number by measuring the voltage drop across a given resistor and then calculating P = V^2 / R. But we still don't know how much is too much because of question #1.

Anyway, I think I'm going to stick with the "works-for-me" approach - but with a few caveats:

• Don't make reproduction resistors for higher wattage parts
  
• Burn in (hopefully not literally!) any radio for a while before calling it done (probably a good idea anyway)
  

[morzh]

To answer the 1st question, if we assume that the temp (or the temp increase rather) is proportional to the produced energy which is proportional to the power, 1/3 of the temp roughly calls for 1/3 of the power.

[Nathan Slingerland]

That seems like a reasonable way to think about it. And by that logic maybe we can 'derate' the power rating of any resistor stuck inside a sleeve like this one as 1/3 of its normal free-air rating.

I wonder though if we need to even assume 300 C as the reference since solder melts at 183 C. It seems unlikely that any of our resistors actually ever run - even at the hottest point - at that temperature.

[Nathan Slingerland]

OK - so where did we leave off?

Right! Getting to the point where we can power it up on a variac for the first time and see if there are signs of life.
I like to do that before replacing too many components so that I can replace things in testable chunks (reduce the amount of wiring I need to check if I make a mistake).

Filter caps are a must to replace first though. Also that one resistor that measured open.

[Image: http://i1072.photobucket.com/albums/w374...0wpdzf.jpg]

I typically use a terminal board - in this case with a standoff - for the filter caps. I know a lot of you guys restuff them, which certainly looks nicer underneath, but I figure somebody (maybe me) will be replacing them someday. And since it's an easily reversible change (I mount using an existing hole) that's what I do.

Here we are with the open resistor and its siblings replaced as well:

[Image: http://i1072.photobucket.com/albums/w374...nltqc3.jpg]

Now it's time to put the tubes back in. All of the tubes that it came with tested good. Yay!

[Image: http://i1072.photobucket.com/albums/w374...idjyaz.jpg]

These look like these could be the original 24 tubes that this radio came with. It seems like there are still quite a few globe 24 tubes around (not so many 80 tubes - mine came with an ST version, so definitely replaced).

[Image: http://i1072.photobucket.com/albums/w374...xljs2q.jpg]

Slowly turning up the voltage....and....we have lift off at around 80V!

So that's great news - it receives a couple of the local strong stations and at pretty decent volume.

[Nathan Slingerland]

Now I'm ready to go through and restuff each bakelite block and replace the out of tolerance resistors.

Several hours later:

[Image: http://i1072.photobucket.com/albums/w374...embucc.jpg]

Wow that wasn't so bad. I tested it at a few intermediate points and managed not to screw anything up...

...that is until I went to align it. The 2nd RF adjustment nut was stuck tight and of course I knew it needed just a bit more torque to loosen it...

[Image: http://i1072.photobucket.com/albums/w374...ue68oh.jpg]

Well that's going to be a bit of a problem.  

It looks like the threaded portion is actually a small bolt that has been soldered or welded to the body of the tuning capacitor. So the next step will be to take that off and apart.

[klondike98]

Its always something... but its looking good!

[morzh]

You're lucky this under-chassis transformer is still alive. Pretty common for them to croak.

[TV MAN]

Nate, are you taking lessons from ... me??? Check my thread in Hi-Fi on the China amp kit Now, ya really got me scratchin my head over something. Why oh why would go to the trouble of making those excellent reproductions of dogbones, then not stuff those cans??? 

[morzh]

I was thinking of asking the same.......

[Nathan Slingerland]

TV Man - Ha ha. I see what you mean about our love of terminal strips.

So you guys got me thinking about trying to restuff the filter caps so I took them off tonight.

[Image: http://i1072.photobucket.com/albums/w374...a6pmvd.jpg]

And then I couldn't resist trying to spiff them up a bit first. At first I tried Mother's Wheel Cleaner... which helped some, but they were still pretty dirty. Then I thought to try some 0000 steel wool on them - and man what a difference that made:

[Image: http://i1072.photobucket.com/albums/w374...2dfmy3.jpg]

I found Chuck's page on how to restuff them so I might give it a shot. The only thing is they look so nice (big mistake cleaning them) that now I'm having second thoughts about trying to 'pop the top' with a screwdriver (lest I make them look not so nice). 

[Nathan Slingerland]

The other thing I tackled tonight was the broken trimmer on the tuning capacitor.

First I took the tuning capacitor off and then disassembled it:

[Image: http://i1072.photobucket.com/albums/w374...wns5qx.jpg]

Then I took a closer look at how that trimmer screw is attached. It is in fact soldered on:

[Image: http://i1072.photobucket.com/albums/w374...boxulz.jpg]

[Image: http://i1072.photobucket.com/albums/w374...ql0qt4.jpg]

The tuning capacitor body makes a pretty good heatsink so I couldn't get the solder to melt with my iron. A butane torch took care of that though.  

I got the remainder of the original bolt off - but it's a finer thread than the normal #4-40 ones I have around. Then I remembered that I have a parts 90 chassis - and it uses a very similar tuning capacitor design - it's just the big boy version.

[Image: http://i1072.photobucket.com/albums/w374...cdrgje.jpg]

I did the same trick with the butane torch and voila:

[Image: http://i1072.photobucket.com/albums/w374...ty4hfm.jpg]

Since there was some rust on it and I had the tuning capacitor from the 50 disassembled I decided to throw the smaller parts into Evaporust overnight:

[Image: http://i1072.photobucket.com/albums/w374...yswczg.jpg]

With the chassis empty it was also a good opportunity to take care of the rust spots there (as best I can) with naval jelly.

[Image: http://i1072.photobucket.com/albums/w374...bhgdqs.jpg]

I've found the result with naval jelly is rarely very pleasing but at least the rust is removed and I can seal it with Boeshield.
[Image: http://i1072.photobucket.com/albums/w374...ydlc0g.jpg]

[Image: http://i1072.photobucket.com/albums/w374...eqeei2.jpg]