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Pre-1936 Philco resistors - Printable Version

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Pre-1936 Philco resistors - 45philcodon - 03-21-2017

I've just started restoration of a Philco Model 620 from 1935. It is totally original; no prior evidence of prior restoration. I've measured all resistor values to determine which parts need replacement. Several of the resistors needing replacement are obviously higher power resistors. The parts list does not specify power so I calculated the approximate power from listed voltages.

The physically largest resistor only dissipates 60 MW. What am I missing? Why would Philco use larger parts than required?

Any thoughts would be welcome.

RE: Pre-1936 Philco resistors - klondike98 - 03-21-2017

Welcome to the Phorum!
Icon_wave

Here's a page from Chuck's Philco Repair Bench you might find helpful
http://www.philcorepairbench.com/carbonres.htm

RE: Pre-1936 Philco resistors - Kenneth F. Besso Jr. - 03-21-2017

Great little radio,,really nice volume out of these units,,I used mostly 1/2 watt resistors,,,BUT a few may have been higher in wattage,,,,

RE: Pre-1936 Philco resistors - Radioroslyn - 03-21-2017

Hi Don and welcome,
If you like to try to keep a more original look I have seen sets that someone used a small resistor solder right under the the out of spec one in parallel to compensate for the drift.

RE: Pre-1936 Philco resistors - jcassity - 03-22-2017

the only problem with that is ohms law.
two resistors in parallel means,,,

if you have a need for 500k resistor and your color bands say green, brown yellow, but the meter says you are at 310k,, the resistor is drifting towards a short.
if you insert a parallel 500k resistor,, ohms law says the the total resistance of 2 or more resistors will be equal to or less than the smallest resistor. inserting a parallel 500k resistor would not fix the issue, it could actually cause the total resistance to be less than 310k!

snip the old resistor out almost flush the the resistor itself, pig tail the old resistor ends, insert your new resistor into the pig tails,, solder up and trim.

if your resistor is spec'd to be 500k and you read 700k,, then yes a parallel resistor of another custom value would be a way to calibrate that particular portion of the circuit. by keeping the out of spec 700k resistor "drifting towards an open state", you would reasoable parallel in a 1.7Mg Ohm resistor to calibrate it down to 500k.

RE: Pre-1936 Philco resistors - Radioroslyn - 03-22-2017

Correct. Most of the time resistor drift up in value unless they have been subjected to overheating. I this case a series may be use to correct the total value.
Or you can build your own BED resistor, a resent thread here on the forum.

RE: Pre-1936 Philco resistors - Steve D - 03-22-2017

I've seen just heating up old resistors while soldering can make then drift up very high.  I always recheck every resistor that I touch with a soldering iron.

Steve D

RE: Pre-1936 Philco resistors - jcassity - 03-22-2017

yup,, its all about how least the amount of time you spend on the target.
In the navy, to avoid this problem we would add a gator clip to one side of the joining parts to act as a heat sync.

clip goes on the "new" component side.

using paste or liquid flux reduced your time you spend on the object by nearly 75%,, some people dont know this because they believe thier solder has flux in it already so why bother. using just a tiny dot of paste flux atop the new joint will amaze you has much less time you actually spend melting down things.

RE: Pre-1936 Philco resistors - 45philcodon - 03-23-2017

Thanks, Bob. That link gave me the info I needed. Much appreciated.

Don

(03-21-2017, 07:16 PM)klondike98 Wrote:  Here's a page from Chuck's Philco Repair Bench you might find helpful
http://www.philcorepairbench.com/carbonres.htm