Replacing Vintage Carbon Resistors In Radio

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
I was originally just changing old electrolytic capacitors in this 1950's Philco 676 Radio. However, when I started measuring the resistors I noticed that they were all reading about 100ohms higher than their intended values.

Most of the resistors I see for sale are rated at about 1/4-1/2 Watt. Is that a high enough rating for this radio? In the schematic I only see wattage ratings on two of the resistors. Ironically those two measured right where they're supposed to be on my voltmeter.
 

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dl324

Joined Mar 30, 2015
16,988
Are you measuring values in-circuit?

Carbon comp resistors are known to drift with age. The amount of drift that's acceptable depends on the circuit. Being 100 ohms higher than marked values is meaningless. Percentage drift is.

You can calculate the required wattage of any replacement resistors by analyzing the circuit. I'd give carbon film resistors preference over metal film because they're lower inductance and the carbon comp resistors are likely 5-10% tolerance.
 

BobTPH

Joined Jun 5, 2013
9,124
Which resistors are measuring off by 100 Ohms? If it us a 1Meg resistor that 0.01%, about as high precision as you can get. whereas, if it is a 100 Ohm, it is off by 100%.

If they are off by no more than 10% they are probably fine.
 

Thread Starter

Jsprite7660

Joined Jan 9, 2024
26
Thanks for all the input everyone. I'm replacing the components for the fun of it and soldering practice. Good to know that these differences in readings aren't very significant tho.

This is all a fun learning hobby for me.

Cheers
 

MisterBill2

Joined Jan 23, 2018
18,934
Given that some of those resistors are marked ten percent tolerance, 100 ohms does not matter at all. And as it is a battery powered tube radio the power levels are certainly OK..
One caution is to assure that the LINE/Battery selector switch contacts are all cleaned, and that the switch mechanism is able to move it fully to either position.
 
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