Philco 38-690 Candohm Resistor Wattages and values? - Printable Version
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Philco 38-690 Candohm Resistor Wattages and values? - NV3G - 07-17-2018
One of the candohm resistors is bad in a Philco 38-690. The values are uncommon. Do you try to come within 20 percent of the original value? And how do you find the wattages of the resistors? The Philco part number is 33-3323 and there is another candohm whose Philco part number is 30-3324.
Thanks,
nv3g
Oscar
RE: Philco 38-690 Candohm Resistor Wattages and values? - morzh - 07-17-2018
I can look tonight what I bought for mine.
(PS. I never put them in - the original ones worked ok, but I bought them just in case).
You mean the ones in the Power/Amp chassis?
RE: Philco 38-690 Candohm Resistor Wattages and values? - NV3G - 07-17-2018
Yes, I have worked on several Philco radios in the past and the candohm resistors tend to go bad. I wanted to also see if there was some information on the wattage of each section.
Thanks,
nv3g
Oscar
RE: Philco 38-690 Candohm Resistor Wattages and values? - mikethedruid - 07-17-2018
To figure out the wattage use Ohm's Law. Take the voltage that appears across the whole candohm resistor. Using Ohms law, you can find the current through the entire candohm, Total current = Total voltage / total resistance. This gives you the current that flows through the candohm. Since the sections are in series the current will be the same through each section. Now, using the formula, Wattage = Current x Current x Resistance, you can compute the wattage for each section. This will give you a value that is close enough for government work. I usually go twice the indicated wattage for the resistor minimum, it runs cooler that way.
RE: Philco 38-690 Candohm Resistor Wattages and values? - morzh - 07-17-2018
Mike,
The very reason the candohm is subdivided in sections is to get different voltages developed. Each section may be loaded in turn so the actual effective resistance of it will be smaller when loaded than the one you see listed. So the total voltage, although it does give you the total dissipation, will not necessarily give you each part's proportionate dissipation as the current is not the same throughout the whole resistor, at least in theory, if you use the listed values.
I did at some point the measurements. I might even have them listed in my 38-690 thread. (have to see if they are there)
RE: Philco 38-690 Candohm Resistor Wattages and values? - morzh - 07-17-2018
I bought: (values are close ones from Mouser that are reasonable and not superexpensive)
225 ohm - 10W
147 Ohm - 3W
47 Ohm - 3W
960 ohm - 3W
370 Ohm - 3W
3200 Ohm - I think I bought two 1600 ohm 10W each (cheap)
2400 Ohm - 2x 1200 ohm 5W.
These are very conservative.
RE: Philco 38-690 Candohm Resistor Wattages and values? - mikethedruid - 07-17-2018
I just looked at the schematic for this particular radio. As with others I have seen, the candohm is used to generate various bias voltages for different parts of the circuit. As such, the additional current drain caused by this will be so small that by using the method I described above, using double the indicated wattage or better for a safety margin, one should run into no real problem. It is the method I have used successfully in the past.
RE: Philco 38-690 Candohm Resistor Wattages and values? - tbone - 07-18-2018
If this helps, here are actual power dissipations calculated from my working candohms which I replaced for reliability.
Candohm 165
46 ohm section .22 Watts used 2 watt replacement
230 ohm section .88 Watts used 5 watt replacement
154 ohm section .54 Watts used 2 watt replacement
Candohm 163
970 ohm section .48 Watts used 2 watt replacement
370 ohm section .21 Watts used 2 watt replacement
2370 ohm section 1.78 Watts used 5 watt replacement
3200 ohm section 6.12 Watts used 25 watt chassis mount dale resistor
My candohms were slightly off but not enough to skew my calculations. I simply measured voltage drop and did calculations using known parameters of voltage and measured resistance. Hope this helps future restorations.