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Zenith with candohm resistor - Printable Version

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Zenith with candohm resistor - Eric Adams - 11-06-2014

Hi guys. I picked up a Zenith 8H023 AM/FM (Schematic)  made in 1946. Besides all of the rubber coated wire that I'll have to replace, it has a candohm resistor that is burned on one side. Looking at the schematic all I see is 30 and 67 (that's also printed on the resistor).  The parts list is so bad that I can't read it. Does anyone know what the values are? Will it be OK to replace it with 10W resistors once I know the correct values?

Thanks

RE: Zenith with candohm resistor - Ron Ramirez - 11-07-2014

I can read it - it says "30 - 67 Ohm Candohm".

They needed to drop the filament string from 117 volts to 86 volts. Ideally, a 103.33 ohm resistor is needed for that, but they also needed to provide for two dial lamps. So they used a 30 ohm resistor in parallel with the dial lamps (which is also part of the filament dropping resistor) and a 67 ohm resistor.

That only adds up to 97 ohms.

Now you're hitting that radio with 125 volts.

I would increase the 67 ohm section to 100 ohms to make up for the higher AC line voltage, since you have to replace this Candohm anyway. The tube filaments will thank you by lasting longer.

Use the power calculation formula to determine wattage, then double the result for safety.

The filament string is drawing 0.3 amps. We know each resistor is 30 ohms and 100 ohms, so we have the numbers we need to do the math.

P = I (squared) R

P = 0.3 * 0.3 * 30 = 0.09 * 30 = 2.7 watts
Use a 5 or 10 watt resistor for the 30 ohm section.

P = 0.3 * 0.3 * 100 - 0.09 * 100 = 9 watts
Use a 20 or 25 watt resistor for the 100 ohm section. You can get a metal clad resistor to save space; it will have to be mounted on the chassis and will run warm.

RE: Zenith with candohm resistor - Eric Adams - 11-07-2014

Thanks Ron Icon_thumbup  I wasn't sure if I was reading the schematic right. It looked too easy.

RE: Zenith with candohm resistor - Eric Adams - 11-07-2014

I'm having a hard time finding these resistors. Any idea where I can get them or a candohm that matches?

RE: Zenith with candohm resistor - Ron Ramirez - 11-07-2014

Mouser is your friend.

Here is a page showing several choices for the 30 ohm resistor, in 5 and 10 watt versions:
http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0x89jZ1z0wui7Z1z0wt4w

and here is the 100 ohm resistor, in 20 and 25 watt configurations:
http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0x8ahZ1z0wo4tZ1z0wn9m

I may be mistaken but I'm fairly sure that Candohm resistors are obsolete and no longer made? I would not trust an old Candohm from an old radio as it could fail quickly or already be bad.

RE: Zenith with candohm resistor - Eric Adams - 11-07-2014

Thanks Ron Icon_clap

The resistors are on their way.

RE: Zenith with candohm resistor - Ron Ramirez - 11-07-2014

Icon_thumbup

Let us know how it works out. If it is like my Philco Tropic 42-761EZ which is also an AC/DC set in which I had to replace two Candohms with two new resistors...and increased the value of one to take care of today's higher line voltage...it should work out just fine.

RE: Zenith with candohm resistor - Eric Adams - 11-07-2014

You got it Icon_thumbup

RE: Zenith with candohm resistor - Phlogiston - 11-07-2014

(11-07-2014, 02:26 PM)Ron Ramirez Wrote:  Icon_thumbup

Let us know how it works out. If it is like my Philco Tropic 42-761EZ which is also an AC/DC set in which I had to replace two Candohms with two new resistors...and increased the value of one to take care of today's higher line voltage...it should work out just fine.

I just called the power co and talked them into setting my transformer down to 119V. CAUTION: I had good reason, it was at 127V and I'm the only one on it.

Have you ever thought about how that extra 8 volts affects your power bill? Let's see - Ohm's Law - - I cant really tell that the florescent bulbs are any dimmer, and I suppose the compressor motor on my fridg is generating a little less torque, but the pop is still cold - - Hum

RE: Zenith with candohm resistor - Geoff - 11-11-2014

Attached is a pretty good article on replacing candohm resistors.


- Geoff

RE: Zenith with candohm resistor - Arran - 11-12-2014

(11-07-2014, 04:43 PM)Phlogiston Wrote:  I just called the power co and talked them into setting my transformer down to 119V. CAUTION: I had good reason, it was at 127V and I'm the only one on it.

Have you ever thought about how that extra 8 volts affects your power bill? Let's see - Ohm's Law - - I cant really tell that the florescent bulbs are any dimmer, and I suppose the compressor motor on my fridg is generating a little less torque, but the pop is still cold - - Hum

 I think I would do the same, even though there are two or three other houses on the same transformer. Fortunately the line voltage seems to be right where it should be, between 119 to 121 volts, though if they were interested in conserving power they could reduce it all to 115 volts. It would save more power then those stupid CFLs ever will, not that they save energy at all since the consume more in the production phase. AC motors won't care, their speed is governed by line frequency, 110, 115, 120, 125 volts, it won't run any faster or slower, but it will run cooler sometimes, and use less current On a brush or universal motor the reduced voltage will make some difference, though pretty negligible, but again it will use less current. On transformers it will do nothing but help them live longer, lower voltage means cooler running. Since the electric meter measures the ratio between voltage and current, yes it will make your bill higher, for power you don't need to use.
Regards
Arran

RE: Zenith with candohm resistor - Phlogiston - 11-12-2014

It seems to have made our bill about 8 - 10% less.

RE: Zenith with candohm resistor - Eric Adams - 11-13-2014

I received the resistors today and wired them in. I brought it up slowly on the Variac and it is working fine. Icon_clap Now for the complete restoration. Almost every wire is rubber coated. They go first, then a recap and resistor check.
Once again The Phorum comes through. Icon_thumbup Thanks to all who replied.

RE: Zenith with candohm resistor - Ron Ramirez - 11-13-2014

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As to one's house line voltage, if you folks feel that your electric company will listen to you, by all means, call them. Nevertheless, if you think you might sell that nice Doomaflotchie 100 XL AC/DC, and you find you have to replace the dropping resistor, then go ahead and add enough extra resistance to take care of the modern 125 VAC standard. The next owner, who may have 127 VAC at their receptacles, will thank you. One man's opinion, that's all.

RE: Zenith with candohm resistor - jcassity - 04-02-2017

resurecting old thread..

Ron,
i am dealing with one of these resistors on my project.

how did you come up with a third of an amp?

I am so amature at these radios, i dont even know jargan you all use,, i have no idea what is meant by "string"
is the "string" the illustration showing the tubes pins 2&7, 2&7 and so on..?

when i look at the diagram , i see the candohm as a voltage divider ckt.  total system demand is only 50 watts.


where did you find the .3A for calcs?

sorry, just asking