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 Post subject: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 12:05 am 
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Joined: Jan Thu 10, 2019 5:39 pm
Posts: 114
Location: Rockbridge Ohio 43149
In working on this radio I have discovered that most if not all the dogbone resistors are WAY out of tolerance. My question is what is the preferred way to replace resistors that are not a standard resistance in modern replacements? For instance it contains a 16K 2 watt. Would you series resistors to make what you needed? Go with the closest available? I'm torn which way to go. TYIA

Steve


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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 2:22 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3092
Location: Lexington, KY USA
Your example is perhaps not the best, since 16k is a standard value at this time, and in stock awaiting your order.

https://www.mouser.com/Passive-Componen ... =Pricing|0

For old household radios, you are almost always OK with the closest modern value among 10% tolerance values.

Since 5% parts hardly cost more than 10%, you can easily get even closer.

If in doubt about a particular resistor, ask here and someone will help.

Actually the power and voltage ratings may be of more concern. And new parts are usually smaller than the old ones with the same power rating.

Be sure not to exceed the voltage rating of any resistor, particularly during warm-up of the radio.

It's often convenient to buy resistors that are roughly the same body size as the originals, which will usually get you more than ample power and voltage ratings. Metal oxide film, carbon film, and metal film are good sorts of resistors for general use in place of dogbones.

Be careful where the old schematics used "M" for thousands, long since replaced by "k". There are other strange symbols used on very early schematics. Again, feel free to ask here when in doubt.

Ted


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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 3:18 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9093
Location: Beaver Falls, PA. USA
Actually, the resistors in the 1930s radios are usually 20% tolerance, with no gold or silver paint on one end. If they measure close to that, I leave them in.

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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 12:09 pm 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 18893
Location: Dayton Ohio
On this whole "Voltage rating" of a resistor, I really question the criticality of it in our line of work.

Consider a 47,000 ohm resistor or 47K. If it is a 1/2 watt resistor, you better not have more than 153.3 volts across it or you exceed the wattage. That is good old Ohms Law at work.

Now as for just having 400 volts on one end and 350 on he other, that would be a difference of 50 volts and all is happy.

I could see a concern in a crowded circuit board where components are in close proximity, but in old radios, there is ususlly plenty of clearance.
No worse than a bare wire going through mid air.

-Steve

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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 12:59 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 4871
Location: Sunbury, Ohio 43074
+1. The only time I've ever been concerned about any voltage rating on resistors, was in truly HV circuits (in the thousands). And even then, usually you'll see these types of resistors, as noted, floating in the air for insulation from nearby parts.

I more or less consider the voltage rating to be more or less just the insulation rating .. .can it be close to other parts and boards, or not. Usually, not except for some very special cases, and components.

Case in point .... my C band transmitter (most transmitters for that matter) have some circuit elements elevated thousands of volts above ground. But they still use standard 50 watt etc resistors.... they are just mounted in elaborate plastic or carbonate structures to keep them insulated from any possible arc path. Couple this insane voltage with high power RF energy and arc prevention becomes something of an art form.

But they are still just normal every day resistors, no special voltage rating.

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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 7:40 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 3092
Location: Lexington, KY USA
Resistors have both end to end and insulation voltage ratings.

These ratings may often be quite conservative. But without extensive testing, you will not know how much extra you have beyond the manufacturer's rating.

In reality, the 1/4 watt size film resistor is usually going to be OK for general use in a table radio.

Some of the early AC powered radios did have quite high B+ voltages that might get you in trouble on one or two resistors.

The larger size resistors are mostly easier to use; their higher voltage ratings are a bonus.

Ted


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 Post subject: Re: Fearnola/Wells Gardner 7J dogbones
PostPosted: Aug Fri 09, 2019 10:16 pm 
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Joined: Jan Thu 10, 2019 5:39 pm
Posts: 114
Location: Rockbridge Ohio 43149
Thank you all for your insight, advice and suggestions! It has been quite helpful!

Steve


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