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 Post subject: Differences between pots/controls
PostPosted: Aug Sat 28, 2021 8:04 pm 
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Joined: Jul Wed 07, 2021 10:44 pm
Posts: 6
Howdy,

Please forgive me if there are lots of messages concerning parts interchangeability; my searches yielded only discussions on tube substitution so hopefully I'm not repeating a question.

My volume control died in middle of restoring a Zenith 6S229 tombstone. I posted an ad in the forums looking for a replacement, and a few folks responded to let me know that I could substitute a modern component. While my intention was to keep the radio as original as possible, their suggestion led me to peruse a copy of the Zenith parts list. I found that Zenith made several different 400K ohm volume controls.

I'm sure that some of them differ in the length of the shaft (get your minds out of the gutter :wink: ), but are there other differences, do you think?


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 Post subject: Re: Differences between pots/controls
PostPosted: Aug Sat 28, 2021 8:47 pm 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 18639
Location: Mountains of Mourne. Ireland.
http://www.oldradios.com/reference/zenith.htm

Rider 1938 • Zenith pages…
8-23 http://americanradiohistory.com/hd2/IDX ... e-0098.pdf
8-24 http://americanradiohistory.com/hd2/IDX ... e-0099.pdf

V.C. (volume control) R-10 400 MOhms
Zenith part number 63-520

1937 Zenith parts catalog. (49-pages)→ https://antiqueradios.com/forums/viewto ... 3#p3178963
V.C. physical attributes not listed.


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 Post subject: Re: Differences between pots/controls
PostPosted: Aug Sat 28, 2021 9:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2479
Location: Lafayette, CO
If Centralab made those, then maybe you could swap resistive elements. Most controls were about 1meg so 'close enough' ohms has worked for me. Some of those controls were junk, especially those with a wood tip being a slider. Craig


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 Post subject: Re: Differences between pots/controls
PostPosted: Aug Sun 29, 2021 2:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10853
Location: Beaver Falls, PA. USA
Mark Oppat will have the correct control for your set.

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Tim KA3JRT


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 Post subject: Re: Differences between pots/controls
PostPosted: Aug Sun 29, 2021 5:47 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11141
Location: Long Island NY
Aside from the obvious mechanical differences, there are a few other things to take note of when looking for replacement/substitute volume controls.

1. Does it have a switch of some kind, or more than one control section? A proper replacement control should have the same features as the original.

2. Does it have a tap or a fourth connection? If so, you'd want the same in a new control. Otherwise the circuitry may have to be modified to work without it. Also, some pots were designed to stop before you got to the end, so they'd never go to zero ohms but always leave some resistance in the circuit. Conventional pots go to zero ohms resistance when the wiper reaches the end of travel. To replace one that goes to some fixed value of resistance, you'd have to put an external resistor in series with one end.

3. What is the taper? Taper means the way the resistance changes per degrees of rotation of the shaft. Linear is the most common taper; it means the resistance changes with the degrees of rotation in straight-line fashion. Tone controls are typically linear taper. There are also logarithmic or audio taper pots which give a logarithmic response. They are typically used for volume controls because human response to audio levels is logarithmic. Reverse taper pots, which are logarithmic but give the opposite effect of audio pots, sometimes turn up in test equipment and other applications. There are some other tapers--exponential for example--but they are more specialized.

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 Post subject: Re: Differences between pots/controls
PostPosted: Sep Fri 03, 2021 5:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 6071
Location: 253 Blanche St. Plymouth, MI USA
There can be up to about 8 criteria to match when seeking a replacement control.
1. the overall resistance
2. any "tap" on that resistance (can be one or two, RCA did one with 3 once in 1938 !)
3. any power switch and what type of switch
4. what is the "taper" of the resistance ? there are up to 6 tapers I have available. "reverse" taper was used in early AC radios that had no AGC ("gain controls, usually 3K-25K value).
5. Shaft type and length as measured from the MOUNTING FACE of the control or panel face. bushing lengths vary so we dont measure from the end of those.
6. If a 'flatted shaft" does the flat have to face a certain way at off postion ? This is important for knobs that are indexed to the panel.
6. special mounting bushings (as used on car radios-- especially important!)
7. special mounting brackets or flanges ?
8. Special insulated shafts for safety ? Some controls used in HV stage of TV's are isolated from chassis.
I can match most any control 1930-1970 or so. After that, the makers had controls made to their particular needs and the aftermarket parts were not made to match.

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Mark Oppat
Antique Audio
Plymouth, MI
http://www.oldradioparts.net


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