T Attenuator Termination Kluge

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Gold
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T Attenuator Termination Kluge

Post by Gold » Thu Jul 25, 2019 12:24 pm

I've run into an issue with one of the Output Amplifiers of my console. The P&G RF15 rotary fader I'm using has a series resistance of about 700Ω at the top of the throw. The other RF15 is less than 25Ω. It took me a while to suss that out.

The signal path for the Output Amplifier is: Balanced to unbalanced Input Buffer - 5KΩ Bridged T attenuator- 10k RF15 Rotary Fader- Line Driver. The Attenuator needs to be terminated with 5KΩ for accurate level steps. The extra series resistance is throwing off the attenuator steps.

My first thought was to put the Fader before the attenuator but the attenuator wants to see a low source impedance so I'm not sure that's a good solution. The next thought I had was to trim the termination resistor to take into account the series resistance of the Rotary Fader. This seems like the best plan to me. Any suggestions besides ordering TKD pots?

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JR.
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Re: T Attenuator Termination Kluge

Post by JR. » Thu Jul 25, 2019 2:10 pm

Gold wrote:
Thu Jul 25, 2019 12:24 pm
I've run into an issue with one of the Output Amplifiers of my console. The P&G RF15 rotary fader I'm using has a series resistance of about 700Ω at the top of the throw. The other RF15 is less than 25Ω. It took me a while to suss that out.
By series resistance are you describing the wiper contact resistance between wiper and top of the pot? 25 ohms seems a little high, 700 ohms seems faulty.
The signal path for the Output Amplifier is: Balanced to unbalanced Input Buffer - 5KΩ Bridged T attenuator- 10k RF15 Rotary Fader- Line Driver. The Attenuator needs to be terminated with 5KΩ for accurate level steps. The extra series resistance is throwing off the attenuator steps.

My first thought was to put the Fader before the attenuator but the attenuator wants to see a low source impedance so I'm not sure that's a good solution. The next thought I had was to trim the termination resistor to take into account the series resistance of the Rotary Fader. This seems like the best plan to me. Any suggestions besides ordering TKD pots?
I'm not sure I understand the question.

I found this data sheet for RF 15 and it looks pretty straightforward. https://www.cw-industrialgroup.com/geta ... nical_data

Can those be opened up and inspected inside for wear or damage? If one measures 25 ohm and the other 700 ohm one is clearly wrong.

I seem to recall linear P&G faders being relatively easy to take apart to clean.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Gold
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Re: T Attenuator Termination Kluge

Post by Gold » Thu Jul 25, 2019 2:37 pm

JR. wrote:
Thu Jul 25, 2019 2:10 pm
By series resistance are you describing the wiper contact resistance between wiper and top of the pot? 25 ohms seems a little high, 700 ohms seems faulty.

Yes that’s what I am describing. I agree 25 ohm seems high and 700 ohm seems faulty. Unfortunately they are not made or supported anymore. I’m trying to use them as they were hideously expensive and the tracking between elements is excellent.



Can those be opened up and inspected inside for wear or damage? If one measures 25 ohm and the other 700 ohm one is clearly wrong.
They look like they can be opened up but I don’t know what’s going to pop out if I do that. The linear faders have a cover and nothing pops out.

My question if I’m stuck with the 700ohm series resistance can I subtract 700 from the 5K termination resistor and bring the gain steps into line. If that will work is there a downside?

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JR.
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Re: T Attenuator Termination Kluge

Post by JR. » Thu Jul 25, 2019 6:14 pm

the 700 ohms is unlikely to be a stable resistive offset.. I would not try to use it.

If you open it up a caterpillar may pop out.

JR

[edit] more likely a moth or butterfly..

Seriously determine what is causing the extra 700 ohms which could inform how reliable I is, but I cannot imagine a stable fault like that.

]/edit[
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Gold
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Re: T Attenuator Termination Kluge

Post by Gold » Fri Jul 26, 2019 1:00 pm

JR. wrote:
Thu Jul 25, 2019 6:14 pm
If you open it up a caterpillar may pop out.
All six decks are measuring the same. I'll consider myself lucky if a caterpillar crawls out. I have a feeling I'm S.O.L.

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JR.
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Re: T Attenuator Termination Kluge

Post by JR. » Fri Jul 26, 2019 10:33 pm

Gold wrote:
Fri Jul 26, 2019 1:00 pm
JR. wrote:
Thu Jul 25, 2019 6:14 pm
If you open it up a caterpillar may pop out.
All six decks are measuring the same. I'll consider myself lucky if a caterpillar crawls out. I have a feeling I'm S.O.L.
It sounds like you had two of these potentiometers.

One with 25 ohms wiper to top of pot travel... and another with 700 ohms from wiper to top of pot travel...

Now do I understand that you have 6 gangs all with 700 ohms from wiper to top of pot travel,,,, and still one other one with 25 ohms from wiper to top of pot travel?

If you have a 6 gang pot all measuring exactly 700 ohms from wiper to top of travel, that seems unlikely to be contamination, or worn wiper.

JR

[edit- is it possible that the shaft has a rotational offset explaining the not hop-off level low end resistance? Might be visible or fixable when disassembled, or not. If that is what it is the 700 ohms could be resistive. [/edit]
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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mediatechnology
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Re: T Attenuator Termination Kluge

Post by mediatechnology » Sat Jul 27, 2019 2:29 pm

Penny and Giles RF15 https://www.cw-industrialgroup.com/Prod ... Fader-RF15

Penny and Giles RF15 Datasheet https://www.cw-industrialgroup.com/geta ... nical_data

Something just doesn't seem right about the 700Ω or 25Ω wiper resistance.
Were these new when you bought them?
https://ka-electronics.com

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Gold
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Re: T Attenuator Termination Kluge

Post by Gold » Sat Jul 27, 2019 5:12 pm

mediatechnology wrote:
Sat Jul 27, 2019 2:29 pm
Something just doesn't seem right about the 700Ω or 25Ω wiper resistance.
Were these new when you bought them?
I agree something isn't right. I bought them new from Dale Manquin who is no longer with us. I'm going to attempt to dis assemble one and see what happens.

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JR.
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Re: T Attenuator Termination Kluge

Post by JR. » Sun Jul 28, 2019 10:05 am

Gold wrote:
Sat Jul 27, 2019 5:12 pm
mediatechnology wrote:
Sat Jul 27, 2019 2:29 pm
Something just doesn't seem right about the 700Ω or 25Ω wiper resistance.
Were these new when you bought them?
I agree something isn't right. I bought them new from Dale Manquin who is no longer with us. I'm going to attempt to dis assemble one and see what happens.
I seem to recall the P&G linear faders were designed to be user serviceable to take apart for cleaning and reassemble, hopefully the rotaries are too.

Look also for possibly a slipped wiper angle that is preventing the top of travel from reaching the low R metal conductive hop off contact.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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mediatechnology
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Re: T Attenuator Termination Kluge

Post by mediatechnology » Sun Jul 28, 2019 2:26 pm

I've cleaned a lot of cocaine out of P&G and Waters linear faders in my day.
"Blow" being very, very corrosive, eats through PC board traces and elements.
Coffee with sugar or soda also requires extreme measures.
Black coffee isn't so bad.
You can vacuum pot out of a console. Pot's my favorite since it doesn't require soap and water.

The P&G rotary faders I have no experience with but they look to have long screws that hold the sandwich together.
The individual elements may be sealed like a Mod Pot.
https://ka-electronics.com

Free Hong Kong.
"We’ve Seen This Movie Before–How Communist China Is Following Nazi Germany’s Playbook Against America" https://www.breitbart.com/politics/2019 ... t-america/
"China Warns Brands to Accept Communism, Freedom of Speech ‘Ridiculous’" https://www.breitbart.com/asia/2019/10/ ... idiculous/
"There's A War On For Your Mind" Banned.Video https://banned.video/

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