Shallco Bridged T Attenuators

This is where we talk about testing, measuring and repairing things. Sometimes we have to repair the equipment we use to test, measure and repair other things. It's an endless cycle of fixing the broken things we need to fix other broken things.
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Gold
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Shallco Bridged T Attenuators

Post by Gold » Wed Oct 31, 2018 12:34 pm

I have eight Shallco two deck 45 position per deck bridged T attenuators in half dB steps. They are for the Input and Output amps of the cutting console. Left Modulation and Preview. Right Modulation and Preview. Six of them worked two of them didn't. I ordered them about ten years ago so asking for support wasn't an option. A few months ago I found a solder bridge on one of the non working ones after many hours of looking and measuring. That one only had one bad position.

The last non working one was difficult. The whole range got screwy. I thought it had to be external to the attenuator to alter the attenuation scale. After going through all those options a few times I figured it had to be the attenuator. I couldn't find a solder bridge or any resistors shorted to each other. Finally I bit the bullet and measured everything. In to Out, Out to In, In to Common, Out to Common. I finally found the bad resistor. Five sheets of graph paper notes later.

Shallco obviously never checked this deck because it couldn't have worked. I actually think it was a perfect storm. The bad value wasn't one that was used in the attenuator. The resistors are the IRC 0.1%. They sent me the necessary values and I ordered the resistors and sent them to Shallco. I think Mouser put the wrong value in one of the bags. Shallco didn't catch it when soldering and never checked it. That was a decent chunk of my life I won't get back.

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mediatechnology
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Re: Shallco Bridged T Attenuators

Post by mediatechnology » Wed Oct 31, 2018 1:39 pm

Glad you found it.
Would love to see a pic of that beast.

I'm always measuring my 1% resistors now before soldering.
Some years ago Mouser's inventory got 4K99 and 49K9 mixed up in their bins.
They had to ship me 4K99's twice before getting it right.
After speaking with an engineer, I finally got him to go into the warehouse and read the parts himself.
I finally got my 49K9s on the third try.

Gold
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Re: Shallco Bridged T Attenuators

Post by Gold » Wed Oct 31, 2018 3:03 pm

This was a 39K2 instead of a 392R.
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emrr
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Re: Shallco Bridged T Attenuators

Post by emrr » Thu Nov 01, 2018 8:33 am

Oh man, that's a serious pain. I love what those do, but the labor time for anything around them is insane.
Best,

Doug Williams
Electromagnetic Radiation Recorders

Gold
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Re: Shallco Bridged T Attenuators

Post by Gold » Thu Nov 01, 2018 9:44 am

It been a thorn in my side for years. One of those things that is a barrier to working on the project because. It’s such a head scratcher. It feels good to have it out of the way.

Next up is getting the crossfader going. After that’s going I “just” have the EQs to do to complete the console. I’ve had a distinct lack of energy the past two or three years. I feel like I have my energy back and am looking forward to working on it.

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Re: Shallco Bridged T Attenuators

Post by emrr » Fri Nov 02, 2018 7:24 am

I feel like I recall you ordering these for the console project. It'll be great to have the finished off.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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mediatechnology
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Re: Shallco Bridged T Attenuators

Post by mediatechnology » Fri Nov 02, 2018 8:35 am

Solving a mystery like that (finding the wrong R-value) that's held up a project is so gratifying.

Paul let me know if you need any help online or off-line with the cross-fader.

Like you I'm feeling motivated to fix and use things I've set aside from repairing bad clip leads to fixing old instruments.

Gold
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Re: Shallco Bridged T Attenuators

Post by Gold » Fri Nov 02, 2018 9:44 am

I looked to see if I had 39R2 resistors to replace the 39K2 resistor. I pulled the bin and found a bag. Most of them were out of spec for 0.1%. I made it half way through the bag before realizing I had the 39K2 bag in the 30R-42R bin. DOH! No one has an IRC/Welwyn 39R2 so I had to order the only kind they had.

Gold
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Re: Shallco Bridged T Attenuators

Post by Gold » Fri Nov 02, 2018 11:13 am

Thanks Wayne for the offer. The crossfader designer forgot to include the logic to switch the push button lamps between A/B/xfade. I think I need flip flops for that. Since I don’t know how to make a flip flop I could use some help with that.

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