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Old 13th January 2010, 02:53 PM   #1
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Default Surplus Voltage Solution

I recently completed a "Retro-Mod" project on a Magnavox console amp. The biggest issue was the high TX output. It was about 40V over spec. I think this is caused because the the load from the other console components was removed from the circut.

What is the best way to tame the TX output?

I have several more amps to rework and expect will have a similar problem with these.

Whe running the amp at the high voltage the sound stage was completely confused and hard to listen to.
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Old 13th January 2010, 03:27 PM   #2
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Quote:
Originally Posted by Troncones View Post
Whe running the amp at the high voltage the sound stage was completely confused and hard to listen to.
Can you give any more details like a schematic and what you removed? If it sounds that bad the problem is more likely that you ripped out something that was necessary for proper function.
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Old 13th January 2010, 04:05 PM   #3
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Not necessairly, He could have pulled out a tuner section, phono section, etc to get down to a basic power amp.

Original schematic and changes you made would be nice to have.

What is your B+ (HT) circuit? Pi filter, CLC?, CRC? You can manipulate the output of the filter section by changing values of the input cap, output cap, and inductor or resistor. Lowering the input cap value would be my first step, followed by increasing (or adding) resistance between the first and second caps.

Model it in PSU designer.

PSUD2

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Old 13th January 2010, 05:15 PM   #4
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Tron, you need to show us a schematic of your power supply, otherwise we have no idea what we're dealing with. Even if you have to hand draw it and scan it in, that's cool; I can draw a schem.gif for you, and the greater lights here will be able to give you some concrete suggestions.

Aloha,

Poinz
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Old 13th January 2010, 06:28 PM   #5
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Originally Posted by TheGimp View Post
Not necessairly, He could have pulled out a tuner section, phono section, etc to get down to a basic power amp.
I disagree, if he did that properly and the only issue was high voltage, then the sound would be good, and the worst symptom would be that the PTX and/or tubes were hot.
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Old 14th January 2010, 12:14 AM   #6
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Default Surplus Voltage Solution

Thank all of you for your responses. I hope you can read the attached. How can you share large files?

I got the amp only so the PTX that powered other components in the console is now just providing power to the amp section.

What I did to solve the problem is add a couple 180 Ohm R's between the Rectifier and the first cap in the PS. This seems to work. I got the B++ down to 157V.

After I made this change the sound stage problem went away. I'm guessing it was either the high voltage/heat or the bias node for the amplifier stage was all screwed up.

The added resistors are adding a little more heat under the chassis than I would like. The other concern is that adding the resistors will reduce performance.

I recently downloaded PSU II (Duncan's Amps). I haven't had a chance to work with it. When I looked at it the first thing that poped up was the load resistance. Can this be accurately calculated or is it better to get the amps at B++ while the amp is running?

Thanks again for your responses. Considering I'm new at this I can use all the help I can get.

Tom
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Old 14th January 2010, 12:26 AM   #7
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My follow up post should read 257V not 157V
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