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PostPosted: 04 Mar 2020, 13:08 
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Hello,

This is my first post. I have an old Magnavox tube amp which I love but has started making crackling/popping sounds. I decided this was the time for me to start learning about tube amps so that I can endeavor to fix it myself, instead of sending it out for repair. So I have been reading books and online, and studying my amp. I have a PhD in Mechanical Engineering, and know my way around 12V DC electrical pretty well from building auto wiring harnesses. I also worked on satellites, rockets, and antennas while at DIRECTV for 13 years, so I am hopefully not completely inept.

The Magnavox amp has a balance control. I will never use it and would like to remove it from the circuitry. I've been studying how it is wired and just cannot figure out how it works. It only has three leads. All the example of balance controls I have found have 4-6 leads. Here are some pictures:

Image

Image

Would anyone have any insight on this?

Another perhaps ignorant question. Can anyone tell me what the rectangular items are in this picture? The Tan and Light blue ones? Are they resistors? Fuses? I've not been able to find anything similar online.

Image

Thank you.


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PostPosted: 08 Mar 2020, 07:31 
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What is the type number of your Magnavox amplifier?


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PostPosted: 08 Mar 2020, 13:20 
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EL504 wrote:
What is the type number of your Magnavox amplifier?

It has the following number ink stamped on the chassis:

Image

And this number hard stamped on the chassis:

Image

Thank you.


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PostPosted: 08 Mar 2020, 14:12 
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I do not have the schematic of this type but you could take a look at these schematics.

The first step could be finding out if the balance control is connected to the cathode resistors of the first stage (like in the 8802 and the 93) or to the grid resistors (like in the 196-00).


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PostPosted: 08 Mar 2020, 14:14 
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A few points before we continue...

Dr G wrote:
Can anyone tell me what the rectangular items are in this picture?

Yes. These are 5w dissipation ceramic power resistors. You would have to meter them to find out the value.

Also, from your picture of the resistors, I see mismatched wire (i.e. some cloth covered, some modern). This, in combination with the nature of the way the resistor on the right has been wired in, leads me to believe that the amp has seen some previous modifications or upgrades. A couple of overall pictures of the underside, a picture of the topside, and a tube complement, would help us with your questions.

Also, the three wire balance control could be a couple of different things. Is it possible to determine where the center terminal of the balance potentiometer is connected? It may just be a floating ground point.

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PostPosted: 10 Mar 2020, 17:39 
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Thank you EL504 and Suncalc. I did not get an email so did not realize anyone had replied.

My Magnavox amp has been worked on in the past, though not by me.

It has worked great for about 10 years and I do like how it sounds with my Smaller Advents.

Lately it started crackling and popping a bit. I thought I would try to see if I could fix it instead of sending it out. I would like to someday build my own amp. Not design it, just build it. I love fabrication.

Some folks on another forum did speculate that this amp is an AA 196 or 197. Apparently it has to do with how it was used in the system inside the console. My amp has a Molex slot on the side tabled "tuner pwr" but it is blocked off, and looks to have been that way since it was made. So that is one thing it could have done but did not. It also has an open slot labeled "changer pwr" which I assume was for a record player?

Here are some pictures of the top and bottom:

Image

Image

I've been informed of the no no's on the AC mains wiring and have bought parts to fix that.

Gustave


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PostPosted: 10 Mar 2020, 18:25 
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EL504 wrote:
I do not have the schematic of this type but you could take a look at these schematics.

The first step could be finding out if the balance control is connected to the cathode resistors of the first stage (like in the 8802 and the 93) or to the grid resistors (like in the 196-00).

Thank you EL504 (btw my parents were from Utrecht). I will study the schematics. I don't have much practice yet looking at an as-built product and backing out a wiring schematic. My attempt at describing the balance pot wiring is shown below. To me it most resembles the Magnavox 93 series. But then not exactly.

Image

What confuses me most is what seems to be an electrolytic capacitor in the picture (the large silver can). Because the mirror image wiring for the other channel is just a resistor (at the very upper end of the picture, 1.2k I believe).

Gustave


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PostPosted: 10 Mar 2020, 18:39 
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Suncalc wrote:
A few points before we continue...

Dr G wrote:
Can anyone tell me what the rectangular items are in this picture?

Yes. These are 5w dissipation ceramic power resistors. You would have to meter them to find out the value.


Thank you Suncalc. The tan resistor measures 13.2 Ohms. The light blue one measures 119 Ohms. Using my Fluke 117 (I know, I should get a 177 rated at 1000V).

Suncalc wrote:
Also, from your picture of the resistors, I see mismatched wire (i.e. some cloth covered, some modern). This, in combination with the nature of the way the resistor on the right has been wired in, leads me to believe that the amp has seen some previous modifications or upgrades. A couple of overall pictures of the underside, a picture of the topside, and a tube complement, would help us with your questions.
Done above.

Suncalc wrote:
Also, the three wire balance control could be a couple of different things. Is it possible to determine where the center terminal of the balance potentiometer is connected? It may just be a floating ground point.
It is difficult to see, but is shown in Lavender in the picture from the previous post. It ducks under the balance pot and the silver can and then connects to a point coincident with the metal can and the resistor leading to the RCA inputs.


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PostPosted: 10 Mar 2020, 19:02 
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Dr G wrote:
What confuses me most is what seems to be an electrolytic capacitor in the picture (the large silver can).
From the way you've marked the wires it looks like the capacitor is actually an AC bypass on the wiper of the balance control. The "yellow loop" is actually a parallel R-C pair on the wiper of the balance control between the cathodes of the two 12AX7 triodes. This ensures that the wiper of the balance pot is always at AC ground and can't cause channel to channel bleed through. The 220Ω resistors in the upper part of the photos are the 12AX7 cathode resistors. However the other cathode circuit wiring is confusing. It looks like negative feedback from the outputs but I can't really be sure without attempting to recreate the entire schematic.
Dr G wrote:
Lately it started crackling and popping a bit.
Can you describe the cracking and popping? One channel? Both channels? Does changing the balance control affect the popping? Really, anything you can tell us about it would be helpful.

As for the crackling, that could be coming from a number of sources:

    1. A bad solder joint from the rework time which has recently corroded enough to introduce noise.
    2. One or more of the many carbon composition resistors going bad.
    3. One or more of the sections on the main filter capacitor going bad.
    4. The balance potentiometer going bad.
    5. Bad tubes.
    6. Something else we can't see.

You'll likely have to work through the list one by one.

Hope this helps.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


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PostPosted: 11 Mar 2020, 12:03 
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Suncalc wrote:
Dr G wrote:
What confuses me most is what seems to be an electrolytic capacitor in the picture (the large silver can).
From the way you've marked the wires it looks like the capacitor is actually an AC bypass on the wiper of the balance control. The "yellow loop" is actually a parallel R-C pair on the wiper of the balance control between the cathodes of the two 12AX7 triodes. This ensures that the wiper of the balance pot is always at AC ground and can't cause channel to channel bleed through. The 220Ω resistors in the upper part of the photos are the 12AX7 cathode resistors. However the other cathode circuit wiring is confusing. It looks like negative feedback from the outputs but I can't really be sure without attempting to recreate the entire schematic.

Please don't bother recreating any schematics. Not worth it. But I do understand what you wrote, and that helps. I wonder if the pot could be replaced with two resistors - to replicate its circuit effect when in the middle position?

Suncalc wrote:
Dr G wrote:
Lately it started crackling and popping a bit.
Can you describe the cracking and popping? One channel? Both channels? Does changing the balance control affect the popping? Really, anything you can tell us about it would be helpful.

I wish I'd been more scientific in documenting it. There would sometimes be a drop in volume, perhaps as if one channel was dropping out. And definitely a "pop" sound occasionally which was quite loud. But the amp would other times play normally for hours before issues returned. Definitely intermittent. The rectifier tube is somewhat loose in its socket. One time I moved it as the amp was playing and swore I saw a blue spark down near one of the sockets. Was not able to replicate though. I lightly tapped on the tubes with a pencil (with the amp on) which did not produce any unusual sounds.

Suncalc wrote:
As for the crackling, that could be coming from a number of sources:

    1. A bad solder joint from the rework time which has recently corroded enough to introduce noise.
    2. One or more of the many carbon composition resistors going bad.
    3. One or more of the sections on the main filter capacitor going bad.
Is this the big aluminum "can" on the topside of the amp? Next to the rectifier tube?

Suncalc wrote:
4. The balance potentiometer going bad.
5. Bad tubes.
6. Something else we can't see.
You'll likely have to work through the list one by one.

Yep, I've sort of gathered that. And that is what I plan to do. Thanks Matt


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