Magnatone 431 - Needs Repair

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wdelaney72
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Magnatone 431 - Needs Repair

Post by wdelaney72 » Thu Apr 05, 2007 3:01 pm

I picked up this little combo and it sounds great, but needs some work. First and foremost, it's got some kind of loose ground, because if you touch a mic stand or anything metal while plugged in you get a little sumthin' special charging through your system.

Also, pots are all noisy as hell.

I found Viroworld, but that appears to be defunct at the moment. If anyone knows where to get a schemtic of this, I'd greatly apprciate it.

Makes for a really nice sounding practice amp. I'll post pictures when I can get away from my lesson room.

darrell
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Post by darrell » Fri Apr 06, 2007 11:40 pm

Is the plug a 2 prong or 3?

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wdelaney72
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Post by wdelaney72 » Sun Apr 08, 2007 11:28 pm

darrell wrote:Is the plug a 2 prong or 3?
It's a 2 prong. I did find a schematic, so I've begun the dismantling.

Image
Last edited by wdelaney72 on Mon Apr 09, 2007 9:21 am, edited 1 time in total.

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Matt
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Post by Matt » Mon Apr 09, 2007 1:40 am

Yea most of those two prong amps have some sort of artifical ground system on them where either the neutral or hot side is connected to the chassis through a capacitor and usually a polarity switch to select which side gets grounded. It wast really that dangerous when it was used back in the day since most every appliance was setup like that, and you would only get a buzz if you touched something that was truly grouded like a metal sink/faucet etc. Now days most everything is grounded so you need to make sure it gets a proper three prong plug too. Ive heard of people almost getting killed when they have a old non grounded amp and touch a grouned mic while holding the guitar, even if it doesnt kill you, getting 120v to lips doesnt feel very pleasent.....
If I was a good student I wouldnt be attending music school.

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Wicksy
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Post by Wicksy » Mon Apr 09, 2007 9:09 am

The method is use is to solder a connector to the earth lead and attach it to the PT chassis bolt. That way you have a nice safe ground. Then you disconnect the polarity switch and remove the cap and you're soughted.
Simon

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wdelaney72
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Post by wdelaney72 » Mon Apr 09, 2007 9:21 am

Wicksy wrote:The method is use is to solder a connector to the earth lead and attach it to the PT chassis bolt. That way you have a nice safe ground. Then you disconnect the polarity switch and remove the cap and you're soughted.
That's been my plan, but there's no polarity switch anywhere that I can see... I'll look more at it tonight. I'm having a hell of a time getting the chassis out.

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wdelaney72
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Post by wdelaney72 » Mon Apr 09, 2007 11:32 pm

Here are some internal shots

Image

Image


Here's the schematic
Image

I can see how to easily fix the power cord with a 3-pronger grounded to the chassis. The Reverb tank connections were undone. Basically, I think all it needs is that... along with new pots. I've never done anything with a reverb unit so this will be a new one.

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Wicksy
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Post by Wicksy » Tue Apr 10, 2007 8:47 am

Damn! You've a fortune in Blue Astrons in that amp! :shock: A Fender collector/restorer would drag their balls over hot coals for them!

Doesnt look like there is a polarity swich, just what looks like a cap. I think Fender called them the "death" cap. You can probably remove it (or at least replace it with a modern one id its paper in oil).

I assume the lamp is mains driven? The bits before the primary taps? Might be an idea to get a mains fuse in there, just for you own safety sake.
Simon

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wdelaney72
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Post by wdelaney72 » Tue Apr 10, 2007 9:02 am

Yeah, Hoffman has referred to that as "the death cap".
http://www.hoffmanamps.com/charts/CommonHookups.htm

Adding a mains fuse is probably a good idea.

The amp has 5 pots.
1) VOLUME: 11-0005, 137.6504
2) TONE: 11-0005, 137.6504
3) INSTENSITY: 11-0005, 137.6504
4) SPEED: 11-0006, 137.6511
5) DEPTH: 11-0005, 137.6504

I know 137.6504 = CTS built 4th week 1965 and 137.6511 is CTS built 11th week 1965. The schematic says volume and tone are 1M and speed is 500K. would these be linear or audio taper. I assume the volume is audio taper and since the others match (except speed) they would be audio as well. Before taking it apart, I played through it quite a bit and REALLY liked it. I'm under the assumption that the pots need to be replaced, as adjusting any of them made a lot of noise and they seemed to "short" out on occasion... in other words, they were very sensitve when you touched them, and not in a good way.

Am I wrong to assume the pots need to be replaced?

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Post by ScottW » Tue Apr 10, 2007 10:11 am

Before you toss the pots, you can try to clean & lube them with Deoxit FaderLube.

Check out this thread:
http://forum.metroamp.com/viewtopic.php ... highlight=

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Post by wdelaney72 » Mon Apr 23, 2007 1:20 pm

I) polarized, 3 prong power cord installed
2) cleaned out all pots with deoxit
3) repaired the broken connection to the reverb tank.

The Deoxit helped A LOT. It didn't completely restore the pots, but they're usuable. All that's left is a replacement set of tubes. 6HU8 power tubes don't grow on trees, but all is good, as TT is on the case.

This is a great little amp. REALLY warm and creamy sounding.

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Re: Magnatone 431 - Needs Repair

Post by Patientzero studio » Thu Apr 01, 2021 11:34 am

How did you get the chassis out . I have the exact amp, and music store I tool it to can't figure it out

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