Masco C-10 Mic/Phono Amp - diyAudio
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Old 31st January 2013, 10:31 PM   #1
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Default Masco C-10 Mic/Phono Amp

Hi,

I have an old Masco C-10 Amp I was trying to get going. I've tried reforming the caps and I've replaced the burnt tubes (specifically the twin diode rectifier tube). I was getting a good output but the transformer was heating up. So I then replaced the multi-cap can with some individual capacitors. Still heating up. I then drew out the schematic and noticed the 20k ceramic resistor going to ground. I had a look and it's on a two terminal block bolted to the chasis and the terminal has been busted off. It is now held in place with a wire to the adjacent terminal. It looks like to me this shouldn't be going to ground. Maybe someone with more knowlege than I can have a look at this circuit and tell me what they think.

Thanks,
Craig
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Old 31st January 2013, 11:59 PM   #2
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Quote:
Originally Posted by CraigNS View Post
Hi,

I have an old Masco C-10 Amp I was trying to get going. I've tried reforming the caps and I've replaced the burnt tubes (specifically the twin diode rectifier tube). I was getting a good output but the transformer was heating up. So I then replaced the multi-cap can with some individual capacitors. Still heating up. I then drew out the schematic and noticed the 20k ceramic resistor going to ground. I had a look and it's on a two terminal block bolted to the chasis and the terminal has been busted off. It is now held in place with a wire to the adjacent terminal. It looks like to me this shouldn't be going to ground. Maybe someone with more knowlege than I can have a look at this circuit and tell me what they think.

Thanks,
Craig
The 20K as shown would pull a lot of current that's why the transformer is heating up, a more reasonable schematic is shown below, perhaps the 20K is sub'd for the choke?

Click the image to open in full size.

Jaz

Last edited by jazbo8; 1st February 2013 at 12:09 AM.
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Old 1st February 2013, 12:14 AM   #3
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Ya, that's what I've assumed. I was thinking it should go from the cathode of the rectifier to the 40uf cap then ground. I tried that, but have less volume output and still a heating transformer.

What about like this?
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Old 1st February 2013, 12:47 AM   #4
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Originally Posted by jazbo8 View Post
The 20K as shown would pull a lot of current that's why the transformer is heating up, a more reasonable schematic is shown below, perhaps the 20K is sub'd for the choke?

Click the image to open in full size.

Jaz
Right now, the center tap from the B+ transformer is going to chasis ground. Also it's using a multi-cap can, so no way to grab the negative of the 10uf/20uf cap. Do you think the center tap should be going to this resistor?
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Old 1st February 2013, 12:49 AM   #5
jjman is offline jjman  United States
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The 20k is a bleeder for safety, assuming the scheme in post#1 is accurate. It drains the dc after turning off. You can use a larger value if you are concerned it's pulling too much current. I'm actually using a 1meg on an amp but that's overly "slow."

Last edited by jjman; 1st February 2013 at 12:52 AM.
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Old 1st February 2013, 03:07 AM   #6
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Instead of us just guessing, perhaps it's easier to change it to the way you want, unless you are trying to restore it to its original condition. What is this amp going to be used for?

Jaz
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Old 1st February 2013, 03:20 AM   #7
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Well, it's been around since I was a kid, so partly just want to get it going. Will use it for guitar. Wasn't looking for a guess... was hoping this would be a familiar circuit to someone.
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Old 1st February 2013, 01:42 PM   #8
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Quote:
Originally Posted by CraigNS View Post
Well, it's been around since I was a kid, so partly just want to get it going. Will use it for guitar. Wasn't looking for a guess... was hoping this would be a familiar circuit to someone.
Check out the Fender 5C1 Champ, or even other Masco for inspiration. Harp players seem really into the classic Masco's for their tone.
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