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Post subject: hi, 1974 Vibr0sonic Reverb mod questions
Posted: Tue Apr 30, 2019 2:01 pm
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Hobbyist
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Joined: Tue Apr 30, 2019 10:20 am
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Hey folks, new guy here.
I have a 1974 Fender Vibr-0-sonic ( not A-sonic) . Im doing some maintenance and came up on this 'mod'?
The white Mallory 25/25 is on the reverb pedal rca circuit , it looks like someone removed the cap from the board to this position. Where there should be 2x 25/25's there is one. The black 35/25 is on the vibrato pedal circuit. On the layout it should be 2x 25/25 on the board, what is there now is this single 50/4.7uf.
Lastly this 80/75 was added. But why?
So, anyone know what these mods were for?
Thanks, Merl

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Post subject: Re: hi, 1974 Vibr0sonic Reverb mod questions
Posted: Sun May 05, 2019 11:47 pm
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Rock Star
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Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
My WAG at these photos. :)

First off, Vibrosonic schematic:

http://www.thevintagesound.com/ffg/sche ... _schem.jpg

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https://i.imgur.com/XQ2dcpq.jpg

The "extra" 80mfd/75VDC cap is the second filter cap of the BIAS POWER SUPPLY. Seen in the schematic as two parallel 80mfd/70VDC caps.

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https://i.imgur.com/Ozqzwq8.jpg

4.7mfd/50VDC cap replaced the 5mfd/50VDC cathode bypass cap off V5B.

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https://i.imgur.com/YCGbfw4.jpg

The 35mfd/25VDC cap is the bypass cap off V5A. A tech just re-routed it off the circuit board and onto the chassis. Much like how Fender wired the OEM 25mfd/25VDC Mallory bypass cap off V4A.


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Post subject: Re: hi, 1974 Vibr0sonic Reverb mod questions
Posted: Mon May 06, 2019 6:58 am
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Professional Musician
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Joined: Tue Nov 27, 2012 6:56 am
Posts: 1677
Location: Coastal Bend, Tx.
+1, what BMW said.
CBS era amps moved some components to the sidewalls/tube sockets. Why, maybe to help aid heat dissipation. It doesn't really matter where some components are located so long as they are in the correct place in relation to the circuit and don't induce any unwanted signal degradation or hum.

I would replace/update all those old bypass/buffer caps for reliability sake. As well as any in the power supply/doghouse of that age.


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