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smuprof
Intermediate Member
Username: smuprof

Post Number: 134
Registered: 12-2003
Posted on Sunday, March 04, 2012 - 1:18 pm:   Edit Post

I have an F2-B with the blue case and the silver knobs, old style 2 prong cord. I bought it from a recording studio going out of business. The paper schematic glued to the inside top says "Alembic F-2B Stereo Preamp" and "Starting with Serial Number 143" so I presume the prior units had some slightly different circuit design.

Channel One works fine; Channel Two will work fine typically at first, but then after some period of time that varies from 2 minutes to 20 minutes will get very distorted and it seems like it loses lower frequencies.

I've swapped and replaced tubes, but the problem doesn't follow, so it can't be the tubes and the power supply is common to both, so that's not it. It also seems the "time delay" is a bit misleading (not a true heating issue, more just an intermittent).

Given the reduced low frequencies, I've wondered if the bass tone pot might be intermittent or something in that specific section going south.

Has anybody experienced anything like this? Thoughts, guidance, direction, suggestions would be much appreciated.

John
dela217
Senior Member
Username: dela217

Post Number: 1105
Registered: 6-2002
Posted on Sunday, March 04, 2012 - 5:51 pm:   Edit Post

Sorry I do not have any input for you on the distortion problem.

But, you mention the schematic and I can give some insight on that. Starting with serial number 143 the unit used an LED indicator instead of a light bulb on the front of the unit. The circuit is basically the same. Except the VERY early ones (the first 10 or so) did not have a mono output and fewer capacitors on the power supply filter.

Michael
jbybj
Advanced Member
Username: jbybj

Post Number: 350
Registered: 6-2006
Posted on Sunday, March 04, 2012 - 10:29 pm:   Edit Post

Most Alembic rack gear, eventually, ends up suffering from oxidation on the I/O jacks. Clean the jacks by exercising them repeatedly. There is a post, probably in the FAQ about doing this.
slawie
Advanced Member
Username: slawie

Post Number: 366
Registered: 8-2002
Posted on Thursday, March 08, 2012 - 2:54 pm:   Edit Post

The high voltage capacitors degrade over time.
The electrolyte encased in the capacitor dries out and effectively become unfunctional.
The alternate caps from Alembic are just the ticket as the original rated caps are no longer available.

slawie

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