Rick Blechta wrote:
> Hi All!
>
> I think this is a topic worthy of comment by those repairers, restorers and general gaddabouts of all things Mellodramatic to address. After all, they've demagnetized everything they can get their hands on. Well, JB should comment. That Martinge character knows nuffink.
>
> So guys, what is the correct way to demag tron head blocks? And do you do it differently when you're in Hollywood? And does Julia Roberts look in person a lot like she does on the screen? Rick
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Hi Rick/Vicki/fam, "Brendatron"and everyone,
Long ago, it was discovered that iron could be magnetised by hammering
it repeatedly in a north-south orientation. Then came naturally-occuring
lodestone,creating compasses to guide us to polar directions.
I shared with you that little bit of trivia,to tell you the following.
Mellotrons can be demagged with either the "Master-Flux" electro-
magnet of JB or individually, one head at a time. It's DC current which
can cause N-S polar magnetism, AC as in JB's bulk unit or small units,
will again disarrange the ferric material to random order.
Looking over the schematics for M400 preamps, there's a 10-47 UF cap
attached to coax connector for the headblock. When headblock is
reattached, there's a chance the charge in the cap will induce a small
transient DC potential on the headblock,possibly magnetising it again.
That's why it's been recommended to short-out (w. switch off, or course),
the coax connection on preamp box. Or to insert a 50K-100K resistor
across that terminal,elim. forever the chance to remag the headblock.
Moving the demagger back and forth several times but NOT touching
each head will work effectively. The demagger itself must not be
switched on-off when the tip is near the headblock. You can actually
"feel" or "hear" the buzzing sound induced when demagger is working.
If you can find a "bulk tape eraser", the job will be completed in
short-order. But make doggone sure it's not anywhere near our
precious tapes ! When restoring "Julia", I discovered that Gordon's
orig. tapes 1-4 on LH and 30-35 on RH side had a few audio
dropouts . Someone/something installed speakers inside or fiddled
with the tapes w/o taking the necessary precautions.
Check with Sir Isaac Newton and the Inverse-Squares Law of physics.
Cheers, Jerry Korb (playing with iron filings,and fluxing all-night)