From muelle–(at)–sl.stanford.edu Mon Jan 31 23:00:39 CST 1994
Article: 13823 of rec.music.makers.guitar
Path: geraldo.cc.utexas.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!morrow.stanford.edu!morrow.stanford.edu!mueller
From: muelle–(at)–sl.stanford.edu (Fritz Mueller)
Newsgroups: rec.audio.pro,rec.music.makers.guitar
Subject: Re: Tech help w/ Fender twin not tremoling
Date: 30 Jan 94 23:37:54
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In-reply-to: frank–(at)–co.caltech.edu’s message of 24 Jan 1994 18:57:49 GMT
Xref: geraldo.cc.utexas.edu rec.audio.pro:7131 rec.music.makers.guitar:13823
>> HELP! I’m working on a Fender Twin that’s working fine other than
>> the tremolo. … What does the tremolo circuit look like, and how
>> does it function?
The trem circuit in those old fenders is an RC phase-shift oscillator
— three cascaded RC stages in a feedback loop between the plate and
grid of one of the triode tube sections. The rate of oscillation is
controlled by varying the resistance of the “R” leg of one of the
stages; the trem is disabled/enabled by breaking/making the ground
connection on one of the legs. The oscillator is buffered through
another triode section and coupled onto the power tube grid bias
supply via a neon-lamp/photoresistor gizmo.
You can find the trem circuit by tracing back from the trem speed pot
or from the trem pedal RCA jack, or just look for the three ceramic
disc caps connected end to end (usually two .01mf and a .02mf.)
Most often these stop working for one of three reasons:
– The caps get leaky. If the circuit oscillates very briefly and
damps out after a cycle or two when you switch it on and off, this
is probably the problem. Just pull ’em and replace ’em. (Note
that these caps may read fine on a capacitance meter even though
they’re leaky, unless you have one of the cool old “magic eye”
type cap meters!)
– One of the RC sections has a bad ground connection. Note that
this includes the leg whose ground connection is used for
switching the trem on and off! Most old fenders won’t trem
without a pedal or a dummy shorting plug installed in the pedal
connector.
– The neon-lamp/photocell gizmo flakes out. If your ‘scope shows
the circuit is oscillating at the grid of the buffering triode
section and you still have no trem, then this is the culprit. You
should be able to see the neon bulb winking at you if everything
is working correctly. It is not uncommon to find that these
gizmos fail because of a short between the leads of either the
neon bulb or the photoresistor beneath the heat shrink that the
thing is wrapped in; this can usually be corrected by spreading
the offending leads with a dental pick or other suitable
instrument (discharging the power supply caps first, as always, of
course!)
Hope this helps!
–FritzM.
