Re: Newbie trying to fix a Johnson FT-2 Tremolo Pedal (for guitar)
- From: "Arfa Daily" <arfa.daily@xxxxxxxxxxxx>
- Date: Wed, 30 Apr 2008 08:37:42 GMT
"DaveM" <masondg4499@xxxxxxxxxxxxx> wrote in message
news:1a6dnYDCq4K1WYrVnZ2dnUVZ_gadnZ2d@xxxxxxxxxxxxxx
stairs.
<Kotti75events@xxxxxxxxxxxxxx> wrote in message
news:2e4e0ced-4a24-453e-84d3-16e9e14b3142@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I'm trying to repair this pedal for a friend. There appeared to be
only one burnt out component and I replaced it but it still doesn't
work. It did look like something was spilled inside the pedal and I've
cleaned that out. When in bypass, the signal passing through is
distorted and garbled, like hearing it on a faint radio station. But
when the pedal is engaged, it doesn't pass signal at all. I'm pretty
certain that this is a copy of the Boss TR-2 because the schematic
seems to fit with the exception of the IC chips.
http://www.schematicheaven.com/effects/boss_tr2_tremolo.pdf
Since the "bypass/engage mode" is triggered by a momentary switch, and
the dry signal goes down a very processed chain of components, I
haven't been able to figure it out. What typically goes bad when there
are spills? From looking at this schematic, how would I go about
figuring out where in the "dry signal" chain things are wrong?
Thank you very much!
Olivia
D1 is in the line from the external power adapter. It's only in the
circuit if the external adapter is in use. Since the 1N4004 has a PRV
rating of 400V, it's unlikely that its PRV was exceeded. More likely is
that something downline from D1 is shorted. Look at D4 and C9. If either
of those are shorted, then the external power adapter's output is shorted
to ground, through D1, thereby toasting its innards.
Since the +9V supply powers IC2, IC3 and IC4, it's possible that one or
more of those ICs might have failed, causing the short across the power
supply.
Since you mentioned that something was spilled into the box, I'd look
closely at the switches and jacks. Even though you cleaned it out, you
might not have cleaned the gunk off the contacts of the switches and
jacks. Go back to those parts and really inspect the contacts carefully
with a magnifier and make sure they are operating room clean. Make sure
that the contact leafs in the jack(s) and switch(es) are moving freely and
aren't sticking when a plug is inserted.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the address)
Interestingly, Dave, it would appear to be a negative tip power supply that
it needs. Notice that the blown series protection diode, is bypassed when
the unit is on battery power only - presumably to avoid the forward drop of
that diode. So if anyone had tried to connect a power supply to the battery
connector instead - maybe one of those reversible universals with a PP3
connector on it, then protection would be via the shunt diode D4, which
would of course fail short circuit if enough current was available, and
which would then blow D1 next time power was applied correctly to the power
socket. (Reminds me a bit of that thread a few weeks ago about positive and
negative tips and shunt protection diodes ... ;-) )
I agree with you that the very first thing would be to check the contacts,
particularly the switch contacts on the input socket, as the 'ring' contact
is used to switch the battery / supply power, when a standard mono jack is
inserted into it. Beyond that, it might be getting a bit tough for a newbie,
depending on what experience level and test equipment is possessed. If this
landed on my bench, I would be immediately reaching for a current limited
power supply, my multimeter, a signal generator, and my 'scope probe ...
Arfa
.
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