Newbie trying to fix a Johnson FT-2 Tremolo Pedal (for guitar)

[[email protected]]

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

 

[[email protected]]

PS: it was the D1 diode that I replaced. It was broken (had burst) and
there's also a burn mark on the perf board. A near by wire was also
singed but I replaced that. I don't have many tools and am new at this
sort of thing, so any help is greatly welcomed!

 

[Martin Whybrow]

PS: it was the D1 diode that I replaced. It was broken (had burst) and
there's also a burn mark on the perf board. A near by wire was also
singed but I replaced that. I don't have many tools and am new at this
sort of thing, so any help is greatly welcomed!

D1 is in the line for the external power supply, if it burned out then
there's a high probability that the wrong power supply was connected to it;
unless you really want the challenge of fixing it, I would recommend your
friend buy a new pedal as many components are liable to have been damaged.
Sorry to be so pessimistic!
Martin

 

[Arfa Daily]

stairs.



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.

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

 

[bz]

I would be immediately reaching for a current limited
power supply, my multimeter, a signal generator, and my 'scope probe .

I have heard about being 'four armed' but it must be hard to find shirts!




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[Arfa Daily]

I have heard about being 'four armed' but it must be hard to find shirts!

Then you sir, must not be self-employed !! It is completely necessary for
any self-employed person to have at least four arms, and an iron
constitution so as to never get ill. You must also have perfect eyesight for
looking at manufacturer's poor schematic prints and pdf's, and a degree in
accountancy and book-keeping. A degree in sociology is also helpful for
dealing with customers, and a modicum of deafness is desirable so that
endless manufacturer's phone-hold music, does not wear you down too much ...
d;~}

Arfa

 

[bz]

Then you sir, must not be self-employed !!

Been there. Done that. Couldn't afford it as my soul[sic] means of support.

Now, I have a full time 'job', work part time in another industry, help my
wife do the same, help her with an on line store AND co-author books with
her in yet another field.

It is completely necessary
for any self-employed person to have at least four arms, and an iron
constitution so as to never get ill.

four-ti-tude!, and temper-ance

You must also have perfect eyesight
for looking at manufacturer's poor schematic prints and pdf's

Used to have fun fixing shipboard radio/radar equipment. Manual in
Japanese, crew from Norway.
Then there were the Greek ships with the boxes of 'used spare' tubes marked
65%, 75% .... etc.

, and a
degree in accountancy and book-keeping. A degree in sociology is also
helpful for dealing with customers, and a modicum of deafness is
desirable so that endless manufacturer's phone-hold music, does not wear
you down too much ... d;~}

And then there are the warranty claims.... getting billed for the parts by
the company and then getting the claim denied.

Oh, and don't forget the degree in law. You need it for small claims court
on the 'stop payment' checks.

Then there is licensing.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap

 

[[email protected]]

So, I was wrong when I said that D1 burnt out. This pedal compared to
the schematic, D1 doesn't exist and it was D4 that burned out. And D4
appeared to be a 1N4007 diode. I'm not sure how big if a difference
that makes for what was said. I did clean the contacts and nothing
changed. And here's where my ignorance will really start to show. My
VOM meter is as cheap as can be (Voltcraft VC-120,
http://www.ciao.de/Voltcraft_Digital_Multimeter_VC_120__2253015 Sorry
I can't find an english page on it). And I know you probably aren't
supposed to do this but I've been using the "diode" mode on it to
check for continuity within a circuit. It reads "1" if there's none
and some other random numbers when it appears there is continuity. So
to take this a step further, (and I'm guessing I could probably ruin
my VOM meter this way?) I check for continuity in this way by touching
the leads on C9 and yes there's continuity. I'm guessing that that
means it's bad?

Another thing is that I don't have a working 9v adapter so I've been
using a new 9v battery to test this. So if that portion of the circuit
with D4 and C9 is only engaged when the power supply is connected,
then C9 shouldn't be the problem if I'm using a battery. Is that
correct?

So that brings me to the next possibly problem, the IC chips.
Is there a way to test IC chips to see if they're broken or not, or do
they just have to be replaced? If I remove one, using the info on the
data sheet:
http://zeta.octopart.com/Mitsubishi__M5218AL__0.pdf
is it possible to test whether it works or not?
There's also a second # on the IC chips under "M5218AL" which on IC2
and 3 is "747007" and on IC4 "750005". Are these #'s significant when
replacing the chips? I also noticed other manufactures make this chip
as well, or at least chips with the same name. Does it matter which
manufacturer I should get them from (are they that different)? I would
be nice to know how many I have to replace and buy them (order them)
all at once.

I really appreciate everyone's help and patience to help me since I'm
so new at this. I'm interested in learning electronics more and this
place seems really helpful.

One more question though, regarding my non working 9v AC power
adapters. They do "work" except that the output voltage is between
12-15V on each, not 9v. And I realize that this should drop when a
load is "attached" but when plugging it in to my guitar pedals, none
of the AC power adapters work with any of my guitar pedals. It is
possibly that all of them never worked (the wallwarts that is, I
bought them all at flea markets. Do they not work because the amps
that the pedals draws is low, forcing the voltage to be too high (the
wall warts support up to 500-600mA each)? It seems fishy that all of
them don't work. I've tried other circuits in my house too, no change.
And here's maybe a bad idea but can you put a secondary "load" on the
PSU to get the voltage to drop to 9V?

Thank you thank you thank you!!!!

Olivia

 

[Arfa Daily]

AC power adaptors are misnamed because they are mostly DC power adaptors,
i.e. they output DC. This means that the centre pin can be either
negative or positive, there is no fixed convention. Which means that if
you plug a positive pin power supply into a pedal which is meant to have a
negative pin power supply, you run the risk of burning out a diode and
possibly one of the 3 IC's and having to post on usenet for help in trying
to fix it.
Do not just go around plugging every wall wart you have into every piece
of equipment you have as you will very soon have several dead pieces of
equipment, dead wall warts, or both. Which is quite possibly what you have
right now. Especially if the wall wart output is a higher voltage than
the equipment requires.

Some wall warts really are AC adaptors in that they output AC. Plug this
into all of your pieces of equipment you have lying around and you have
double the chance of burning out your equipment or wall wart, as they
output both positive and negative voltages at the same time.



Blimey.

I'd say this was a good possibility. For the last 8 or 10 years, the nearest
thing to a 'convention' that equipment with coaxial DC connectors has had,
is that the centre pin is "+", so that's the way that the wall warts are
configured. However, that pedal that you are repairing, defies this
'convention' in having a negative centre pin, and this is actually quite
common on pedals. So, if you plug in a common 'modern' wall wart, that's
*likely* - although not guaranteed - to have a positive centre connection,
then the result will be at best that the pedal just doesn't work (D1 being
in circuit for an external power supply input will 'block' and allow no
current to pass), and at worst, will damage a shunt protection diode if one
is fitted without a series diode, as you seem to be suggesting is actually
the case, contrary to what the schematic says. Worse, if there is no
protection of any type, then the result will be destroyed IC's in the
twinkle of an eye ...

Note all of what Gareth has said. It's all good stuff. Should you have any
trouble laying hands on 4558 opamps, you can also use bi-fet low noise types
such as TL072 or TL082 in most all dual opamp applications in this type of
equipment.

Arfa

 

[James Sweet]

They're not misnamed, they're called AC adapters because they adapt the AC
mains to whatever the equipment being powered requires, whether that be AC
or DC.

 

[Gareth Magennis]

They're not misnamed, they're called AC adapters because they adapt the AC
mains to whatever the equipment being powered requires, whether that be AC
or DC.

Well it's just a matter of opinion really. It doesn't make a lot of sense
to call them AC adaptors if there are no such adaptors running on DC, the AC
term is totally superfluous. It's like calling bicycles "Pedal Bicycles".

Anyway, in this context I was trying to make the point that it would be
better to think of Wall Warts as providing DC of either polarity, and to be
aware that occasionally you do get a Wall Wart that produces AC. I think
my terminology works a lot better.



Gareth.

 

[[email protected]]

Note all of what Gareth has said. It's all good stuff. Should you have any
trouble laying hands on 4558 opamps, you can also use bi-fet low noise types
such as TL072 or TL082 in most all dual opamp applications in this type of
equipment.

Ok, I did replace the IC chips with TL082 chips and now the clean
signal passes just fine but the tremolo signal is distorted and not
vibrating like it should. Also when the tremolo is engaged, the volume
jumps considerably. I'm guessing it's not just the IC chips that were
ruined but other things as well? Any guesses for how to figure out
what they might be?

Thank you for all your help!

Olivia

 

[jakdedert]

Well it's just a matter of opinion really. It doesn't make a lot of sense
to call them AC adaptors if there are no such adaptors running on DC, the AC
term is totally superfluous. It's like calling bicycles "Pedal Bicycles".

Nothing much about wall warts makes sense. As I've ranted here several
times, there should be some kind of standard set for these devices.
With the myriad of connectors, voltages, polarities (or lack thereof for
AC devices), it's way too easy to fry either the equipment, or the wart
by simple inattention or ignorance. Likewise, it's easy to spend good
money on an unneeded replacement supply when the uniformed public has a
perfectly serviceable replacement gathering dust in a drawer...because
of the label which says 'USE ONLY A [BRAND X] REPLACEMENT SUPPLY!'.

If the output voltage was of lethal potential, there would be
regulation; but since we're dealing with low-voltage supplies, the
various manufacturers are free to supply whatever most enhances their
bottom-line, without regard to the consumers' best interest.

It's of little concern, and indeed economically advantageous, for those
companies to deliberately confuse the consumer with supplies which will
cause damage if plugged into the wrong device. If the supply itself
fails out of warranty, the company makes money by 'requiring' a
proprietary plug architecture, or odd voltage. Unregulated supplies,
while outputting the correct voltage into a device which draws a
specific amount of current, can damage a device which requires the same
rated voltage at a different--less, or even more--current.

Then there are the millions of perfectly good warts ('good' being
somewhat an oxymoron) that end up in landfills, because the device they
were meant to power died before the wart did. I have at least a hundred
such in a box, that I mix and match with new plugs when the need arises;
but I'll never use them all....

Anyway, in this context I was trying to make the point that it would be
better to think of Wall Warts as providing DC of either polarity, and to be
aware that occasionally you do get a Wall Wart that produces AC. I think
my terminology works a lot better.

It would be better if there were about (maybe) ten different
configurations, each with its own standard plug, which would fit the
majority of devices.

[Rant Mode: Off]

There is a little light at the end of the tunnel for those with skills.
Most cell phones and other small devices these days require 5VDC at
around half an amp or less. Likewise most of the (switchmode) supplies
are regulated, so current is not such an issue. I've replaced many,
many warts with ones from obsolete gear, simply by popping the case and
swapping out the proprietary cord from the defective wart. It's a lot
easier than trying to troubleshoot the sm supply; and thrift stores
carry a plethora of replacement warts from phones that people have
discarded...for a buck or two.

jak

 

[[email protected]]

No new thoughts? Anyone? Pleeeease?

Olivia

Ok, I did replace the IC chips with TL082 chips and now the clean
signal passes just fine but the tremolo signal is distorted and not
vibrating like it should. Also when the tremolo is engaged, the volume
jumps considerably. I'm guessing it's not just the IC chips that were
ruined but other things as well? Any guesses for how to figure out
what they might be?

Thank you for all your help!

Olivia