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


<Kotti75events@xxxxxxxxxxxxxx> wrote in message
news:66ca1bd9-7b59-4db3-98a5-652fe475060d@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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?


That meter should be perfectly adequate for this application. Does it not
have a continuity check that buzzes or beeps? If not, use the Ohms range to
determine a short or near short circuit.

No it doesn't necessarily mean that C9 is bad, because there are many things
connected in parallel with C9, any one of which could be bad. Well actually
there is only D4 and the three IC's that are directly connected across C9,
so if you are reading a short or near short circuit, it should be easy to
establish which of these 5 components is the culprit.



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?


No, only D1 is not in circuit with the battery, everything else is.



Note that for it to work with the battery, you must insert a mono jack plug
into the input jack - this switches the battery in circuit so it doesn't run
down when the pedal is not in use. (like an automatic on/off switch) Do
not connect the battery until you are sure you have removed the short
circuit or you will drain the battery very quickly.

In fact, you had better measure the battery voltage with your meter, in case
your only problem is a flat battery.




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 ***:
http://zeta.octopart.com/Mitsubishi__M5218AL__0.pdf
is it possible to test whether it works or not?


No. These are standard Op-amp chips - the data*** says 4558 is identical,
and these types are very cheap and very available.



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.


Just replace them with 4558 chips.



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?


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.


.

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