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.
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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?
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.
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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?
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
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.
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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 ...
My diagnosis too. Its also possible that one of the IC's shorted out the supply for a while before burning itself into an open circuit, so the power supply may no longer be shorted, and replacing the diode has fixed it. The only solution then is systematic replacement of the IC's. (Note that a distorted signal is passed, probably wouldn't get this with no power supply at all)
Use IC sockets when changing IC's, a good desoldered IC can be re-used easily or moved around, and there is less overall heat damage risk to PCB and IC's.
Its also possible that a shorted IC has burnt the power supply trace to it into an open circuit instead - you often are able to see some discolouration of the track concerned.
God advice, but it is also possible that this spillage happened a while ago and has nothing to do with the current fault. It's easy to be misled by things like this.
"Arfa Daily" wrote in news:qdWRj.17931$244.4187 @newsfe1-win.ntli.net:
I have heard about being 'four armed' but it must be hard to find shirts! :)
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bz 73 de N5BZ k
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Then you sir, must not be self-employed !! It is completely necessary for any self-employed person to have at least four arms, and an iron constitution so as to never get ill. You must also have perfect eyesight for looking at manufacturer's poor schematic prints and pdf's, and a degree in accountancy and book-keeping. A degree in sociology is also helpful for dealing with customers, and a modicum of deafness is desirable so that endless manufacturer's phone-hold music, does not wear you down too much ... d;~}
"Arfa Daily" wrote in news:jR7Sj.61$ snipped-for-privacy@newsfe1-win.ntli.net:
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.
four-ti-tude!, and temper-ance
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 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.
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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,
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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:
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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?
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.
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.
No. These are standard Op-amp chips - the datasheet says 4558 is identical, and these types are very cheap and very available.
Just replace them with 4558 chips.
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.
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.
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.
AC adaptors "adapt" battery powered equipment to AC mains. If they output AC, then they are not AC adapters but rather are transformers. If the equipment can't run off batteries, then the device is best called a DC power supply. Wall wort is a useful slang term to cover all of them.
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?
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....
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.
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