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Almost newbie needs to troubleshoot electric guitar effect

First, I misinterpreted the original post, especially since this is the basics newsgroup. I read it as meaning you'd found a schematic and built it, which throws in wiring errors and untested parts into the mix.

Since it's actually a commercial unit, and was working but suddenly not, it's actually easier.

But then the chances are good that you didn't connect the wire to the right place. For it to be the right place and yet it still not working seems too odd a coincident.

And this is where I stalled. Any of us could see immediately upon looking at the unit whether it was reconnected in the right place or not, could easily check that voltage is reaching the ICs, and then quickly isolate what part of the circuit isn't working.

Translating that into steps that you can follow becomes harder. The schematic helps, since then we can see what's there. But I'm also concerned that even if you can point to the schematic you found, that there is still a small variable that the schematic may not be correct; either because someone drew the schematic from the circuit board and introduced error(s), or because there were small variations through the production of the unit that may not match your unit.

The schematic that seems to be all over the place shows the input signal going through two stages of op-amps. If the unit gets no power, those op-amps won't work, and you'll hear nothing. Since you hear nothing at all, either one of those stages have gone bad, or they aren't getting power. It's far easier to expect them to not have power, especially since you had a broken wire, than to look for something more complicated.

You get a voltmeter, measure the pin on the op-amp where power goes, and see if it's getting voltage. Easy, except maybe the schematic I found doesn't match your unit and then we have to describe how to find pin 1 so you can count pins properly, and maybe describe how to find that IC.

If there's no voltage there, then that reinforces the notion that you reconnected the wire to the wrong place. Then it requires finding the exact schematic, and then seeing where one should connect the wire.

And then there may be worry that when you reconnected it to the wrong place, it's done damage. ONe hopes not, but that's always there.

Michael

Reply to
Michael Black
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I hope to be able to post the front / back of the PCB tonight:

Here's what I have which I believe is representative - Feel free to take a look:

Stellan's EH Electric Mistress schematic

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Reply to
goldenhound

schematic

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(solderside of EH Electric Mistress 1979)
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(inverted component side of EH Electric Mistress 1979)

I've inverted the component side so that it matches the solder side. IOW, think of the component side as a transparency that's been turned around so that the traces on one side of the board, match the component locations when they're compared. Thought it might help. YMMV. I'm sure I could've done something even better in Photoshop, like overlaying the images and making one transparent, etc. But, you get what you get.

Does this help?

Reply to
goldenhound

1) Try to work with a maximum line length of ~72 characters on Usenet. 2) Rather than linking to the *page*, just link to the *image*.
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(If you can cut & paste the URL into a blank page/tab and it still works, then deep linking is allowed.)

Nothing is obviously weird on the component side. Putting a bit more light on it before snapping it would have been good.

3) How anyone expects to inspect a board when it's covered in rosin, I don't know (though looking at the cheezy technique on the schematic

--as mentioned by Black, I shouldn't be surprised by the poor workmanship).

An acid brush with bristles trimmed to ~3/16" and some alcohol and elbow grease does the trick.

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Reply to
JeffM

Will do.

*image*.
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Good information.

Actually, I reduced the exposure because it was too bright.

This is Electro Harmonix, circa 1979, NYC. All about sweat-shop production. The cheapest, noisiest fx on the market at the time. Very budget, but had some pretty brilliant hippie designers. And you're complaining about a little rosin on the PCB? :o)

trick.http://www.google.com/images?q=Disposable-acid-brush

Thanks for that.

Anyway, I found that pin 5 of the 4558 received guitar signal. But no signal on output of any of the pins.

Checking with VOM set to DCV - I've been using the black wire off the battery to the guitar in jack as the ground and checking all the pins, with the fx turned on (fx is turned on by inserting a guitar cord), and the stomp switch engaged. I get no voltage anywhere. If I probe direct to the battery ground and touch components, everything shows like 17+ volts DC. I know this is a dumb question, what am I doing wrong?

Reply to
goldenhound

check all of the cables inside the box, especially the one for battery negative.

check the switch-action of the guitar jack that it actually works.

because from your desctiption above it seems that atleast one of them are not doing their job.

Bye. Jasen

Reply to
Jasen Betts

goldenhound blockquoted (badly):

*image*.
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If you are going to use Google to post, recognize that, by default, it does a lousy job (long links, whitespace, etc). There IS a trick.

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*-*-4-ways+monospaced-*+To.read.ASCII.prints.from.Google+zzz+Before+%26fwc-*+Show.original+Fixed.text+Notepad+Proportional-*

Silly me. Actually, I was commenting on the prep work by the repair depot in Portland that's handling this job. ;-)

Again, with the crap job they did on the schematic,

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the pinout of the circuit isn't clear.

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I'll ASSuME we're talking about | |8 5 | \\ ----|+ \\ 7 3 | \\ | >---------|+ \\ 1 ----| / | >---- 6 | / ----| / |4 2 | / | Do confirm or correct. How to view ASCII art:

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*-*-4-ways+monospaced-*+To.read.ASCII.prints.from.Google+zzz+Before+%26fwc-*+Show.original+Fixed.text+Notepad+Proportional-*

Ever wonder why they didn't call those things "jills"?

Unless the switch uses more than 2 terminals, it is mostly meaningless.

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Something is "open". That jack has more than 2 elements and it sounds like the juice isn't passing from one to the next.
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The *wb4hfn* item is good. You may also be able to knock off the crud by just inserting a rough piece of paper between the contacts, engaging the contacts, and dragging the paper thru. Well-used paper money is one such material.

A clip lead can temporarily jumper a defective set of contacts.

The word you seek is "reference".

...because there is a complete circuit (of sorts) then.

Reply to
JeffM

If I'm reading this correctly, it sounds like the wire between the battery ground terminal and the board is broken somewhere - likely in the battery connector.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
Reply to
Peter Bennett

Ahem...guilty as charged!

Thanks for the info on viewing ASCII art. Looking at your schematic of the pinout of the 4558, what you've expressed looks right.

Not until you brought it up - but yeah. And the 1/4" phone plug - call it "Jack"? I don't know.

Hmm. I'd love to try that right now, but I'll have wait until I get home. The input jack has a stereo plug that, when a guitar plug is inserted, juice then flows into the circuit. I did check it, but didn't try to clean it.

I'll try that as well. Thanks for all the tips and suggestions, Jeff.

Rob

Reply to
goldenhound

Sounds reasonable. I'm going to check them tonight.

Thanks, Rob

Reply to
goldenhound

During checking last night, (which didn't yield much - the stomp switch actually is working), a wire broke. This is the yellow wire (18v) from the wall wart jack that connects to a pad on the PCB (see the solderside photo link.), which in turn passes thru the 741 opamp, and then passes to an electrolytic 10uF cap.

Once I re-soldered the wire, I was checking the 10uF electrolytic cap that is fed from that yellow wire, and found voltage on one side, but not the other. Unsoldering it and checking continuity I found it open.

I said to myself, "Self, this isn't helping things". Tonight I'll pickup a new 10uF electrolytic and try it out. I won't be able to do it tonight (band practice), but will work on it Wednesday.

Reply to
goldenhound

But, capacitors aren't just used for coupling signals (and blocking DC), they are also used to bypass and filter. When they are bypassing and filtering, one side in the circuit is usually going to ground, so not finding voltage on the other side means nothing.

And capacitors will not show continuity between terminals, at least not after a brief time (the length of time dependent on the value of capacitance, the more there is the more delay).

If this is the 10uF capacitor connected to the arm of the 100k "trim" pot, then all it's doing is keeping noise off that point, and one side is indeed grounded. That capacitor not working or not being there will not cause the unit to fail, at best there will be noise.

The trick to troubleshooting is to find the general area, and then narrow down the fault. Just going through components will take forever, and is open to misinterpretation since in circuit they may appear different from when they are outside the circuit. You don't want to have to desolder and resolder every component finding this problem.

So you look at the collector of that 2N5037 connected to that 741. If there's the right voltage, the schematic says 12volts, then that section cannot be the fault. If there's no voltage there, then something before that point is not right.

If the proper voltage is there, you need to look elsewhere, looking at the unit as multiple stages and seeing where the DC voltage disappears or where the audio signal disappears.

Michael

Reply to
Michael Black

You're thinking too much in black-and-white terms. It's possible to have no DC voltage present on something and have it still significant AC-wise.

...usually called "Ground" or "Return" or "Zero-volt reference".

aka "normal". Look at the symbol for a cap. The plates are seperated by an INSULATOR. When you first hook a (discharged) cap to an (analog) ohmeter, you should see a small "kick". When you then reverse the leads, the needle should move in the other direction

--perhaps pinned against zero for a while. Capacitors should NOT give a *constant* reading.

If you have valid supply voltages on the parts, it's time to start doing an AC test on the circuit. Put a signal in and see how far it gets. An oscope is ideal; an amplifier with input leads and a speaker is good; an ac voltmeter works too.

Reply to
JeffM

Soldered the 10uf ecap back into place. I then hooked up an mp3 player into the input and started checking it with an audio probe. It seemed I was getting audio everywhere.

I hooked my guitar up and found that when the input was not making contact with the box, I'd get no signal. I would also get no signal if I bolted it down to the case. But if I made contact with the case, and held the input jack in my hand, and strummed my guitar, I'd get audio to the amp. No flanging, but audio nonetheless. So, now I get clean (unengaged switch) bypassed signal, and a slightly lower audio with the footswitch engaged? But no flanging?

Reply to
goldenhound

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