Build Your Own Clone Message Board

Hi, I built the Analog Chorus about two months ago and it has been working perfectly via a 9V battery or a 9V power adapter.

Yesterday, someone plugged in a different 9V adapter from Line 6 with the opposite polarity +/- reversed. When the pedal was turned on there was a buzzing sound. I turned off the pedal after only a couple seconds. I disconnected this adapter and tried the pedal with a good 9V battery (and later with my original 9V adapter) the light no longer comes on nor does it make any sound. It's dead!

I recently fixed a Boss digital delay with a similar issue. Boss puts in a diode backwards to protect the main circuit from frying in a case like this. My question is, does this pedal have any kind of circuit protection (diode???) that I can easily replace and get the pedal working again? If not, does anyone have any troubleshooting tips?

Thanks in advance!!!

At the very least you probably took out IC1 & possibly IC3 (if not more). If you have a spare dual op-amp lying around replace IC1 with it. If that doesn't get it up and running again it's very likely another chip or chips are shot now too.

The chorus has a protection diode but even so it's possible to take out a component or three if you plug in a reverse polarity power supply. While D3 helps to protect the circuit it really needs to be placed in series with the incoming 9V rather than shunting to ground. You lose .7V across the diode using this method but IMO it is the better option.

Stephen, thanks for the help. Is there a way I can test the IC's with a multimeter? Also, which diode is the protection diode? I have a diode testing setting on my test meter.

Thanks!

Your best bet would be to use a signal probe. You could try taking AC voltage readings at the output pins of the op-amps with a meter but you'll need a signal source. I don't know how easy that would be with only a guitar as the source. If you had access to a function generator you could use that. With a guitar it's much easier to use a probe which is nothing more than a glorified 1uf lytic cap...

http://buildyourownclone.com/signaltest ... ctions.pdf

Ok, I'm not sure I want to tackle the IC testing so instead I ordered one of each IC and the D3 Diode from Mammoth Electronics. Parts total = $5.78 (and shipping was about the same). Only downside is I have to wait for two weeks for the parts to show up here in Canada.

Thanks for your help!

D3 should be fine. The 1N4001 has a peak reverse voltage rating of 50V. The op-amps on the other hand will fry in an instant which is why D3 probably didn't do anything to protect them.

My only other thought that is if the LED no longer lights, the 9 volt power rail is likely shorted out. If the op amps fried, but the diode held, the LED should still light. First remove all of the op amps from their sockets and see if the LED lights. Then remove D3 and see if it works with a battery. It won't hurt anything - just pull all the IC's from the sockets. If you remove D3 and the LED lights, D3 burned opened. At that point you can reinstall IC1 and see if you have dry signal. If the LED stays dark, try pulling C20 as well.

Morgan, since I have time to kill while I wait for parts to arrive I tried your suggestions. Pulled all ICs and the diode D3. Plugged in 9v battery, guitar cable to IN jack, clicked the foot switch a couple times and the LED stayed off. Also tried a straight jumper across D3's location. Still no light. Did not pull C20 however.

Hm - I don't think the LED should be harmed by backwards voltage, but could be remembering incorrectly.

Here's a tutorial on taking continuity readings: http://www.ladyada.net/learn/multimeter/continuity.html

Here is a tutorial on taking voltage readings: http://www.ladyada.net/learn/multimeter/voltage.html
And here is the pinout of a DIP IC for reference:

Don't put a jumper in the D3 spot, you're just shorting out all power to the pedal. Do take a continuity reading in the spot where D3 was. If you do get continuity, C20 is likely burned open and needs to be replaced. If not, reinstall IC1 and take and report DC voltage readings. We may be able to determine if it's dead or not that way.

Ok, I removed the D3 jumper.

I tested the LED directly with a small 3V calculator batter...the LED works. I just wanted to make sure the LED wasn't burned out.

I tested for continuity across D3 and it was an open loop so C20 should be fine.

Re-installed IC1 dual op amp, plugged in 9V battery (measured directly gave me 8.18 volts), plugged in guitar cord to INPUT and tested IC1 pin voltages (with black lead on the ground location beside R38). Pin test results were:

1 - 5.25 Volts
2 - 5.26
3 - 2.63
4 - 0.00
5 - 5.09
6 - 5.12
7 - 4.99
8 - 8.16

Can you explain a little how you performed this test? Did you remove the LED from the circuit, or test it in circuit?

If the LED is functioning but doesn't light when you click the pedal on, you almost certainly have a shorted component in the voltage supply circuit somewhere. Keep all of the IC's out of the sockets until we get the LED lighting up. And check voltage on both sides of R38.


Theses voltages actually read a little wonky, but should be good enough for it to work. Leave the chip out of the socket until we chase down why the LED isn't working.

R38 is 8.20V on the top side and 8.16V on bottom side (assuming the board is orientated like the photo in this thread where the LED is at the top and the D3 diode is bottom left)

The LED test was done while it was still soldered into the board. Just used two small alligator clip jumper wires on the LED leads then touched + and - of the CR2032 3V battery.

Measure continuity to ground from the negative side of the LED. You should have continuity in effect mode, and no continuity in bypass. It's very strange that it works and is getting power, but is not lighting up...

Yes, just like you said, I have continuity with one click of the footswitch and an open loop with another click.

Is there a troubleshooting guide for this chorus pedal? I just finished building the Ping/Pong Echo Delay and the end of that PDF manual has a bunch of test points to check voltage readings etc. for troubleshooting. This might help narrow down where a fried component is hiding.

Nope - no troubleshooting guide.

Try continuity to ground on both sides of R3 R36, and R37, and across R37. Should all be open.

Ok, all were open loops EXCEPT:
R36 bottom side was 0.935
Across R37 was 0.046
Note: on my little RadioShack meter I have to use the diode mode to do a continuity test. An open loop reads 0L otherwise it's usually -0.000

Not sure what's going on with the LED. Let's wait for the new chips to chow up.


BTW - I have an old radio shack meter. On mine, if you push the diode mode button twice it cycles over to continuity and gives you the beep.

Got Back from summer vacation and installed the new diode and all the chips. LED still not working nor the pedal itself. Must be some other component fried. I am just not sure how to test each cap and resistor???

Resistors should be fine, but you might have to worry about the electrolytic capacitors. I've had moderate luck testing those for continuity (ie, had a few that beeped as open, so I new they were dead).

Where in Canada are you? I'm in Toronto!

I tested the alum. electrolytic caps for continuity. I tested with the meter leads in both directions (+ to +, - to minus then + to - , - to +). Here are my cap test results...not sure what it means however:

C15 Open Loop in both directions
C16 Open Loop in both directions
C21 Open Loop in both directions

C22 Open Loop with test leads matched up with cap (meters red positive lead on cap positive)
C22 .828 with test leads reverse of cap (meters red positive lead on cap negative)

C20 Open Loop with test leads matched up with cap (meters red positive lead on cap positive)
C20 .850 and value seemed to keep climbing as I left the leads on the cap...with test leads reverse of cap (meters red positive lead on cap negative)

I am located in Edmonton.

Stephen is right about the diode protection across the supply lines being ineffective. I think the idea is that it is supposed to protect against accidental and momentary reverse connection of only a battery where the dioe acts as a short to ground. When an external supply is connected with a relatively high current capacity the diode still acts as a short but is likely to get very hot and burn out open circuit - then the external supply fries the rest of the active parts.

As Stephen says the series diode provides a more certain protection.

LEDs will sustain for a short period when reverse biased, but will eventually go open circuit if more than a few volts are applied. Which explains why you can get away with using them as back to back clipping diodes - the reverse voltage isn't high enough to be a problem.

Testing resistors in circuit often produces misleading results because the surrounding resistances in the circuit influence the reading obtained. Also the voltage from the meter itself may damage active components.

Testing caps for continuity with meter will only tell you if they have gone dead short and again, trying to test them in circuit will often give misleading results. Connected with the right polarity electrolytic caps will appear briefly as a short and then as rising resistance as the cap charges up to the voltage across the meter test leads. Small values will charge quickly while caps of several 100uF and above will show a slow rise. Connecting electrolytics in reverse polarity is not a good thing to do because they don't tolerate reverse voltage well and will eventually fail.

Most of the time the only effective thing you can do with a multimeter and a completed circuit is use it to measure operating voltages.

As far as your pedal is concerned - the simplest active path through the pedal is the clean path through IC1a and IC5b (this is a numbering mistake IC5b is really IC1b). Do you get any signal through the pedal at all ? If not, even the clean signal path is not working and you can concentrate on getting that to work. Note that the op-amp is biased from VR3 25K, so the pin voltages particularly on 3, 1, 5 and 7 should move up and down as the trimmer is adjusted. If these voltages do track the trimmer movement it is a reasonable indication that those two op-amps are OK. Try turning the trimmer down to about 1/3rd and measure the voltage on its wiper. The same voltage should appear on pins 3 and 5 of the op-amp, in fact is should appear on all the points on the circuit diagram marked 1/2V.

Watch out for pin differences between the circuit diagram and the physical PCB. The BYOC PCB layouts do not always follow the pin numbers shown on the circuit diagram, the halves of dual op-amps may be swapped around.

I would take Tark's advice more than this, but anytime I've had an electrolytic that measured open consistently it was bad and needed replaced.

The point I'm trying to make is that for DC any capacitor is ideally an open circuit.

Connected the right way around and measured out of circuit (usually switching to the resistance ranges on a multimeter puts a positive voltage on the red lead) a medium to high value electrolytic (that is already discharged) will momentarily look like a short and then you will see the resistance value rise until eventually, if the electrolytic is in good condition, it may rise to as high a resistance value as the meter can measure.

Non-electrolytic (which are normally not used in bigger values than 1uF) and small value electrolytic (measured out of circuit) should look like open circuits right away.

Any cap that remains a dead short or a low resistance has failed. A cap that settles at anywhere from tens of K to 100K may be leaky (as in leaking current - effectively having an internal high value resistor in parallel with the capacitance).

Tantalum caps in particular do not like reverse voltages, don't behave well in audio signal applications and tend to fail as dead shorts.

Ok, I plugged the pedal into my amp (without any ICs installed but 9v battery plugged in). I do in fact get a nice clean signal with the pedal effect off. I installed all ICs again and tried the pedal into the amp. Again clean signal is fine however LED does not light when effect is turned on and the audio signal is completely silenced...not even a hum.

I did the voltage tests on the op amp pins 1, 3, and 5 as Tark suggested. The voltages do change as expected when the trimmer is changed.

So I can confirm the dry signal path is working fine. What do I test now?

Hold on a second lets just examine what you have done so far.

With the pedal foot switch switched to bypass mode the switch connects the tip contacts of the two jacks together. This is a very simple circuit, you can have your input jack lead and output jack lead plugged in to the pedal and it does not matter which sockets you use for either lead - the signal will pass through (providing the jack sockets, wiring and switch are OK of course). No active electronics is involved.

The LED circuit is also very simple and is largely isolated from the rest of the pedal, all you have is one third of the foot switch, a resistor, a LED and the power source (battery). There is very little there to go wrong and the most likely component to have blown would be the LED.

Getting to the pedal switched to active, with power applied, you have the input signal coming in through the input jack, going through the input amp (1/2 of the op-amp) splitting, with one path (the clean path) going to the output amp and then out of the pedal. The other path (the delayed path) goes through the modulated delay chips and is mixed with the clean signal at the output amp. The combination of the clean and modulated delay signals produces the chorus effect.

If the input amp and the output amp are working, even if the delay path is broken, you should hear the clean signal. So with the dual op-amp plugged in, in either state of the foot switch the signal should not disappear. If it does disappear then the clean path is not working and that limits the problem to the two op-amps and a handful of components.