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Circuit Troubleshooting?


donbenjy
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hey, I've had a reaally slowly moving Fuzz project on the go since the summer, and a break in my regular fuzz kickstarted me into mounting my circuit into a cake tin the morning before a gig :D anyway it works about 80% of how it should (when it was on the protoboard) and I've never had a problem like this before (usually it either works perfectly, or it doesn't, n i just have to find a short or loose connection) but I have no idea how to check this is running well!

here's the circuit:

opampfuzz.gif

Basically, it's soldered to a stripboard using a painstakingly drawn up spreadsheet, along with some help from Stripboard Magic. Anyway, it fuzzes a little bit: about where you'd expect a normal fuzz to work if it was set at 9 o'clock. The octup knob does do the octave effect, but it sounds dodgey with the rest of the circuit right now...On the protoboard, the feedback knob would start feeding back at about 12-1 o'clock, but now it only works maxed out, with the tone/gain up to just below max...and this is a single, very high pitched, uncontrollable squeal. Turning the pedal on results in a drop in volume even with the volume at maximum. Oh, the 2 clipping LEDs don't light either, whereas they would do at higher gain settings before.

What would an overheated opamp sound like? I didn't have any IC mounts so I really quickly soldered direct to the board - not the best connection, but it seemed ok! How can I test it to see whether it works or not? (resistance, expected voltage readings etc?)

So bascially, any suggestions as to what the exact problem might be would be really welcome, but I'm expecting just a few pointers for how to test my circuit works well using a multimeter? and how to check if the diodes, caps, opamp are in working order?

I can post my soldering for the stripboard too, if anybody would be kind enough to check it over?

would the type of pot(i.e. log/lin) affect this? I doubt it, as with all the pots maxed, there would be neglible resistance along the signal path?

cheers for any help! I'd relly appreciate any info/advice as I've never had to troubleshoot an audio circuit, and I'm not even sure which components do what!

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Cool thanks! Only problem is, Stripboard Magic is abit crap at showing the actual layout (i.e. all flyleads appear as generic grey dots!) so I've been using an excel spreadsheet to lay it out. The sheet is too long to take a screenshot so I've uploaded both an excel version (far easier to follow) or a CSV version (viewable without excel, but very hard to read)

excel

CSV

I know it's not the easiest to follow, I'll have a poke around later to see about getting a Jpeg sorted! There are far to many flyleads coming off the board to take a good quality picture, but I'll try that later too.

Oh and the highlighting on the excel spreadsheet:

pink is just the ground rails to remind me

yellow are connections that I made off the board directly to each other

green are components that I havn't soldered: they were all for an on/off LED, and I haven't got a 3PDT handy to test that. The two 1K resistors in parellel dropping from the 9v rail are to supply the LED with a decent voltage.

1.Name of the project: T.M.K (too many knobs)

2.Links to the source: Schematic

3.Followed schematic exactly by copying into stripboard magic and then using the automated layout to draw a digram for stripboard.

4. 500k pot is 470k, 250k are 220k, 300k is 220k, 50k is 47k. The 10k is 10k. I used the following types for each pot (numbered on the schematic)

  1. p1 -can't remember, it's coded as B470k
  2. p2 - can't remember, "B220k"
  3. p3 - can't remember "B47K"
  4. p4 - "log B"
  5. p5 - "log B"
  6. p6 - can't remember "B10k"

5. I guess it's a negative ground...the black 0v lead from the battery goes to signal ground.

6.ok, I've had a more extensive play and, tbh, I'm slightly unsure if it's broken or not, due to it's randomness and the craziness the pedal already has! :S The things that are concerning is the feedback/oscillations- I seem to remember that it used to be much more "friendly" to control and broke up easier than now, where it seems that there is a threshold somewhere when feedback is full, input Z is high, and gain is just short of full...what I get is a really high pitched squeal, that doesn't build or change pitch nicely...sounds kinda broken :D which I['m sure it didn't before...also, the amount of fuzz seems quite small (i.e. still on the point of really breaking up, and I'm sure that the sound suited Muse's Plug In Baby really well, whereas now it's too clean still!) It seems like the upper end of the fuzz - the high gain stuff, doesn't seem to appear! But with 6 variables, it's hard to find every sound anyway!

1.What does it do, not do, and sound like? = fuzzes, a little - not too much. Oscillates badly.

2.Name of the circuit = TMK (Too Many Knobs)

3.Source of the circuit (URL of schematic or project) = Schematic

4.Any modifications to the circuit? Y

5.Any parts substitutions? yes, different pot values, as above

6.Positive ground to negative ground conversion? N

Cheers for this!

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I can't really help with the troubleshooting side of things...

But I thought I could suggest an alternative to stripboard magic and a better way to share layouts. Member bancika has developed a program called DIY Layout Creator with a lot of input from the guys at Aron's and there is a library of submited and verified layouts for a number of projects (but unfortunately not this one yet). It is extremely easy to use and allows very easy transfer to a JPEG for posting on sites like this through photobucket or the like.

I have used stripboard magic but found it to be buggy and the auto layouts not always the best. While this will not work directly from a schematic nor automatically as in SBmagic, it may prove of use so I can recommend it...plus it is free!

Anyway, ad over, hope it may be of help in the future and your fuzz works out in the end... pete

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This might help you to verify your offboard wiring. I drew this up when I made my own T.M.K. -

http://www.diyguitarist.com/PDF_Files/tmk-wiring.pdf

cool thanks, we have P4 wired differently, but mine has both the wiper and left lug running to pin 1 of the opap (as on the schematic), whereas you only have the left. I assume this would just reverse the direction that the control works though? it shouldn't cause the circuit to go wrong! Otherwise, everything is fine! thanks for that though. Just out of interest, how does the feedback oscillation on the pedal sound? is it pretty responsive and usable or just a high pitched tone?

Try disconnecting the wiper (middle) lug of P1 and tying that wire to ground, and see if the circuit works correctly that way ( I'll bet it doesn't, but what it does may point us toward the problem).

ok thanks, I'll try that once I've got a new battery, as I want to make sure it's not anything as stupid as that! :D

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Sorry, I forgot to link you to the actual PCB layout that I used (with components shown)... :D

http://www.geocities.com/diygescorp/tmk1.3layout.gif

Just out of interest, how does the feedback oscillation on the pedal sound? is it pretty responsive and usable or just a high pitched tone?

It's sort of like what you can do with the Z. Vex Fuzz Factory.

Edited by Paul Marossy
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lol, just me then! :D hey what settings give you a good, saturated fuzz? It'd be useful to know what is meant to sound good than try an second guess my own circuit when there's something wrong with it! That way I can leave the pots all set up and test it without having to fiddle for 20 minutes!

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ok, so I've taken voltage readings across all the pins of the IC and the diodes (wasn't sure about the LEDS, but they measured the same voltage across both pins):

ok:

Voltage across battery: 9.57

Voltage at circuitboard end of red fly lead-9.54

Voltage at end of black flylead-0.01

IC1

  1. 0.76
  2. 0.76
  3. 0.71
  4. 0.00
  5. 4.75
  6. 4.80
  7. 5.01
  8. 9.55

note: I numbered the diodes in order from left to right, and the 3 last ones go from top to bottom

D1

A-5.0

K-4.8

D2

A-0.00

K-0.00

D3

A-0.00

K-0.01 (?)

D4

A-0.01

K-0.01

D5

A-0.01

K-0.00

Now, as I mentioned before, I don't know too much about audio circuits and the effects of certain components on the sound, but I would assume that the diodes should have a greater voltage across them? I get that the "backwards" one that goes from ground up to the circuit would have a high threshold before the voltage can flow the "wrong way" but shouldn't the other 3 be showing something?

Or will I need to set the pots at anything specific? E.g. P6 might need to be one way or the other to get the voltage from the opamp?

Oh btw, just whilst I've been messing with the circuit, I can't work out what exatly P4-crossover distortion is meant to do...it looks like when I turn it to full, the output of the first opamp runs through the paired LEDs and when it's dialled back, hardly anything passes through them. So which way is crossover distortion "on" and which is "off" if that makes sense? Also it seems that when it's turned fully one way, there doesn't seem to be alot of difference between the circuit being on or off...not sure which way this is though... :D

thanks again to anyone that's helped out!

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IC1

  1. 0.76
  2. 0.76
  3. 0.71
  4. 0.00
  5. 4.75
  6. 4.80
  7. 5.01
  8. 9.55

There's part of your problem - the left opamp (pins 1, 2 & 3) isn't biased properly. Something is miswired - those three pins should be between 4 and 5 volts (like pins 5, 6, & 7). Make sure you havent wired something to ground (0V) that should be connected to Vr (4.5V). Did you ever get that spreadsheet converted to a JPEG? A look at your layout would sure be helpful!

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*kicks self*

...guess what was wrong? I'll give you 3 goes.

Yep, a short! it was across pins 1 and 2 of the opamp! Removed it, noticed the pulsing LEDs so I plugged it in and pure fuzzy, oscillating bliss! :D Very happy! :D

Oh, one last thing- the gain control works oppositely to the way i'd expect...Did i just wire it wrong, or does the wiring need to be reversed? i.e. if the wiper was fully left in the schematic, is that full gain?

oh and if someone wouldn't mind explaining what the "crossover distortion" does? Does full to the left mean that the LEDS are fully on, and the clipping creates the crossover distortion?

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oh and if someone wouldn't mind explaining what the "crossover distortion" does?

That's kind of hard to explain without showing a graphic, but it basically an offset that occurs where the sine wave crosses 0 degrees where there normally wouldn't be one. It causes an ugly sounding distortion kind of like a badly misbiased tube amp will do.

Glad you got it working. Pretty fun circuit, huh? :D

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