HELP:big muff resistor

[mongo]

My Vintage big muff died  last night and I discovered that a resistor fried.it is connected to the ON/OFF  switch of the unit.











My Muff is an old one with IC's in it so I dont think some of the schems I find here are the same.


 I can't read the fried resistor because it turned BLACK!!  

so If ANYONE has the same unit or can tell me what kind of ressitor is I'll be forever thankfull.

Andy

[mrsage]

Shoot.

I just sold mine.

I checked my old pictures, and I didn't take any of the top of the board.

Sorry man.

[mrsage]

I have a Euthymia ICBM fuzz, which is a clone of the Op Amp muff. It should have similar resistors.

I'm seeing an 820k resistor connected to pin 7 on that IC.

[nelson]

I have the schematic of the Big muff op amp version, email me and I will send you it
. my email is hempathy_@hotmail.com

[sir_modulus]

Nish

[analogguru]

Here you can find a better schematic:

http://forum.musikding.de/attachements/EH_BigMuffPi-77op.gif

And here is your picture with the according component numbers:

http://forum.musikding.de/attachements/BigMuffPiOP-77_components_c2.jpg

The resistor is R17 and will be something between 22 ohm and 100 ohm.
My Estimation is 47 ohm.

It would be nice if you could compare the component values and tell me eventually the values you have on your board.

Especially all capacitors and electrolytics, R12 and R15.

regards,

analogguru

[Ge_Whiz]

Sorry to say it, but a more important question is what caused that resistor to fry - proceed with extreme caution.  

[mongo]

thank you very much for your help everybody!


yeah that "why did that resistor fried" question worries me.

 thanks for the schems and I'll let you know  i everything goes back to normal.

[NaBo]

Well, if nothing else fried (quite possible), and you haven't messed with power supplies or made any modifications, then I suspect it must have been a short to ground somewhere shortly after the resistor.

I'm no expert, but here's my theory.  Someone more knowledgeable can and should correct me if I'm at all wrong - I gotta learn this stuff good, I'm starting an Electronics Technician distance education course soon!!! :wink:

Looking directly after the resistor in the schem, we see C13, a 220uf electrolytic.  You say its a vintage pedal?  How vintage?  Electro's can start to behave "erratically" after 10 years or so, and I wouldn't be surprised if that was your real culprit.  Hard to tell from your pic, but check C13 for signs of any "bulging".  Unfortunately (I mean... fortunately...) electrolytics don't normally explode or catch fire like tants, so it might not be as easy to spot the problem.

It's possible that it may have gone seriously "leaky", as in letting some serious amounts of DC current through.  This would effectively create a short circuit from battery's positive end through the resistor, through the bad cap, to the battery's negative end.  The resistor couldn't stand up to the amps, overheated trying to burn off all that energy, and just sizzled.

So, if I'm right (and I could be completely wrong  ), before you just pop in another resistor and fire it up, you might want to replace that electrolytic or the same thing could happen again!

You can do a simple test on the cap that will probably give you the answer (remove it from circuit first) like this:

For electrolytics in the uF range or above, you should be able to see the cap charge when you use a high ohms scale with the proper polarity - the resistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with VOMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode test) indicates that the positive lead is on the anode (triangle) and negative lead is on the cathode (bar).

If the resistance never goes very high, the capacitor is leaky.

*from http://repairfaq.ece.drexel.edu/sam/captest.htm#ctelc

Hope I'm at least half right about all this, and hope it helps!

[mongo]

Well, I replaced ALL electrolytic Caps, put the resistor on and the Big muff CAME BACK TO LIFE!!!!  


  working fine, I'll take it to reharsal this evening and see how it behaves.

This is my favorite Muff, I have a Russian, One of the first soviet reissues (bought almost 13 years ago, ouch!!) and a brand new NYC reissue but this old one is definately my favorite.

Is there a PCB layout for the OPAMP big muff somewhere??? I'd like to make one to take on the road  and leave this one home.


I'll post the list of components and values for the person who asked for it later tonight.




THANK YOU VERY MUCH everyone!! Man I love this forum!

Andy

[NaBo]

Is there a PCB layout for the OPAMP big muff somewhere??? I'd like to make one to take on the road and leave this one home.

I don't think so, not that I've seen  I've been passionately seeking out big muffs and big muff variants so I can get a nice variety of muffly goodness to choose from... I've finished the GGG version, and I'll have a green russian and swollen pickle on my plate over the summer.

But hey, it seems you have a working layout right there in front of ya... Shouldn't be too hard to trace by hand, and you only really have to worry about the spacing a bit for the opamps.

Very cool that you're gonna figure out the part values n post them.  EH wasn't exactly consistent from unit to unit, as I'm sure you know... So the only way to figure out how the circuit is "supposed" to be is to kinda average out those differences... and perhaps along the way find a muff that fits your taste

Anyway, glad you got it working again, and thanks for those pics!!!  Perhaps I might take it upon myself to make a layout in ExpressPCB...  but...... maybe I should finish this 2500 word essay that was due three weeks ago first  :lol:

[NaBo]

 I must've been right!!!  I USED OHM'S LAW SUCCESSFULLY!  Or... um... Joule's Law?  EITHER WAY!!--- I'm finally back at the level I was 6 years ago in high school!  :lol:

I decided to calculate the wattage the resistor would have had to contend with if that power filtering cap went nuclear and shorted.

Power (Wattage) = Voltage^2 / Resistance

So:
P = V^2 / R
P = 9^2 / 47
P = 1.723 W (!!!)

Most resistors in stompboxes are rated at just 1/4 watt... 1/2 W if you're lucky for the small value one involved in power filtering.  So yes... sizzle sizzle  :?

Ah, the simple pleasure of realizing "Yes, I am learning"... Good, Brain.  Good.