big muff pi stopped working recently

[fatfoot51]

Hi~ Im a newbie to the boards ... I am more of a guitar player than a hobbyist but I stumbled upon this by accident and actually repaired a few of my vintage gear .. Rather than shotgunning the board I was audioprobing around and lost my signal in the effect path @ the .0047 cap but I'm not sure why ... So I plugged my 1/4 " cable in the input and measured voltages instead.. Considering that i may be getting power to the board my voltages are as follows for the ic version of the big muff which came out in the late '70s... Thanks so much guys...  I need all the help I can get! 
the LM741                                                                       4558NB
Pin:  1    ovdc                                                                  Pin :    1       .209vdc                       
        2    4.4  and steadily decreasing                                            2       .180vdc
        3    4.41 vdc                                                                       3       4.15vdc
        4    0vdc                                                                             4        0
        5    0vdc                                                                             5       .184vdc
        6    3.6 and steadily decreasing                                             6        7.34vdc
        7    8.47   battery voltage                                                     7        7.1vdc
        8    0 vdc                                                                            8        8.47vdc

[petemoore]

the LM741 has single opamp pinout                        4558NB [see data sheets for pinout info, 4558 has dual OA pin   
                                                                                               assignments
Pin:  1    ovdc                                                                  Pin :    1       .209vdc                       
        2    4.4  and steadily decreasing                                            2       .180vdc
        3    4.41 vdc                                                                       3       4.15vdc
        4    0vdc                                                                             4        0
        5    0vdc                                                                             5       .184vdc
        6    3.6 and steadily decreasing                                             6        7.34vdc
        7    8.47   battery voltage                                                     7        7.1vdc
        8    0 vdc                                                                            8        8.47vdc
   One of these opamps is probably assignable to the pin-hookups on the board you've populated, with a schematic guide or layout, the orientation of an opamp that will operate [when external needs/conditions are met] can be figured out.
  Probably a dual opamp layout...especially if there's only a place for 1 chip on a BMP board.

[newfish]

Have you thought about re-flowing all of the existing solder joints?

This would be my first port of call...

Cheers, Ian.

[petemoore]

  Our best bet is for you to use:
  DEBUGGING - What to do when it doesn't work
  >Sticky<

[runmikeyrun]

@ the .0047 cap

Ok, WHICH cap?  where is it?  If you can post a schematic and indicate which cap it is we can probably help you better.

EDIT:  does this schematic look like it's correct for your pedal?  I see a 0.0047 uF cap off Pin 7 of the 4558, is that the one you're talking about?  If so, that cap might be bad... check for signal at pin 7 with your audio probe and if you don't have any, try replacing the 0.0047uF cap.  Also, it seems weird to me you have 7.1v at pin 7, does that seem wrong to anyone else?  sorry i'm not the best with ICs.

[fatfoot51]

Yes the audio probe lost its signal at the .0047 uf capacitor starting from the input by pin 7 of the 4558...and I changed the cap and the same result happened.. All the diodes seem to be okay, not shorted acording to the DMM.. reading one way etc..after desoldering one side of the chain hoping to keep from reading anything else... In bypass i get a clean signal all the way through but in effect I dont get even a full unaffected path..It stops and if I stack a new 741 ( i think its new)  I get a clear path but no distortion... I may have a bad coupling cap? Except when I probe the rest of the circuit I get nothing near the LM 741 at all.. I don't want to assume that the LM741 is bad but @ this point I'm not sure what else it could be..  I may have to get back to you guys later with anything I might find if everyone still wants to know.. thanks again!

[arawn]

I know on the 4558 everything but pin 8 and pin 4 should be near vbias or half of supply pin 8  should be battery voltage and pin 4 should be 0 volts. the 741 should be seeing bv at pin 7 and 0volts at pins 4 and 8

[runmikeyrun]

are you getting audio signal at pin 5 of the 4558?  If not C2 could be shorted.  I still think there's a problem if you have 7.1v at pin 7 of the 4558.  Can you replace the 4558 and see if the voltages change?

[fatfoot51]

Thanks to everyone that read my post and replied..  :icon_biggrin:I appreciate that!  Mike, I do get a clean signal at pin five but nothing around it when i probe other  points... Im not sure what I'm supposed to hear on the other side of the .01uf cap or the .oo47uf cap because I do not hear anything.. The sound stops!  Bad 4558?  It seems to be reacting more differently than before voltage-wise...  Why isn't there a clean signal all the way through at least even if there is nothing affecting it? Hmmm...Last night I decided to use a new battery and take readings again... The outcome was different... So i plugged the cable into the input and this was what I read off of the ic chips in the big muff with my DMM ( black lead on ground and the red lead on pins 1-8)..  I hope this data is more helpful now.  Thankyou much!
LM 741                                                        4558NB
pin 1         0vdc                                                   pin 1        1.3vdc and decreases
     2       4.1vdc  and decreases                                  2        1.3vdc  reacts like pin 1
     3       5.0vdc                                                        3        4.56vdc
     4          0vdc                                                        4            0vdc
     5          0vdc                                                        5         1.1vdc  and decreases
     6       3.8vdc  and decreasing                                 6         6.5vdc  and decreases
     7       9.63vdc  new battery voltage                         7         6.4vdc  and decreases
     8          0vdc                                                        8          9.63vdc battery voltage

[runmikeyrun]

It still seems weird that you have so much voltage on pin 7 of the 4558.  I'm no expert on 4558s but that just seems like too high a voltage.  If it's not beyond your skill level and you have an extra 4558 AND an 8 pin socket, try unsoldering the 4558 and solder in the socket.  Then try another 4558.  Make sure to get the chip orientation correct.  You can also do the same with the 741 if you suspect that, but it seems like the strangeness is originating in the 4558, I would try replacing that first.  It's odd that those voltages are decreasing like that- at first I suspected the power supply circuit but all the chips are getting steady voltages at va, vb, and vc. 

Sorry I can't be of much more help. 

[fatfoot51]

Thanks mike for the advice! I think that's a fair assessment since there are more points of having strange voltages being a dual op-amp I suppose ... If that doesn't work I guess next would be the 741... I do have an extra 4558 and socket somewhere. I'll start there first and let you know what I get!  It's time to operate! 

[lopsided]

hello,
I have a working opamp bigmuff clone I build, so I checked the voltages, all pins (except 4 and 8 of course) of the 4558 shoud be between 4,1 and 4,6.
I am not sure if I understood all the info you gave here right, but if you do not have any audio at pin 7 of the 4558 and  you got higher voltage there I would go with Mike and suggest trying to change it for a fresh one. Broken opams tend to leak voltage at their outputs, as I have learned at my old MXR equalizer. I would worry about the 741 later if this doesn't work.

A coupple more notes:

Why isn't there a clean signal all the way through at least even if there is nothing affecting it?

when something is broken in a circuit you very often will not get an unaffected signal, you will get no signal at all.

what I'm supposed to hear on the other side of the .01uf cap

It is okay to not hear anything on the other side of the .01uf cap. It is connected to ground, therefore no audible signal is ok.

Finally be very careful by desoldering the opamp it can get a little tricky.

[fatfoot51]

Mike, you are a genius!  Last night I decided to look for my sockets and ics with enough time to desolder the old RC4558NB and put the new one in with the socket careful not to overheat the traces or rip them off the board.. We have life!  The pedal has distortion! It is a bit lower on volume even with the tone bypass switch but it's a marked improvement! In tone bypass the distortion is less muffled..It's amazing how you picked up on the 4558 even before the second voltage readings!  I may change out the 741 also...  Thanks jakub also for your input. I may put in a new 741 to see if I can recover the volume drop and let you know.. Its strange that there is a pull on the voltage..Thanks guys again for your help on my first thread. 
LM 741                                                              RC4558P   (more stable voltages now)
pin 1          0vdc                                                         pin 1          4.53vdc
     2      2.54vdc and decreasing                                        2          4.57vdc
     3      4.89vdc                                                              3          4.52vdc
     4          0vdc                                                               4              0vdc
     5          0vdc                                                               5          4.53vdc
     6      2.2 vdc and decreasing                                         6          4.57vdc
     7      9.38vdc                                                              7          4.53vdc
     8          0vdc                                                               8          9.38vdc
                 

[Barcode80]

You may want to replace that 741 too, those voltages look odd. 2, 3, and 6 should be sitting at around half supply if I'm not mistaken (I could be, my opamp knowledge is shaky tonight). It could be the cause of your volume issue.

[runmikeyrun]

you got it buddy, I'm glad it lives!!     Thats what we're here for, to help each other out.  and BTW I am FAR from a genius!  lol.  Sometimes I just get lucky 

[fatfoot51]

So last night I got home a bit late so I postponed replacing the LM741 until this morning... Once again trying not to lift traces I desoldered the old one and installed another socket.. At last! We have distortion and volume! Good as new!  So now its together!  Thanks again gentlemen! 

[arawn]

it was our pleasure

[Barcode80]

'twas nothing