How do you tell when a chip is shot? (Op Amp Muff content)

[mrsage]

A friend asked me to work on an Op Amp big muff he found for an insanely cheap price. It isn't working, and when I compared the voltages to my Euthymia ICBM Fuzz (an op amp muff clone), the voltages on the 4558 chip were way off.

Here's what I measured off the clone:

4558
1: 4.57    8: 9.19
2: 4.63    7: 4.57
3: 4.59    6: 4.58
4: 0.0      5: 4.55

741
1: 0.003  8: 0
2: 4.98    7: 9.15
3: 4.77    6: 4.99
4: 0        5: 0.003


Here's what I measured on the original I'm fixing

4558
1: 0.5     8: 9.72
2: 0.48   7: 5.3
3: 4.80   6: 5.3
4: 0        5: 0.49

741
1: 0.004  8: 0
2: 5.36    7: 9.72
3: 5.13    6: 5.38
4: 0        5: 0.004


So clearly the 741 readings are within normal parameters, but the 4558 readings are way low on pins 1, 2, and 5.

Could this mean the chip is bad? I'm sure it could be something else, but I'm not sure where else to look right now...the resistors in the area seem to be okay, and I'm not sure how to test the caps without replacing those.

Any suggestions? Should I start by replacing the 4558 chip? My only concern is losing "mojo" on a vintage pedal...

Thanks.

[petemoore]

  I'd start at pin 1 of 4558, test that it's pin actually connects to what the schematic says it does, then test the resistor it uses, and trace around like that , checking continuities and resistances for pin 2 and around 5 also.
  "Suspect' lower reading resistances [using DMM] may be just an alternate path of the resistor in the circuit, in which case:
  a.Disregard the reading as an 'anomoly'
  b.Do Math to figure out what equation matches or doesn't match the readings to the resistor value code
  c.Pull one end of the resistor and re measure it.
  Electrolytic caps may be worthy of suspiciion also.
  The best way [only way I know of] to test an opamp is in a circuit, setup for 'a' function, if 'test opamp 2' performs there, and 'opamp 1' doesn't, it failed.

[Johan]

..to me, your reading suggest one out of two thing..either there is a bad soldering somewhere around pin 1-3 on the 4558 or the 4558 is dead...
"going DC" ( compared to "ground= 1/2V in this case) is the most common type of failure among opamps, in my experience..

johan

[Garrett]

I would use the ol audio probe for this job.

I have come acrossed a 4558 dual op that didn't output squat.  There so cheap to just replace anyway.


                                     have fun debugg'n    , Garrock

[mrsage]

Thanks guys.

Keep in mind that this isn't a circuit I built myself, so bridged traces and solder tails aren't the problem. This is a 1979 Electro Harmonix Big Muff Pi that did work at one point. I've re-heated a few cold solder joints, but I don't see any more on here.

I suspect it's one or more of the components.

My first inclination is to replace the opamp and try again, but he wants me to try to keep it as original as possible...I have a feeling I'm going to need to switch out a few things, though.

[petemoore]

  not working isn't what I'd call original'...
  OA ... it's like a 8cyl engine, except it has probably even closer 'specifications' ... less difference from one to the next, I think only those *selling 'mojo' opamps *say they can hear a difference.
  I'ts like telling your mechanic 'can you fix this dead car? but without changing anything...?'

[mrsage]

Yeah, that's my take on it, as well.

If it were mine, I'd dive right in and rip out that chip...

I suppose that's probably the best course of action!

(it's not even a "mojo" 4558 chip!)

[Johan]

Yeah, that's my take on it, as well.

If it were mine, I'd dive right in and rip out that chip...

I suppose that's probably the best course of action!

(it's not even a "mojo" 4558 chip!)

..well, you could tell him his got a choise to make..either an all original, but non workin muff, or a working muff with a replacement 4558..not much else to choose, right?..
johan

[mrsage]

I just decided to replace it, and it turns out that the opamp was bad after all.

Thanks guys. I just needed a little reinforcement that I was doing the right thing!