Need Help Testing LM741CN Op Amp Measurements?

[Steve Mavronis]

Help me out. I am trying to repair a near mint condition 1979 grey DOD 250 Overdrive for a friend. He got it off eBay as "not working" condition really cheap ($163) for this pedal, just because the PCB looked so clean and undamaged. It looked like it could be easily repaired with maybe an offboard problem so he sent it to me. I suspect the IC is bad even though it looks perfect. Basically with the SPDT footswitch bypass works and effect on is no sound. I even temporarily tried a working footswitch from my 1980 Dist+ but same symptom. I DC voltage measured V+ pin 7 of the National Semiconductor LM741CN and it reads the battery voltage. Input pin 3 reads half the battery voltage as expected from the voltage divider circuit. Output pin 6 reads the battery voltage. When plugging in my guitar and measuring for AC voltage changes when playing, the input pin 3 voltage increases but no measurement on the pin 6 output signal.

I also tested resistance of all the resistors and paths in between parts - all normal as expected readings.

Maybe I'm testing this wrong. How do you properly test that the 741 chip is functioning while in the circuit? I don't want to replace the original chip if I'm doing something wrong and if I do the owner wants to try and find a NOS 741 from 1978 or 1979 not because a modern 741 is any different but to keep the pedal as period original as possible with dated parts.

[CodeMonk]

I would pull that 741 out and put another one in there just for testing purposes.
Then you can worry about finding an NOS one if it works.

Good luck in finding an NOS 741

[armdnrdy]

Instead of pulling it...use an audio probe to check the signal before and after the IC.

If there is a signal before...and no signal after...the 741 is bad.

Audio probe the circuit from input to the 741. See if you can find a problem before pulling the IC.

[Scruffie]

Didn't I send you an NOS RCA741 a long time ago? Alright not exactly the same but it's definitely 70s.

Post full voltages for the chip and build an audio probe just to be sure signal is at the input pin and not the output.

Also check continuity between the chip legs and the traces coming from them... then you'll know for absolute certain the chip is dead.

Edit:armdnrdy beat me to it on the audio probe.

[CodeMonk]

Instead of pulling it...use an audio probe to check the signal before and after the IC.

If there is a signal before...and no signal after...the 741 is bad.

Audio probe the circuit from input to the 741. See if you can find a problem before pulling the IC.

Yeah, what he said is better than my idea.
My brain isn't awake yet. Needs more caffeine.

[Steve Mavronis]

Didn't I send you an NOS RCA741 a long time ago? Alright not exactly the same but it's definitely 70s.

I got RCA metal can 741's. I don't remember getting a chip version or I would have used it in my DIY build instead.

Here are the grey's LM741CN Op Amp voltages with 8.9V test battery connected and guitar cable only plugged into input jack to enable the battery in the circuit:

Pin 1: 7.46V
Pin 2: 7.55V
Pin 3: 4.29V
Pin 4: 0.00V
Pin 5: 7.46V
Pin 6: 8.35V
Pin 7: 8.90V
Pin 8: 0.00V

[Edit] As a comparison I measured my working grey spec clone using a 1979 RCA CA741T metal can Op Amp with 8.9V test battery connected and guitar cable only plugged into input jack to enable the battery in the circuit:

Pin 1: 0.01V
Pin 2: 4.40V
Pin 3: 4.20V
Pin 4: 0.00V
Pin 5: 0.01V
Pin 6: 4.47V
Pin 7: 8.84V
Pin 8: 0.00V

[DrAlx]

Bias level for the input signal (pin 3) looks OK at roughly half the supply level (pin 7).
The output pin 6 seems to have hit its positive limit, which is why pin 2 is also high, and nowhere close to pin 3 (which is where it would be if the feedback was working OK).
Pins 1 and 5 are the obvious suspects. I haven't seen these OFFSET NULL pins wired to anything on any of the schematics for the DOD 250.
See where those 2 traces lead, and if they actually go to any other components.

[duck_arse]

n my 741 drawer .....

2 x uA741TC, fairchild, "F7747" and "F7708" - unused
1 x uA741CP, TI, "M810" - unused
30 x CA741CE, RCA, "RCA123" and "OPUS 6604" (and others) - unused, given to me about 1980, actual datecode ?

how original do you want? pm me if interested.

[Steve Mavronis]

Bias level for the input signal (pin 3) looks OK at roughly half the supply level (pin 7).
The output pin 6 seems to have hit its positive limit, which is why pin 2 is also high, and nowhere close to pin 3 (which is where it would be if the feedback was working OK).
Pins 1 and 5 are the obvious suspects. I haven't seen these OFFSET NULL pins wired to anything on any of the schematics for the DOD 250.
See where those 2 traces lead, and if they actually go to any other components.

Pins 1, 5, and 8 are unconnected. They are soldered to standalone pads.

[DrAlx]

Bias level for the input signal (pin 3) looks OK at roughly half the supply level (pin 7).
The output pin 6 seems to have hit its positive limit, which is why pin 2 is also high, and nowhere close to pin 3 (which is where it would be if the feedback was working OK).
Pins 1 and 5 are the obvious suspects. I haven't seen these OFFSET NULL pins wired to anything on any of the schematics for the DOD 250.
See where those 2 traces lead, and if they actually go to any other components.

Pins 1, 5, and 8 are unconnected. They are soldered to standalone pads.

Either the op-amp is fried, or something else in the rest of the circuit is causing it to latch-up. 
I don't know if old/leaky electrolytic caps can cause the op-amp to latch-up on powering up.  My guess is the opamp is fried.

[PRR]

Any electrolytic cap is a MUCH more likely problem than a '741.

And a dang sight easier to pull, and less likely to reduce its collector value.

You only have two. VERY careful solder-suck should let you lift one leg of each. If the DC voltages pop up to "correct", replace that e-cap.

[Steve Mavronis]

Any electrolytic cap is a MUCH more likely problem than a '741. And a dang sight easier to pull, and less likely to reduce its collector value. You only have two. VERY careful solder-suck should let you lift one leg of each. If the DC voltages pop up to "correct", replace that e-cap.

Thanks I'll try that first before signing the 741 death sentence!

[Steve Mavronis]

Same DC voltage readings on the 741 with each electrolytic disconnected one at a time.

[Steve Mavronis]

It's definitely a bad 741 chip. I unsoldered it from the grey 250 PCB with solder wick and put it into my DIY clone since it has an IC socket. I get the same bad measurements. Then I put back my metal can 741 and test results show the same 'normal' numbers as before. I guess my friend will have to search high and low for a late 1970's LM741 if that's what he wants. I have plenty of 80's LM741's here and even another RCA 1979 metal can IC but he wants to maintain what the original would have had in it. So for a 1979 grey 250 he'd have to find an RCA or National Semiconductor LM741 within the date range expected for this model.

[R.G.]

I'll keep biting my tongue.

[Steve Mavronis]

I'll keep biting my tongue.

Uh oh I must have done something wrong?

[Scruffie]

He'll probably say something about it being a 741 so it not making a blind bit of difference.

Find some 70s audio equipment at a flea market or charity shop and open it up, sure you'll find a 741 to de-solder.

I did send you an RCA one actually, along with a vintage RCA3080 prior to that (checked my PMs) but it must have been lost in the post if you didn't get it.

[Ice-9]

In all honesty a 741 is a 741, put one in and try it to see if it works. then look for an old chip if really needed. Hint on old chips, have a look in the loft (attick USA) find an old cassette player etc, probs find a few 741's of that era in those machines.

[Steve Mavronis]

He'll probably say something about it being a 741 so it not making a blind bit of difference.

I did send you an RCA one actually, along with a vintage RCA3080 prior to that (checked my PMs) but it must have been lost in the post if you didn't get it.

Yeah with the 741 new vs vintage it doesn't make a bit of difference to the ears. Getting a vintage part to me is more about paying homage to an era by dating it.  It's cosmetic like color coordination or something, nothing more. I'd probably put an IC on it for future chip swapping but it's not mine so I'm trying to help the owner with what he wants if at all possible.

Hmm, I'll go look at my PMs and emails. I remember the RCA 3080. Not sure what happened with the 741 unless I gave it away. It wasn't the metal can RCA version I suppose. You guys have always been helpful. Thanks for that.

[bluebunny]

Not sure what happened with the 741 unless I gave it away.

I think you may have put it here: