MXR Distortion + Op Amp Bias Voltage

RedViper · Post by **RedViper** » 10 Apr 2018, 23:18

Good evening everybody. I’m repairing a vintage MXR distortion + for someone. I probed it and found that the IC was bad. I ordered a new one and socketed it. It’s a 741 by the way in case anybody was wondering. The pedal works now and seems to sound like it should but I decided to check to see what might have fried it in the first place. Its been weeks since I replaced the op amp and it still seems to be working fine. I assumed the voltage to pin 3 should be about 4.5 volts but I’m getting just shy of 1.9. I’m reading 9v like normal to pin 7. This is all with the IC taken out of the socket. Could I have an open circuit somewhere? Or is 1.9v normal. I would suspect I should be getting around 6.9v on the other side of the voltage divider but I’m not seeing it anywhere. But the traces are a tad old and look like they may have had a layer of something on them so it’s kind of hard to get continuity on the traces and some of the pads. I considered reflowing some of the joints to get a solid reading. My next step was to desolder and check the resistors to see if there was enough drift over the years to throw the bias off. But first I wanted to see if my inexperience is telling me incorrectly that I should be getting 4.5v to pin 3. Thanks! Disclaimer: I used the search function, tried wording it a few different ways and couldn’t find anything.

RedViper · Post by **RedViper** » 10 Apr 2018, 23:56

I since found another post that said that the resistance of the meter itself can account for the incorrect voltage reading? It’s still lower than it should be by a bit since it says I should’ve seeing about 2.25v. But .35v seems more possible due to drift than over 2.5 volts. Interested to see if others have additional input.

alexradium · Post by **alexradium** » 11 Apr 2018, 05:16

if you had the IC damaged by reverse polarity,try changing the filter capacitors.

RedViper · Post by **RedViper** » 11 Apr 2018, 12:15

I had considered that initially but it's the older version with no DC power supply jack, just the 9 volt battery so it seems unlikely that it would have been hit with reverse polarity. Would it still be worthwhile to check the filter caps?

alexradium · Post by **alexradium** » 11 Apr 2018, 13:13

new cap is always welcome because after many years it dries up.
Check the 2 resistors that make the 4,5V divider if they are equal then reflow all the solderings up the 1M resistor that connects to input

RedViper · Post by **RedViper** » 11 Apr 2018, 14:45

You're referring to the 1M resistor to pin 3 of the opamp and the one 1M that's parallel to that going to ground? Those are the two that make up the voltage divider correct? Just want to make sure I'm checking the right ones. And do you check your resistors while they're still in the circuit? I usually take them out to make sure I'm getting an accurate reading. That's what I was told in my EE circuits class and that's the way I've always done it. It would be nice if I could test them without removing them though.

Fender3D · Post by **Fender3D** » 11 Apr 2018, 17:15

If voltage on pin 6 (output) is ~1/2 Vcc your bias network... works.
Replace the old tantalums: a couple bucks more won't lead anyone to bankruptcy...

RedViper · Post by **RedViper** » 11 Apr 2018, 18:32

I'll pop the op amp back in tonight and check it. I was taking my voltage readings going to the socket without the IC in it. The pedal sounds like every other Distortion + I've played, that's what was making me think the measurement was misleading. I would think if it wasn't getting enough voltage to pin 3, it would sound wrong or not work at all right? I'm still pretty green when it comes to some of this stuff so forgive me if I'm way off. Most of the repairs I've made so far have been super easy probe, find and replace bad component or replace switch, re-solder bad joints, replace pots, jacks, etc. Thanks to both you and alex for the help you've given me so far. I really appreciate the education.

RedViper · Post by **RedViper** » 16 Apr 2018, 01:30

I am getting the 4.5 volts at pin 6 so I guess everything is working alright. I haven’t replaced any caps yet. I don’t have any tantalums in stock. I asked the guy I’m repairing it for and he said not to worry about it. I remember reading something a while back that you can use either side of a single op amp? Am I remembering this right?

Fender3D · Post by **Fender3D** » 16 Apr 2018, 08:26

I don't know what you mean by "either side of a single op-amp"...
since a single op amp has just one side (aka 1 op amp).
Surely you must install it correctly oriented, it has a "hole" indicating pin 1

It may be true for the various clones you find on the web where lazy "engineers" use dual op amp instead of single, thinking it's another Tube Screamer Mojo thing

...

microbailey · Post by **microbailey** » 17 Apr 2018, 23:20

RedViper, maybe by "either side" you mean either input? (usually pin 2 and 3 are the input pins on a single 8-pin DIL op-amp (like 741)
If so then that's true - there are 2 kinds of simple op-amp amplifier circuits:

one called inverting (since signal phase at the output is opposite of the input) - that feeds the input signal into pin 2 (- pin)
one called non-inverting (since signal phase at the output is same as the input) - that feeds the input signal into pin 3 (+ pin)

If you Google these circuits you'll see the resistors which set the gain are connected differently in the 2 cases.

If the pedal you've been fixing is MXR Distortion + then its the non-inverting type.

Anyway if you measured something which is way off half-battery voltage at pin 3 in this type of circuit then something's gone wrong with the bias (means the little divider that makes 4.5v from 9v).
I know you said there is no external supply, but is there any protection (like a diode in the supply) against reverse connecting the battery, even for an instant?
An accidental reverse supply will fry the op-amp and probably any polarized (like tant or electrolytic) supply rail caps too.
Other than that, most faults in old PCBs are caused by solder joints gone bad or sometimes a hair fracture in a PCB track (where it joins a component pin) which wasn't picked up when the pedal was inspected new.

One other thing - if you want to measure a resistor value on a PCB accurately, just lift one leg and measure, you don't need to un-solder the whole thing (maybe you knew that anyway)

Maybe some of that helps???

Manfred · Post by **Manfred** » 18 Apr 2018, 01:58

Fender3D wrote:If voltage on pin 6 (output) is ~1/2 Vcc your bias network... works.
Replace the old tantalums: a couple bucks more won't lead anyone to bankruptcy...

I agree with you, I had have issues on other device, a defective tantalum drops down the voltage.