Boss OD--1. Need help with repair. no effected output.

[Mr.Grim]

?? Huh. So yeah I have good sound on pin 7 end, so I replaced sed resistor and results are exactly the same, no sound on the other end (pin9).  And I tested the original resistor and it's good.

[ElectricDruid]

Posting a schematic for the circuit you're looking at would be a good start:

https://www.hobby-hour.com/electronics/s/od1-overdrive.php

Now we've got the basics in place, you say you've got a signal at pin 7, the output of the first op-amp, right?

Have you got a signal at pin 1 of that op-amp, the output of the following stage?

Don't worry about getting signal at pin 2, since that's a virtual ground and you won't hear anything there. The 10K resistor is fine, and no signal at pin 2 doesn't mean it's broken.

[fowl]

^ That's a later version with transistor buffers.  I've been referring to OP's schematic in reply 11.

?? Huh. So yeah I have good sound on pin 7 end, so I replaced sed resistor and results are exactly the same, no sound on the other end (pin9).  And I tested the original resistor and it's good.

I'm lost then.  Maybe a bad 3403?

[ElectricDruid]

^ That's a later version with transistor buffers.  I've been referring to OP's schematic in reply 11.

Oh, sorry. My bad. I missed that, somehow.

?? Huh. So yeah I have good sound on pin 7 end, so I replaced sed resistor and results are exactly the same, no sound on the other end (pin9).  And I tested the original resistor and it's good.

Same thing applies - the pin 9 end is the virtual ground end of the 10K resistor, so you *won't* hear anything there, and the resistor is fine, I'm sure. The question is whether you've got any output on pin 8.

[Mark Hammer]

I recently repaired a Tube Screamer for a buddy.  There was appropriate output in the clipping stage, but the tone stagedidn't yield any output.  I changed chips and it sprang to life.

It's not always the case, and I certainly don't know why it happens, but dual and quad op-amp chips can sometimes have one op-amp bad while everything else is just tickety-boo.  If the chip is socketed, try sticking another quad, like an LM324, or TL074, in there, just to verify that it is, or isn't an issue with one of the op-amps.

[ElectricDruid]

+1 agree with Mark. It certainly wouldn't be the first time there's been one fried op-amp on a dual or quad package.

I got caught out with this recently on my breadboard, when the hard-to-find fault turned out to be re-using a dodgy op-amp chip that's been kicking around my workshop for clearly far too long.

[fowl]

Same thing applies - the pin 9 end is the virtual ground end of the 10K resistor, so you *won't* hear anything there, and the resistor is fine, I'm sure. The question is whether you've got any output on pin 8.

Pin 9 should have signal, no?

[Mark Hammer]

Yes, it will have signal.  It can provide a test of continuity, and verify that signal is indeed reaching the 2nd op-amp.  But it is not a test of whether that 2nd op-amp is functional, because any signal you observe at pin 9 is coming from the previous op-amp.

[Rob Strand]

I agree with ElectriDruid, normally you would not expect to hear signal at pin 9 (the -ve input of the second opamp).  (There is signal there but it is absolutely tiny.)

You only get significant signal at pin 9 if the second opamp is clipping.   With the signal levels involved that shouldn't happen.  However, if the battery was very low the opamp could clip and you could hear signal.    If the second opamp was faulty then you could get clipping as well.


One thing worth checking is the DC voltages.  The supply voltage.  The output of the opamps.  The DC voltages on the opamp input wouldn't hurt either - sometimes these need re-interpreting due to multimeter loading..   If the DC voltage on the inputs ot outputs are off then it would indicate an opamp problem.    If the outputs aren't biased at mid rail then that could cause the opamps to clip and then signal would appear on pin 9.
The post page chopped the top post, the DC voltages don't look bad.

Datasheet,
https://pdf1.alldatasheet.com/datasheet-pdf/view/125059/FAIRCHILD/RC3403A.html

[fowl]

I'll defer to y'all's expertise, because I have none.  I would have expected signal at pin 9 to be roughly the same as at pin 7.  Pin 10 connects to virtual ground.

[Rob Strand]

I'll defer to y'all's expertise, because I have none.  I would have expected signal at pin 9 to be roughly the same as at pin 7.  Pin 10 connects to virtual ground.

When you have feedback (and the opamp isn't clipping so feedback is working) the opamp -input follows the opamp +input.   That's true for any opamp (amplifier) configuration.   It's true because the lone opamp has a very high gain.
So for the second stage:
- the 10k on pin 10 doesn't do much for AC signal so it can be ignored. 
- so, pin 10 is connected to Vref (~4.5V).  Vref is DC and has no AC.
- pin 9 tracks pin 10 so the feedback forces pin 9 to be stuck at Vref, hence no AC there.

  For this reason the opamp -input is called a virtual ground.  In this case it's 4.5V and not ground as such
  but for AC signal any DC source is considered a ground (actually a short but the short goes to ground) and the AC is zero (this extra detail comes up a lot).

Another way to look at it is if the output from the first stage is 1V peak AC the peak adds to the 4.5V Vref
and so the total voltage on output of this stage is 1V+4.5V = 5.5V.   The AC gain of the second stage is -1.
That means the output swing below 4.5V by 1V, so the total voltage on the second stage is 4.5V-1V = 3.5V.
So the 10k resistor that goes to the output of the first stage is at 5.5V and the 10k resistor that goes to the
output of second stage is 3.5V.   If you think of the two 10ks as a divider this divider has 2V across it 5.5V-3.5V.
That means each 10k has 1V across it.   So the voltage at the  "divider" output is 5.5V -1V = 4.5V, or 3.5V + 1V = 4.5V
depending on which resistor you use.  The point is the voltage at the junction of the 10ks (ie. pin 9)  is 4.5V, the same as the
voltage on pin 10.    That's kind of a long winded way to describe it but you should be able to see it's true.
Normally this type of feedback circuit is analysed with currents which is a little more mathematical, you will
find plenty of derivations on the web (under inverting amplifier analysis).

[fowl]

I'll have to sleep some and then re-read your post, lol.

If there's no signal at pin 9, how does it enter that opamp to be amplified?  Why even connect pin 7 to 9?

Or are you saying there's signal there, but very low amplitude because the opamp is trying to hold pin 9 to 4.5VDC...something like that...?

[Mr.Grim]

I have no signal at pin 9.

[ElectricDruid]

If there's no signal at pin 9, how does it enter that opamp to be amplified?  Why even connect pin 7 to 9?

This is actually the key point.

The *voltage* input enters through the resistor R8. Since a voltage is passing through a resistor, there's a *current* flowing (V=IR, and all that, right?). At pin 9, the current from R8 is cancelled out by the feedback from the output through R10, so there is no net current flowing into or out of pin 9, and hence no voltage at that point. Pin 10 is held at a virtual ground, and one of the basic rules of op-amps is that the op-amp acts to hold both its inputs at the same voltage, so the op-amp makes sure that the output signal is whatever is required to cancel that input current. That's why the output is (a) inverted (cos it has to cancel the input), and (b) why the feedback resistor sets the gain (since if the feedback resistor is four times -for example- bigger, the output has to be four times bigger to cancel the input).

So despite the fact that *no signal appears at pin 9*, the reason that happens is because the signal has *already* made its way to the output and the negative feedback signal from the output has completely cancelled it! I know, it's a bit weird, but honestly, that's how it works.

I hope that makes sense!

[fowl]

I hope that makes sense!

A bit I guess.  Thanks to you and Rob for at least attempting to educate me, lol.

But how does this all apply to OP's problem?  Still look like a bad chip?  I'd hate for him to try to remove that thing and find out the problem was something else.

When I was building my clone, I found some Raytheon 3403s for sale in Europe, about $15 plus shipping.  Small Bear had the JRC version for about $5, so I went with that.

[Rob Strand]

FYI Tom, the schematic needs to be the RC3404 version but hobby hour has the RC4558 version.  The op has the correct schem in Reply #1.

https://postimg.cc/ZBnjGvtc

But how does this all apply to OP's problem?  Still look like a bad chip?  I'd hate for him to try to remove that thing and find out the problem was something else.

When I was building my clone, I found some Raytheon 3403s for sale in Europe, about $15 plus shipping.  Small Bear had the JRC version for about $5, so I went with that.

Agreed, you want to be quite sure the opamp is damaged before swapping it.

I have no signal at pin 9.

Good news, that at least gives some confidence the Opamp is working.

So far,

When engaged, there is no effect. It passes a weak dry signal that is unison when level is maxed out.  All results are same with adapter or 9V battery.

So yeah I have good sound on pin 7 end

Pin 8 is slightly louder than 14, and pins 1/4 are loud like bypass. 14 is very quiet.

Opamp outputs pins in order through the pedal are:
pin 1 (input buffer) --> pin 7 (clipper) --> pin 8 (inverter/filter) --> pin 14 (output buffer)

No signal at pin 9 implies opamp with pin 8 output is probably working.

So the big question why is pin 8 so quiet?

- A bad connection on between pin 7 to R8 or a bad connection from R8 to pin 9.
  If possible from the top side of the board check continuity with the DMM from
IC pin 7 to R8 and from IC pin 9 to R8. 

This fault allows pin 9 to have no signal and low signal at pin 8.

- A short at pin 8.  Unlikely as this would cause signal on pin 9.

- Faulty R10 or C6.  If anything C6 more likely.


By nature the signal at pin 14 is going to less than pin 8.  The question is there an unnatural loss on pin 14 and pin 8 is actually OK ?

The signal at pin 7 is going to be buzzy the signal at pin 8 has all the buzz removed.  Pin 8 will sound lower because of that.  So we are talking finer points to judge the signal at pin 8 is bad.

One more thing we can check is
- turn the LEVEL pot to full
- check if the signal at the clockwise terminal of the LEVEL pot is the same level as the signal on pin 14.
- check the signal level on the wiper of the LEVEL pot is the same level as the signal on pin 14.

That would at least confirm the opamp with pin 14 on the output is working correctly.

If these signals are at different levels it could be dirty LEVEL pot, or a faulty Q2, or even a faulty opamp pin 14.
When the signal drops in the chain tells you where the fault is.     If there is a problem check the signal at pin 12.

If all that checks out OK we can focus on pin 8.

[Mr.Grim]

Thanks sooo much for all that info Rob!  And everything checks out fine that you mentioned.

And just to reiterate, pin 8 is just a hair quieter than pin 7. When I A/B them it's almost as if you hit a bright switch on an amp when you go back to pin 7. 8 sounds slightly muffled, as if a slight filter was applied.

[Rob Strand]

And just to reiterate, pin 8 is just a hair quieter than pin 7. When I A/B them it's almost as if you hit a bright switch on an amp when you go back to pin 7. 8 sounds slightly muffled, as if a slight filter was applied.

Yes, that's absolutely what is expected.

So you still have the volume loss problem, yes?

I was thinking later, in bypass the last opamp, with output pin 14, is used for bypass so that opamp must be working.   So that means all opamps checkout.

However, if you still have a volume loss problem it has to be in Q2 or the LEVEL pot.

With the LEVEL pot on full, when you progress through signals on,
- LEVEL pot (clockwise terminal)
- LEVEL pot wiper
- opamp input pin 12
- opamp output pin 14

They all have to same level, right?
*BUT* that level is too low, yes?

Maybe it's all working and you just don't like the natural level from the pedal.  You can get more level.  Just change R11 4.7k to say 470 ohm.  That's more of a mod than a fault fix.

[Mr.Grim]

Yes all the same level, and yes all low.

Definitely not a personal dislike of the output level, it's definitely not working.

When I try to use the pedal and turn it on it's more of a volume pedal and knocks 1/3 of my total volume down, not up (when both knobs jacked up).  And no OD at all.

[Rob Strand]

When I try to use the pedal and turn it on it's more of a volume pedal and knocks 1/3 of my total volume down, not up (when both knobs jacked up).  And no OD at all.

OK got it.  That doesn't sound right.

The fault can only be from pin 8 to the LEVEL pot.   That leaves only C7 and R11.

Maybe listen to the point between C7 and R11.  It should sound the same as pin 8.
If not maybe C7 is the problem.   It is an old unit so perhaps the C7 cap has dried out.
As a fault, that makes more sense.