Big Muff - v5 Op-Amp Probs

[vmaxvmax]

HI

I hope it is fine to ask for advice regarding my op-amp Big Muff EH1322 v5 with tone bypass switch on this forum?

I have also posted on DIYStompboxes.

The unit no longer works properly.

It seems as if there is a very extreme noise reduction circuit working - i.e. only faint output unless a loud note is played; the initial attack of a note sounds but dies quickly and has an unnatural 'buzziness' to it. Almost as if the unit is starved of power. This with either external or battery power. With the sustain pot at max. If the pot is less than max, then very, very low and clean output.

If I push the input hard with a booster pedal, it works a little better but sounds like a buzzsaw and decays suddenly.

The pots all seem to do what they are meant to albeit with a much reduced output.

I have checked inside for obvious faults and nothing obvious.

I have attached schematics I found on the web.

I know enough to be dangerous - basic but far from expert with electronics!

Resistors are all within expected limits, the caps measure OK but I can't measure the 220nf power supply cap my dvm won't give me a reading. My DVM doesn't seem to give me readings on any higher value electrolytics... Diodes OK.

The RC4558 pins with a rechargeable 8.8v battery measure:
1 - 4.31
2 - 4.32
3 - 4.28
4 - 0
5 - 4.29
6 - 4.31
7 - 4.32
8 - 8.65

For the 741:
1 - 0
2 - 2.05
3 - 1.93
4 - 0
5 - 0
6 - 3.73
7 - 8.63
8 - 0

The measurements are marginally greater with an external 9v psu attached.

Any ideas?

I am about to order replacement IC's and caps - but as I live in the south of France, I have no local stores so thought I'd get some ideas first!

Also, is an RC4558P a viable swap for the existing RC4558NB?

Many thanks in advance,

[Digital Larry]

I don't see a schematic or a link to a schematic.

[vmaxvmax]

How stupid of me!

Here it is!
https://www.dropbox.com/sh/5frcocihgm96tmq/NDG8KGMyYP/vintage%20muff%20.pdf

[GibsonGM]

Looks like a problem around the 741, my friend.    Have you audio probed the circuit, and determined where the trouble starts?   I am suspicious of that area of the circuit, and would 'listen in', betting you'll find reasonable signal levels up til that point. 

Then you can double, tripe and quadruple check that your connections are OK and no shorts (which seem to be the major reason we have these troubles!).    And that your diodes are in there correctly, as well as having the right connections to the switch!

[vmaxvmax]

Thank you for your reply!

What makes you think it is the 741? - Are the voltages suspect? I don't know what they should be as I have enough electronics knowledge to be dangerous...

How do I go about listening in? - Do you mean that literally - taking a signal and feeding that to an amp from various locations with a source/guitar plugged in? I don't have an oscilloscope.

I've checked connections etc and they all seem OK - I've also checked for continuity from point to point in order to make sure that there are no broken tracks.

From the sound I'm getting - i.e. faint with normal signal and distorted with a loud signal but the 'wrong' type with a very rapid decay like a vicious noise reduction circuit - I'd assumed possibly a fault in the smoothing caps. But I'm no expert!

[lopsided]

Hello,
Check the debugging sticky for how to make an audio probe. The 741 does indeed look suspicious. In most to the applications you would expect the inputs and output of an opamp to be at around half of the supply voltage.

[DiscoVlad]

Do the voltages around the 820k and 1Meg biasing resistors for the 741 read ok?

With a 9V supply, the 820k should read 9V at one end, and approx. 5V at the other (connects to Pin 3 of the 741).
The 1Meg should have 5V at one end (741 Pin 3), and 0V at the other.

[vmaxvmax]

Wow - thanks everybody for the replies and help.

I'll build the audio probe, what a great idea!

I'll check the voltages and signal out today and report back.

[GibsonGM]

Do the voltages around the 820k and 1Meg biasing resistors for the 741 read ok?

With a 9V supply, the 820k should read 9V at one end, and approx. 5V at the other (connects to Pin 3 of the 741).
The 1Meg should have 5V at one end (741 Pin 3), and 0V at the other.

^^ Yup.   A tiny error around that 741 can easily throw things off.  So could having the diodes backwards, or the presence of a small solder bridge...ask me how I know, ha ha!  Around the socket that is common too, since the pins are close together...investigate!

Audio probes are just about indispensable!!  I suggest using a NP blocking cap with as high a voltage rating as you can get your hands on, so you could use it on everything up thru tube amps....mine has a 600V, probably .68u or something like that (higher than 1uF at that rating are bulky).   But for stomps, 25V rating is fine (so is 16V for almost everything).   As long as you're not letting DC into your probe  ;o)   

[vmaxvmax]

I built the audio probe - great tool!

I'm getting 9v on the input side of the 820k (R11) and only 1.93v on the other side, likewise with the 1m (R12) I get 0v on the ground side but only 1.93 on the other side. The resistors have the correct values.

The audio probe gives me a signal at the input that drops to virtually inaudible after R1 and on pin 2 of the 4558, then massive signal at pin 1 (4558), R5, R6 pins 5 & 6 on the 4558 and pin 6 side of R87 and through to R10 on the C7 side.  Pin 7 and into the sustain pot has an even bigger, distorted signal. At this point the 'sustain' pot seems to work as a volume pot. The other side of R10 and pin 2 of the 741 has that 'blatty or gated sound' as described in the debugging page and that sound remains to the output.

I hope this makes sense...

[Johan]

A rule of thumb when working with opamps in audio circuits is that +input,-input and output should all be at the same dc voltage. if they are not, the opamp is suspect...."rule of thumb"of course suggest there are times when things are different, but most of the time they are not
J

[duck_arse]

if you have the IC's in sockets, remove them and measure the resistance of the 820k and 1M on pin 3 of the 741. then apply power and measure the voltage at the 820k//1M junction. post results. the only other part in that area is the 220nF cap, which is a strange value for the purpose.

[edit:] we always like to look at photos, they are great helps.

[Digital Larry]

What are the stripe colors on the 1Meg resistor?  The voltage is too low, but it's not completely shorted out, which makes me think it might be 100k.

[vmaxvmax]

I have again removed the resistors R11 & R12 (just to be sure) and measured them:
R12; 1m = 993k
R11; 820k = 860k

Neither seems wildly out.   

I next removed the 741 and measured with an 8.99v rechargeable battery psu as follows:
1 - 0
2 - 3.32
3 - 4.5
4 - 0
5 - 0
6 - 8.81
7 - 3.18
8 - 0   

With the 741 in place the voltages where (my original posting had pins 6 & 7 wrong way round!):
1 - 0
2 - 2.05
3 - 1.93
4 - 0
5 - 0
6 - 8.77
7 - 3.71
8 - 0

Certainly different to the voltages with the 741 in place, particularly pins 2 & 3... I don't know what they should be but the difference looks promising?

Thanks again for you continued support - I feel we are getting there and the solution can't be too far off!

[vmaxvmax]

Ahhhhrrr!

I didn't get pins 6 & 7 wrong first time - the days have been too long...

So the pin voltages are with the 714 in place:
1 - 0
2 - 2.05
3 - 1.93
4 - 0
5 - 0
6 - 3.71
7 - 8.77
8 - 0

The current op-amps are uA 741 CP and RC 4558 NB. Can these be replaced by any 741 and 4558's respectively?

[duck_arse]

a "741" is generally a 741. the same goes for the 4558's except they are "4558". I'm not really helping, am I?

your empty pin 3 voltage looks about right, it should be a bit higher than half the battery voltage. you are, of course, putting the IC in the right way? have you a replacement you can try? a 741 or a TL071 or a LF351 or probably a couple of other single opamps I don't recall will all go in the same spot.

have you shown us any pictures of your work yet? both sides, nice and clear, up close is good.

[vmaxvmax]

I should explain, this is an original '77/'78 op-amp Big Muff version 5 with a 1322 board, not a self build.

The board, components and wiring are all as original. I haven't taken any photo's as there is nothing different to photo's etc already published.

I have ordered op-amps and caps - so I'll report back when I've started to swap parts, but I have to wait for the parts by post.

Thanks again!

[duck_arse]

ahhh, well, an old pedal gone bad, that is a slightly diffrent story. now we are looking for parts gone over to the dark side, and we always start with electros. pictures, show us pictures!

[vmaxvmax]

Replaced the 741 - it works! I'd forgotten just how dirty this pedal is. It is a beast! Totally at odds with my playing and tone now... I might yet use it though. The years have passed.

Now to replace the caps...

Thank you all for helping!