Milk Fuzz (Demo Tape Fuzz) Build Not Working - Now Fixed!

[rmjlmartin]

Hi, and thanks in advance for your help.

I'm building a Demo Tape fuzz clone called a Milk Fuzz. I'm not sure exactly where the pcb came from, since I got it from an acquaintance who builds pedals. I've attached the build document that came with it. I snapped a picture with my phone, but it's way too large to upload, so I'll try to take a smaller one tomorrow and upload it, if need be.

It's all assembled, and it passes signal in bypass mode, but with the effect engaged, there's no output.

I've checked for voltage and traced the circuit with an audio probe. The last point that I can get a signal is at what I believe is the input (pin 3) of the first stage of the op amp.

The op amp has 8.8v across pins 4 and 8 (4 being negative, and 8 positive), 4.22v on pin 3, and 8.13v on pin 1. If I'm reading the schematic correctly, pin 3 should be the input of the first stage, and pin 1 should be the output of the first stage. Is that correct?

I asked the Google, and I came up with a lot of info, but I'm a bit out of my depth trying to troubleshoot this. I did try another op amp and tried this one in another pedal. The second chip didn't make any difference, and this one works in the other pedal, so I guess that means this one is ok and the problem is in the board somewhere...

I've built and modded a few pedals, so I'm not a complete noob, but I don't completely understand how everything in the circuits work, either. Any help and/or advice would be much appreciated!

[rmjlmartin]

Here's a picture of the build.

[MadMike]

Is it possible to show the bottom of the PCB? Tough to say without seeing the connections.

Here's some general stuff I'd check:
Is the chip in the right direction?
Is either chip getting hot or did they overheat at anytime before?

Check visually and with a multi-meter for solder bridges.

There could also be a problem with the 3pdt switch. Have you tried running the FX signal directly to the output or are you just flipping the switch off to get the bypass signal?

Cheers. Good luck!

[MadMike]

Also the - + connections at the bottom look really close together...

..Also, I am I wrong to say it's normal for the voltage to be 0 at the input on an inverting amplifier. I believe that is called 'virtual ground'.

[matmosphere]

I think you switched r4 and r5.

[rmjlmartin]

You got my hopes up, but no, R4 and R5 are where they should be. I did discover something equally embarrassing, though - I double checked all the resistors and caps, and discovered that I managed to install C5 backwards. But turning it around didn't change anything.

I'll try to get a picture of the other side of the board up later.

The chip is installed correctly, and it doesn't get hot. As far as I know, it has never been hot. It's new, and I used a socket, so I haven't soldered on it. It also does work in another pedal.

I have checked for solder bridges,  but haven't found any. And, the + and - wires are not shorted to each other.

Like I said, I followed the circuit with an audio probe. Also, jumping between the input and output wires at the board with the effect engaged will pass the signal through.

Thanks for the responses so far. I appreciate it!

[reddesert]

OK, step 1, is to check DC levels at the dual op-amp.  Pin 4 should be V- (0V relative to ground), Pin 8 should be Vsupply, about +9V relative to ground. +8.8V is close enough. However pin 1 at 8.1V is not where it should be.  The second step is to check the AC (audio) signal, you've done that, and it also indicates a problem at pin 1, the output.

Pin 3's DC level should be held at Vsupply/2, or about 4.5V, by the bias resistors R2 and R3. We should expect the DC levels of pins 2 and 1 to also be about 4.5 V (because an opamp is a machine for equalizing the inputs, and because we want our audio signal to oscillate around the 4.5 V level for headroom).

A similar logic holds for the second opamp - pin 5 is at Vb = 4.5V, and pins 6 and 7 should be at a similar voltage.  But we need to solve the problem of the first opamp.

You have 8.1V at pin 1 (output), and no signal.  Pin 1 is connected to pin 2 through R4 and then to ground though R5, C2, and the trimmer. There is no DC path from pin 2 to ground, which is a little unorthodox maybe, but also true of the original Demo Tape Fuzz.

If pin 3, the +input, is higher than pin 2, the -input, the op-amp will push the output high, up to as close to Vsupply as it can get. That seems to be happening. With no headroom, you may get no signal output at pin 1. The voltage on pin 2 would be informative.

My guess is that something is wrong in the R4-R5-C2-trimmer chain, possibly something bridged or unsoldered. You can try taking DC voltages all along that chain, testing for continuity, and reflowing joints.

[rmjlmartin]

Thanks reddesert, that's very helpful. Something like that is what I was hoping to get. Understanding what should be there and hopefully how the circuit works goes a long way toward being able to troubleshoot the problem.

Pin 2 had 2.55v on it, which is obviously way too low if it's supposed to be about the same as pin 3.

And it's solved. And I'm fairly embarrassed. (Maybe I'm still a bit more of a noob than I thought! )

I went over the R4-R5-C2 part of the circuit again thoroughly, and discovered that I used the wrong capacitor. C1, C2, and C3 are all supposed to be 1uf ceramic capacitors. I used tantalums, and didn't realize that tantalums are polarized.

I don't remember if I ordered the tantalums on purpose, thinking that they're supposed to be very good quality caps, or if I meant to order the ceramics and clicked on the tantalums by mistake, but however it happened, that's what I ended up with.

I swapped them out for some film caps in all 3 slots (they don't really fit in the space, but they're in there well enough to test), and it works. Pins 3, 2, and 1 on the op amp all have approximately 4.5v on them, and the pedal seems to work.

And again, thanks to all of you that took the time to read this and offer your advice, especially reddesert. All of you rock!

[reddesert]

Great!