MiniTaur v3.1 - It only took 10 years...

[mmlee]

...and of course it didn't work.

I was tidying up the workbench and found this Klone half finished and then found a random bag with all the missing parts in it.

I don't know if it was past me or current me that's screwed up here but any help appreciated.  It's outputting at a very, very low volume in both bypass and on.

Photos of the board can be found here:
https://photos.app.goo.gl/Ei6HHkGNMeM9cRZ77

Voltages of IC's as follows:
IC1 - Bottom TL072
1: 4.86V
2: 4.86V
3: 3.73V
4: 0V
5: 4.85V
6: 4.84V
7: 4.80V
8: 9.72V

IC2 - Middle TL072
1: 4.84V
2: 4.86V
3: 4.85V
4: -9.13V
5: 4.86V
6: 4.86V
7: 4.88V
8: 17.35V

IC3 - Top TC1044SCPA
1: 9.74V
2: 4.88V
3: 0V
4: -4.57V
5: -9.13V
6: 7.12V
7: 7.09V
8: 9.73V

Looking at the schematic it looks like the voltages for IC1 aren't right and should match IC2 - should I swap this out?

I've been through and reflowed all joints and ran the iron between all pads to make sure nothing was shorting too.

Thanks all!

[mauman]

The power on IC1 and IC2 are correct, IC1 runs on +9V and ground, while IC2 runs on +18V and -9V for more headroom.  The voltages on IC3 also look OK. 

I think the key is that the output is low both during bypass and when it's active.  That narrows it down to the following, which is the buffered bypass path, along with the foot switch wiring.  You might want to use an audio probe to see where the signal falls off.

• input jack
• 2 resistors and a cap on the input side of IC1
• IC1 which is the buffer
• 4.7u cap, 560R and 100k resistor in the bypass path (unlikely)
• 68k and 100k resistors at the output
• output jack

[mmlee]

Thanks Mauman, checked everything out and audio traced and found out why I had no real output - I had no continuity between the output pad and the output 100k/Switch 5.  Looks like the output pad had detached from the PCB - Jumpered to the wire and I had proper bypass again.

Alas, it's not all good news, when the effect is engaged I've basically got a spluttery fuzz pedal.

The signal seems to get messed up after the 47k/before IC2. 

[mauman]

After that 47k, there are two loopback paths, one to the gain pot lug 2, and one to the output of IC1.  I'd check everything in those paths, if the signal is good up until the 47k.  IC2 could also be faulty, try pulling it and see if the signal is good all the way up to it's empty socket.

[mmlee]

Thanks again for your help so far.

I had figured it might be something in the feed forward paths so that was going to be my next step.  Have checked all the components in those paths and they all seem correct although I did change out one or two components as their tolerances were quite far off their proposed value.

Unfortunately, it still sounds messed up from the same point.  I've pulled IC2 now and put a socket in.  With no IC2 in place I get seemingly correct sounding audio but as soon as I put an IC in, of which I've tried several TL072's and a couple NE5532's, it sounds wrong again.  I've checked all components around this IC now as well and they seem to be correct.

Connecting directly to the output I'm getting a high pitched whine, oscillation that interacts with the guitar signal.  I'm guessing IC2 is clipping too hard and latching up from the summing of the two feed-forward networks and the main gain stage.

[jessenator]

I can't quite make it out from the pictures (sometimes the printing on ICs don't like being photographed), but is your charge pump the "S" spec/SKU? I.e. TC7660S or 1044S? Sorry if that's there and clear and I cant see it.

The first klones I made were using the non S sku and they will heterodyne/whine like mad if the switching frequency isn't high enough (out of audio range). Also ensure that pins 1 and 8 have continuity. Even of you have an S spec, you'll need those bridged to "boost" the frequency.

I've taken to just using the 7660H, where the frequency is in the >100kHz range, no boost required.