BE-OD troubleshooting

[slashandburn]

Hey all, so after a fairly lengthy hiatus ive been building the odd pedal again and as a result im back begging for help with this one. Built from a (verified, allegedly) PCB from i site I mostly trust from past experience.  In my haste i managed to reverse polarity on power this up first time. Likely fried a few things but nothing smoked, so i fixed poliarity and on second boot, nada, nothing.Good signal on bypass. Gathered voltages which ive since lost, and did some audio probing, at which point the 10ohm series resistor on the Vin after the protection diode burnt out. Since then i've replaced the  10ohm resistor, removed the protection diode (just jumpered those pads, swapped out all three opamps and also both of the electrolytic caps. I didnt replace the clipping diodes as none of them appear shorted. 

I'm pretty certain i have a short to ground somewhere on the signal path or just a bad solder joint, but i sure hell cant see one and ive reflowed any suspect pads i could see. Thinking tonight ill do some audio probing and likely replace the trimmer and the volume pot if ive no luck. Then ill have a few beers and while deciding whether its worth continuing to fight with this one or just dump it in the drawer of shame and move on.  You folks have brought me back from the brink a few times so i figured i'd try my luck here again before resorting to more drastic measures!

Only parts substitutions were the three Antilog pots (used logs, wired backward) and a couple of resistors series/parallel caps and resistors to get to the right value. Heres the Schematic, pcb and gutshots complete with current voltages. Bypass signal passes fine, no effect audible when engaged. Theres a few descepancies in values between the schematic and pcb B.O.M but nothing that looks circuit breaking, at least to my eyes.











Voltages
Vin.  12.5

IC1:
1.1.26
2.0.47
3.0.9
4.0
5.0
6.0
7.0
8.11.86

IC2:
1.1.39
2.0.6
3.0.08
4.0
5.0
6.0.58
7.1.39
8.11.9

IC3:
1.1.38
2.1.35
3.1.32
4.0
5.0
6.0.65
7.1.37
8.11.9

Cheers in advance!

[ElectricDruid]

There's all sorts there that looks weird, but let's start with that second op-amp on IC1. Everything is grounded? That's not right....
The power to the chip looks ok, though. The first op-amp also looks weird, but at least there's something there. I'd definitely check that the 4.5V Vbias voltage is where it should be though, because it doesn't look like it.

[alexradium]

change all the ICs

[slashandburn]

Thanks Tom!

Great shout, Vbias was zero and led me to find short to ground on that trace. We now have voltage bias that reads......2.25v. Bugger. Need to do some more poking around, I don't see anything obvious but I'll scan for some wrong value resistors and then report back with new voltages if I don't see anything.

I've labelled the ICs wrong if you compare to the schematic. IC1B per my voltages is IC3B per the schematic.

I've already swapped out the IC's two or three times. Even tried some 5532s just to check I hadn't got a batch of dud/fake 072s.

[slashandburn]

Corrected that short, I now have 5.7v Vbias on the output pin of IC1A. (shown as IC3B).  My incorrectly numbered IC's aside, Im at a bit of a loss as to what's wrong with this one.
Updated voltages:

Vin.  12.5

IC1:
1.5.7
2.2.15
3.2.05
4.0
5.0.65
6.0
7.0
8.11.79

IC2:
1.1.38
2.1.35
3.1.33
4.0
5.0
6.0.64
7.1.36
8.11.77

IC3:
1.1.38
2.0.6
3.0.08
4.0
5.0
6.0.58
7.1.39
8.11.76

[ElectricDruid]

Well, the Vbias seems a lot more healthy, so that's a good start.

But there's still a lot wrong in those voltages. Your IC1 second op-amp still looks like everything is grounded. Both the clipping stage and the following stage have the +ve input tied to Vbias, so we ought to see...well, Vbias at those pins, right!
What's going on there?

Maybe turn power off and check continuity between all the points in the circuit where Vbias is connected?

T.

[slashandburn]

I think you're right man and I'm beyond confused.  Per the layout i should have Vbias just shy of half Vin across pins 5, 5 and 7 of each IC. Yet its.not there, and somehow instead its found its way to pin 1 of the wrong IC.

I've outdone myself with this one. I'll have to look at it with fresh eyes tomorrow. Not sure how but I've really ballsed this up!

[Slowpoke101]

Have a look here;

[slashandburn]

Cheers Ian, well spotted! I'll have a proper look when I get home tonight but I'm pretty sure thats the short found the other day. The trace on the right leads to the two 10k resistors and is where I'd have expected to see half supply V.  Before correcting this short I was getting 0V, fixing only bumped this up to around 2.2V, not the roughly 6V I'd hoped. This trace leads to the 2k2 series resistor that gives or should give Vbias. The 2k2 bumps my 2.2v down to 0.6V which explains why I'm seeing 0.6v on all the pins I'd expect to see Vbias.

Getting warmer at least!

Edit: more to the point, what the hell is that 2k2 resistor doing there anyway, if the goal for Vbias is half supply V, should I ditch it and just bridge those pads? Still doesn't solve my issue and would leave me with Vbias of 2.2V.

[Slowpoke101]

OK, an interesting result. The 2k2 resistor serves as a bit more noise filtering for the Vbias voltage divider and buffer op-amp. Not really needed but if it was used in the original circuit you may as well keep it there.
The 2.2V at the voltage divider junction is a dead give away that the 2k2 resistor is shorted to ground at pin 5 of the IC but there may be no sign of a short. Take that IC out (I think you refer to it as IC1 ), apply power and measure the voltage on pin 5 of the IC1 socket. If it comes up to 1/2 supply voltage then IC1 is toast - replace it. Also consider replacing the ICs anyway as applying reverse power to them is not good for their health.

[slashandburn]

Very insightful, thanks man!

That all makes some sense, even to me! So if the 2k2 was short to ground it would put it parallel to one of the 10k's that for the voltage divider, giving me Vbias around 2V.

So I measured the empty socket, lo and behold, 6.6V on pin 5. Result! Well, sort of. So I threw in yet another new tl072, measured pin five.....0.6V. Damn. I eventually managed to find some 062's, and we're back in business, at least as far as Vbias on the right pins and traces. Just about to plug it in and see where we are.

Cheers man, appreciate you taking the time. Also apologies to Alex, I should've listened! In my defense I'd already swapped out all the ICs after the reverse voltage incident (twice I'm sure!). Either I been putting dud/fried dual opamps back in my parts drawer or I've somehow ended up with a collection of dud tl072s and 5532s.  So I've swapped them all out for tl062, Vbias there on pin 5 now at least but everything else still seems wrong. I'm seeing 13v on pin 8 and 12v on pin one of that IC. Complete voltages to follow. Short city!

Cheers man.

Edit: voltages, this time with a 9V battery

Vin.  9.5V

IC1:
1.8.74 (huh?)
2.3.28
3.4.49
4.0
5.4.71 (rejoice!)
6.0
7.0
8.9.41

IC2:
1.8.67
2.8.15
3.0.04
4.0
5.0
6.8.6
7.8.78
8.9.4

IC3:
1.1.58
2.1.58
3.1.63
4.0
5.0
6.8.73
7.8.63
8.9.4

Also collected voltages with empty sockets if that helps.
All 0V except:

IC1
3. 4.53
5.  4.7
8.  9.52

IC2. 8. 9.52

IC3.
1. 0.1
2. 0.6
8. 9.52

[ElectricDruid]

I'm still not seeing Vbias on either pin 3 or pin 5 on IC2, so I'm not yet convinced that Vbias is ok. Perhaps it's alright at the output of IC3B, in which case...why's it not connected to where it should be? Have you checked the continuity of the Vbias tracks from IC3B pin 7 out to each of the points where it should turn up?

[slashandburn]

I've mislabelled my ICs! IC1B per my voltages should be IC3B on the schematitc.

Vbias on there on pin 5, and sort of nearly there on pin 3.  Still not quite right though!

I'll have another look when I get home from work.

[Slowpoke101]

You are making progress. Isn't troubleshooting fun ?......

Your Vbias rail is still unwell. IC1B (pins 5, 6 & 7 ) is the buffer for the Vbias rail. Pin 5 (which goes to the voltage divider ) is now at the correct voltage but pins 6 and 7 are at 0V. Remove power and set your multimeter to a low ohms range. Referring to the picture below check the track highlighted in green (Vbias ) for a possible short to ground. The yellow arrows point to possible shorts. Another thing to keep in mind is that the Vbias rail also goes to the "Tight" pot - check that there are no stray wires shorting to ground.



The track highlighted in blue is the voltage divider rail. Pin 3 of IC1A is biased from this rail (and not the Vbias rail ) through a 10K and a 470K resistor (red line in picture ). This means that pin 3 has the correct voltage voltage applied to it but there is something odd here - pin 2 is connected to the Vbias rail which is not working correctly so the output of the op-amp (pin 1 ) will not be at the correct voltage. The voltage that you are currently measuring on pin 1 is correct given the circuit conditions. Get the Vbias rail working and all the other voltages will fall into line.

One other thing to keep in mind is that the schematic that you are referring to is incorrect. There are some errors, none of which are serious but a more accurate diagram can be found in this document ; https://www.pedalpcb.com/docs/ThermionicDistortion.pdf  . The component reference numbers will not be correct but the schematic is.

[slashandburn]

Nice one! We're in business.

I looked over those traces and couldn't see anything but reflowed them anyway then ran a knife between them just to be sure. Damn thing fired right up!

Thanks again! Just in time for the weekend too. Think I'm gonna go give it a whirl and annoy my neighbours a bit before I leave for work!