Blues Driver BD-2 Debug Help

[ListenLoudly]

1&2. Just got this Boss BD2 Blues Driver built on perfboard based on this link.
3. I followed the layout exactly: front of board & back of board
4. My only part substitution was a 1k1 resistor in place of the 1k2 resistor at the top right of the board.
5. Regular negative ground circuit.
6. No sound with volume all the way up and gain low. As gain is increased I get oscillation starting at about 1/4 turn of the pot, and it gets worse as gain is cranked. I don't get any audible guitar regardless of the pot settings.
7. Powered with DC jack. Measuring 9.3ish V on power lead, 0 on ground.
Here are the rest of my voltages:
Q1: G - 4.54 S - 5.86 D - 8.68
Q2: G - 5.55 S - 5.86 D - 9.33
Q3: E - 9.33 B - 5.86 C - 5.56
Q4: G - 4.22 S - 5.46 D - 8.68
Q5: G - 4.61 S - 5.46 D - 9.33
Q6: E - 9.33 B - 8.68 C - 4.61
Q7: E - 3.57 B - 3.99 C - 9.33

IC Pins:
1 - 0.15
2 - 4.64
3 - 4.43
4 - 0.002
5 - 0.15
6 - 4.64
7 - 9.33
8 - Starts at 0 and slowly creeps up in mV

I've taken an audio probe through the board. The green circled areas in this pic do not produce an audio signal. I've also re-wetted a few leads that didn't look so good at first, but that didn't do anything. Also, I swapped out the TL071 chip just to check, and also quadruple checked the transistor pinouts with a Peak Atlas DCA55.

I've posted on the tagboard site as well, but figure there's more people on here. I really appreciate any help! Doing this build for my little brother for Christmas, so hope to get it debugged and boxed soon!

[ListenLoudly]

Just realized I haven't measured voltages at the diodes. I'll post those shortly.

[duck_arse]

listen - can we have a circuit diagram, please? one that matches your layout designations would be best. then we can know where the audio probe will produce.

your transistor voltages are bad. each base voltage needs to be ~600mV higher than the emitter voltage. and the collector needs to be a coupla volts higher than the base for anything interesting to happen.

[edit :] erm, maybe ask a moderator to move this thread up a level, outta members only (they are a stuffy lot, those members ..... ).

[ListenLoudly]

Thanks for tip, duck. I'll try to get it moved up a level.

I was able to track down the schematic. The only difference I see is C9 (1u) in the schem is an elec, but I used a film like the perf layout suggested.

I have tried to trace the audio signal (my yellow dashes), but I don't understand how to trace the signal through active components. I took a guess at the IC based on the pinout. I think I could tackle this with the audio probe a bit better if I knew how to finish tracing the signal. Any tips to trace the audio path through active components?

Thanks!

[duck_arse]

well now, I'm no expert in the use of the audio probe, as I use a sig gen and oscilloscope. this is where we need GibsonGM to appear magikally ..... also, my pc shows your yellow dotted as cyan dotted, but nevermind.

basically, you put a signal at the input, like an mp3 player (I think that is the modern equivalent) or cd player or tranny, then connect the pick-up to your amp (turn down low, probe, then turn up to monitor, each time) and probe "the parts". your dotted line is the right route to take, there should be signal pretty well all along except pins 2+3 of the opamp, where funny virtual earth things happen.

now you have to go back thru yr voltages as well. Q3 base is tied to Q1 drain: they must be the same voltage or they are not connected correctly. you can suely see that the Q1, Q2, Q3 section is the same/similar to the Q4, 5, 6 section. and the 4, 5, 6 section voltages look good(er) at a glance, so use them as a guide to the Q1, 2, 3 volts.

I think Q7 base is a bit low as well ...... I'll have another look at your board overnight.

[GibsonGM]

*poof*  Hi guys, how's it going?

Yes, DA is 100% spot on, the voltages are way off. So,  I would make finding out why a priority - too much abuse, and the FETs could go down, complicating things.  BTW, the yellow dashes do show where you want to probe.   

Thru an active device such as the opamp, you will want to listen to pins 3 (in +) and 5 (out), then maybe pin 2 (in -) to see what's happening in the feedback loop.  As long as your probe has a series capacitor in it, you won't do any harm placing it anywhere (but do NOT touch more than 1 point at a time, or you can short out the IC, etc!!).


Same process for listening "thru" transistors...just check the input (usually the base, and the gate for FETs) and the output (usually the collector, but sometimes the emitter!  Same thing for FETs, input on gate, output on D or S, the schematic tells you).

If the device isn't working, you will get nothing at the output; or perhaps, a section that should amplify will be quieter than the input.  Be aware that often FETs and BJTs are used as buffers, which can have less than unity gain!  Those would tend to have the output at the S or the E.

How far are you hearing your signal to? Did you socket anything, such as the transistors?  Are you SURE the FETs/BJTs are installed using the correct pinout?

OK, homework, to get going on voltages....please check your grounds; make sure all points that should go to ground do so, and check your reference voltage - see that at the Vr end of components that connect to it, you get nearly 4.5V. 

[ListenLoudly]

Thanks for the tips, Gibson! Just got done going through everything and here's what I found:
-All grounds are showing less than 3mV
-All Vref points are at 4.64v
-Most 9v connections (those shown on the schematic) are at 9.33v. The ones that differ are after R3 (8.7v), and the base of Q6 (8.87v).

Using the audio probe here's what I hear:
-unity to Q1 gate
-quieter at Q3 collector
-gets louder at C8 (just before Q4 gate), but not quite back to unity
-very quiet at Q6 collector
-a bit louder at Tone 3 than at Q6, stays constant to IC3, still not at unity
-IC2 is insanely loud
-IC5 is back to unity
-IC6 (which looks to be the out to me) has no volume at all

I have swapped out the opamp to make sure it's OK...unless both are bad somehow. I'm positive of the FET and BJT pinouts, but I did socket them so I could try others.

You guys got any other ideas??? I've checked the layout and component values several times already. Maybe a bad solder joint? I have reflowed several dull looking ones, but I could probably go over it again. I don't typically have that issue though.

Thanks again!
JG

[Quackzed]

looking at this schem, one side of the opamp output is/should be the same as the negative input, (see how the negative side of the triangle is at the same spot as the output of that opamp?)so either ic1 and ic2 are the same V or ic6 and ic7 should read the same V... (which side of the opamp is which, i'm not sure...)
i'd guess if your output is where the signal stops, trace back from the output... even jumper the last bunch o sh*te, after the opamp output ic1?(or ic 7?)  there is a 10uf, try jumping after that 10uf right to the output jack's tip... you should get signal...

[duck_arse]

your transistor voltages are bad. each base voltage needs to be ~600mV higher than the emitter voltage. and the collector needs to be a coupla volts higher than the base for anything interesting to happen.

when I say this, of course, I'm blindly assuming npn's. as thePNP Q3 has its E tied to V+, its B will be about 8V3, give or take. that means the Q1 drain will be the same. seeing as the Q3 C has a DC path to the gate of Q2, gate and collector should be very nearly the same reading. can you give another measure on the Q3 pins? maybe do a swap of this transistor, check your volts again.

if you "inject" at C1 and listen at R7, you should have similar to if you inject at C8 and listen at R17. if you inject at the top of the level pot, you should be getting signal at the op-amp pin6 and output at out. I'm buggered if I know what Q7 does, except maybe dynamic tone-shelf-rubber diode stuff.

[Transmogrifox]

I'm buggered if I know what Q7 does, except maybe dynamic tone-shelf-rubber diode stuff.

Q7 is part of a gyrator, which is a simulated inductor.  This just creates something of a low-frequency recovery (looks like a broad hump centered at ~120 Hz).

I am most interested in the first stage surrounding Q1, Q2 and Q3.  The voltages show it to be unstable, or you have the wrong resistor value for R7.  If you had something like 22 ohm, or even 220 ohm you may have this problem.  Measure that one and confirm.

[ListenLoudly]

Well, guys...I just took another look at this thing (I couldn't for a bit because of work and then Christmas stuff). I appreciate all your input. I checked all your suggestions, and then realized i swapped my input/output wires from my testing pedal. As soon as I changed them everything was working great. So dumb, but I guess it's better than some larger issue. Thanks again for all your help!

Jeff

[duck_arse]

so now that is is working correctly, can you remeasure your transistor and ic volts, just so we can see - or is it, as is the usual case, buttoned-up and gone?

[ListenLoudly]

No problem - I should have posted this. The new values aren't that different except for the B of Q3.  It sounds great, but these voltages still don't seem to make sense based on your suggestions.

Q1: G - 4.57 S - 5.89 D - 8.73
Q2: G - 5.58 S - 5.89 D - 9.39
Q3: E - 9.39 B - 8.74 C - 5.60
Q4: G - 4.25 S - 5.54 D - 8.87
Q5: G - 4.70 S - 5.54 D - 9.39
Q6: E - 9.39 B - 8.87 C - 4.71
Q7: E - 3.63 B - 4.03 C - 9.39

IC Pins:
1 - 0.16
2 - 4.67
3 - 4.46
4 - 0.002
5 - 0.16
6 - 4.67
7 - 9.39
8 - cycles b/t 0 and  -0.2 mV

[Transmogrifox]

Voltages look perfect to me.  Before I didn't see anything really badly wrong except some showed the evidence of what a DC meter might pick up on an oscillating circuit.

I think you might have caught some confusion about the difference between what is expected from a PNP vs NPN transistor. 

With PNP such as Q3, voltage from Emitter-Base should be somewhere between 0.5 and 0.7 volts.
On an NPN this should be 0.5 to 0.7 volts Base-Emitter, such as Q7.