SOLVED Cobalt Drive problems…..SOLVED……..FINALLY!

Jeff C

Jeff C

Active member
New member here and also new to building pedals, as you will shortly see. I previously built a BYOC Reverb pedal kit successfully and it sounds great. I’ve also built from scratch a Strat, a Tele, a 59 LP, and an LP Junior. So, I wanted a blues driver pedal and thought I’d take a whack at the Cobalt Drive from PedalPCB. The directions are not as detailed as in a kit so I’ve had some difficulty. Clearly in over my head. I managed to procure the parts from the list (not without trial and error on my part) from Tayda. I mistakenly bought a switch without solder lugs so after consulting a patient, helpful friend, bought a daughter board and did my best to figure out how to marry the two - having no skills whatsoever in electronics. History major. I have managed to assemble it but I get nothing from it except static pops when I plug it in. Bypass doesn’t work. No idea what I’ve done wrong but I suspect it revolves around needing to do more with the daughter board on the switch. But what do I know? Here’s a couple of pics (the power jack is not there but I did connect it to test). And I haven’t installed the LED yet. Any and all suggestions are eagerly welcomed. You may recomemd I should just start over but I hope not….. Thank you!
855EE297-D464-47F8-880D-F0D8B9C40E15.jpeg 15E06C0D-93DB-4AE9-944D-BA1BED3416FB.jpeg
 
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Jeff C said:
Reflowed. Tried without the bridge. No dice. Put a new bridge in. Audio to Q3 restored but still no audio at gate. Audio now pretty good at all the pots though (!?). 🤔🤔 Should I put a bridge between source and gate on Q3?
Click to expand...
Don't put a bridge between source and gate! That would completely mess up the circuit. I feel like the audio you are getting may come from ground loops/components acting as an antenna rather than the working parts of the circuit. Keep doing the debugging in order without shotgunning solutions :)
 
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Jeff C said:
Well, I’m supposed to get audio on gate, right? So, that seems to be where the bug could be, so how to address it if reflowing didn’t work?

no, but will
Click to expand...
The audio on the gate comes from Q5. So you should focus on that. You should have signal on the base and the collector. Then on the various component connecting the collector back to the gate of Q3. While you’re at it, measure voltages on Q5 too.
 
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I get signal on base and collector at Q5 now. also, Q7 collector but not on base or emitter, but lots of static. Nada on Q6. Voltage: Q1 drain: 7.8; source: 4.28; gate: 4.55; Q3 drain: 9; source: 4.27; gate 0.33; Q5; drain: 9; source: 7: gate: 0.33.
 
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Q7’s collector should not have signal. The fact that you are getting signal there but not on base or emitter means that you have a ground loop where the signal is coming in. I would neglect this issue for now, we have bigger fish to fry.
It seems that Q5 is working as intended. Next I would make sure the network between it and the gate of Q3 is good. Reflow connection if needed and test various points with the probe.
 
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Q5 left to right is emitter base collector

Did you check audio from Q5 collector through C5 to Q3 gate

I'm assuming Q5 collector is 0.33, the same as Q3 gate, emitter 9 and base (middle pin) 7v?
 
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The continuity trail from Q5 to Q3:
Q5 collector: has continuity to the top pin of C7 and C5 (also, btw, to two right bottom pins of gain pot). But not directly to Q3 gate. C7 top pin connects to a C5 top pin. C5 bottom pin has continuity to Q3 gate. (is this how Q5 collector signal gets to Q3 - passing from top to bottom pin of C5?).
Q5 emitter: has continuity to bottom pin of C5 and to Q3 drain.
Q5 base: has continuity to bottom pin of C7 only.

yes, you are correct on Q5 voltages. Q3 voltages are same except base has 4.3v.
C5 has .33v on both pins. C7 has .33 on top, 7v on bottom pin. R8 and R10 have .33 on both pins. c4 has .33v on top pin, zero on the bottom pin. (Gain pot has .33v on bottom pins, 5v on upper pins.)

Audio probe: One thing that drives me mad about this is that I always seem to get some different reading each time I start up again. And of course that confuses the crap out of you patient helpers. This morning I’m getting audio on Q1 gate but nada on source or drain. And no audio anywhere on Q3 or 5. Continuity from source to source Q1-Q3 is fine. Aaarrggh. Wait, check that. Didn’t have the power on. 🤪
Let me start over: audio on source of Q3 and base of Q5 and bottom pin of R7 (slightly less on Q5 and R7). Faint or nil elsewhere on the Q5 to Q3 trail. C7, not R7.
 
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Jeff C said:
The continuity trail from Q5 to Q3:
Q5 collector: has continuity to the top pin of C7 and C5 (also, btw, to two right bottom pins of gain pot). But not directly to Q3 gate. C7 top pin connects to a C5 top pin. C5 bottom pin has continuity to Q3 gate. (is this how Q5 collector signal gets to Q3 - passing from top to bottom pin of C5?).
Q5 emitter: has continuity to bottom pin of C5 and to Q3 drain.
Q5 base: has continuity to bottom pin of C7 only.

yes, you are correct on Q5 voltages. Q3 voltages are same except base has 4.3v.
C5 has .33v on both pins. C7 has .33 on top, 7v on bottom pin. R8 and R10 have .33 on both pins. c4 has .33v on top pin, zero on the bottom pin. (Gain pot has .33v on bottom pins, 5v on upper pins.)

Audio probe: One thing that drives me mad about this is that I always seem to get some different reading each time I start up again. And of course that confuses the crap out of you patient helpers. This morning I’m getting audio on Q1 gate but nada on source or drain. And no audio anywhere on Q3 or 5. Continuity from source to source Q1-Q3 is fine. Aaarrggh. Wait, check that. Didn’t have the power on. 🤪
Let me start over: audio on source of Q3 and base of Q5 and bottom pin of R7 (slightly less on Q5 and R7). Faint or nil elsewhere on the Q5 to Q3 trail. C7, not R7.
Click to expand...
And there is continuity between Q1 drain and Q5 base and emitter. But no audio on Q1 drain. Shouldn’t that path be how the audio gets to Q5? I have audio on Q5 base but it ain’t coming from Q1. Scratch that. Getting audio on all 3 Q1 pins.
 
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Q3 drain and Q5 emitter would have continuity both being connected to VCC they'll have continuity with everything connected to VCC

The shop pcb image shows some traces

CobaltDrive_copy_408x480.jpg

Check Q5 base and emitter pins don't have continuity is it definitely 2N3906 you're using

You shouldn't have continuity from Q1 drain to Q5 emitter if you're continuity probing directly on the transistor pins
 
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Mcknib said:
Q3 drain and Q5 emitter would have continuity both being connected to VCC they'll have continuity with everything connected to VCC

The shop pcb image shows some traces

View attachment 27033

Check Q5 base and emitter pins don't have continuity is it definitely 2N3906 you're using

You shouldn't have continuity from Q1 drain to Q5 emitter if you're continuity probing directly on the transistor pins
Click to expand...
With the power on, base and emitter DO have continuity. Weirdly, only when I touch them on the bottom side. When I test directly on the pins on top they do not…….. With the power off, they do not either way.
now when I check q1 drain to emitter with power on, there is no continuity, but there is continuity between drain and base (With power on or off).
it is definitely a 2n3906.
 
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Jeff C said:
And there is continuity between Q1 drain and Q5 base and emitter. But no audio on Q1 drain. Shouldn’t that path be how the audio gets to Q5? I have audio on Q5 base but it ain’t coming from Q1. Scratch that. Getting audio on all 3 Q1 pins.
Click to expand...
Are you sure you are measuring always in the same condition? Keeping your ground the same? There is no other path for audio to get to Q5’s base other than through Q1’s drain. All 3 pins of Q1 should indeed have audio so that’s good.
 
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0.33v on Q5-C. 4v on Q6-C. IC1: 3.31v; 2: 5.6v; 3: 5.58v; 4: 0v; 5: 3.52v; 6: 0v; 7: 9v; 8: 0v.
well, I’ve done a ton of visual inspection but am a rank amateur. There are various photos earlier in this Tolstoyan saga…..but all help is welcome!
 
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The first stage (Q1, Q3 & Q5) is not working.

The 2nd stage is working.

IC1:
pin 1 don't care
pin 2 is too high, should be close to +4.5V
pin 3 is too high, should be close to +4.5V
pin 4 0V is correct
pin 5 don't care
pin 6 0V? Really? Makes no sense.
pin 7 9V is correct
pin 8 don't care

If pin 6 was really at 0V, pin 2 would also be very close to 0V.

Recheck your measurements on IC1 and only measure pins 2, 3 & 6.

Let's try to figure out why the 1st stage is broken. Measure & report these voltages:
Q1-G
Q1-S
Q1-D

I don't see any good pix. If you can't read the IC part number in the pic, it's not good enough. I'd like to help, but you need to meet me half-way.
 
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Sorry, I see the data now.
Jeff C said:
I get signal on base and collector at Q5 now. also, Q7 collector but not on base or emitter, but lots of static. Nada on Q6. Voltage: Q1 drain: 7.8; source: 4.28; gate: 4.55; Q3 drain: 9; source: 4.27; gate 0.33; Q5; drain: 9; source: 7: gate: 0.33.
Click to expand...
Q5 is a BJT, it has E-B-C. Going by the other measurements, E = 9V (same node as Q3-D), B = 7.8V (same node as Q1-D). That leave C = 0.33V. Q5 is not working. The difference between E&B is too large and the collector voltage is way too low. Are you sure it's a 2N3906? Better check Q5 again.
 
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