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

[Jeff C]

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!

 

[Jeff C]

It’s odd that you are getting 9V at the gate and source of Q3. Do you get signal at the drain of Q1?

Nope.

 

[Jeff C]

I haven’t analyzed this circuit but anything much higher than 4.5V on the drain of any of Q1-Q2 is probably bad news. Those are all common source so the drain is typically biased around 4.5V. If you see 9.3V that means you are at the power rail value so there is no way to get signal through. I’m not entirely sure what could be causing the problem. Let me take a look at the circuit.

Maybe the missing D100 is causing it?

 

[giovanni]

Maybe the missing D100 is causing it?

No, the diode is only for polarity protection. It doesn’t affect bias or tone. I wonder if R4 has some issue? Can you check continuity?

 

[Jeff C]

No, the diode is only for polarity protection. It doesn’t affect bias or tone. I wonder if R4 has some issue? Can you check continuity?

It has continuity to the gate of Q5 and the drain of Q1.

 

[giovanni]

Uhm what about R6?

 

[Mcknib]

That set up is a long tailed or differential pair Q1, 3 and 5 and Q2, 4 and 6 work together like a basic op amp

This isn't your usual kinda stuff you've also got gyrators making an appearance with Q7 so far from your normal frequently used pedal circuitry

Your 2nd posted op amp voltages look ok 2,3 and 6 maybe slightly high but should still work, you're only interested in pins 4 ground 7 power 2 and 3 inputs and 6 output

Anyway I'm not clued up on this circuit far from it but for probing I'd initially just probe your active device outputs Q1 drain Q5 Collector Q2 drain Q6 collector and pin 6 IC1.1

As I say not 100% I think Q3 and 4 are part of the negative feedback network forget them for now

 

[Jeff C]

Uhm what about R6?

It has continuity to the source of Q1. No other Q’s. But the same side of the R6 also has continuity to ground (?).

 

[Mcknib]

It has continuity to the source of Q1. No other Q’s. But the same side of the R6 also has continuity to ground (?).

One side should have continuity with ground the other Q1 and 3 source

Reflow Q3 solder joints they don't look great @giovanni may have directed you to hopefully your only problem

 

[giovanni]

If Q1’s source also has continuity to ground you have a short somewhere.

 

[giovanni]

One side should have continuity with ground the other Q1 and 3 source

Reflow Q3 solder joints they don't look great @giovanni may have directed you to hopefully your only problem

It’s so hard to debug over forum! So many things can go wrong in a circuit, and this one has so many components! Hopefully this is all helpful though and a good learning experience. I am learning a ton.

 

[Jeff C]

It’s so hard to debug over forum! So many things can go wrong in a circuit, and this one has so many components! Hopefully this is all helpful though and a good learning experience. I am learning a ton.

Not as much as me! I’ll get on it tomorrow! Tks, guys!

 

[Mcknib]

Yeah you learn a lot and remember some of the stuff you've long forgotten

You've just got to use a systematic approach always a visual check first then voltages

A quick resistance check between Q3 source and ground will tell you if it's not connected you should expect R6 4K7 value as mentioned before if it's infinite resistance it's open circuit or a poor solder joint

Same for Q1 continuity check source to ground

 

[Jeff C]

Yeah you learn a lot and remember some of the stuff you've long forgotten

You've just got to use a systematic approach always a visual check first then voltages

A quick resistance check between Q3 source and ground will tell you if it's not connected you should expect R6 4K7 value as mentioned before if it's infinite resistance it's open circuit or a poor solder joint

Same for Q1 continuity check source to ground

Ok, I was wrong. Q1 source is not connected to ground. Also, it’s the lower pin of r6 that’s connected to ground, so that’s ok (I checked all the ground connections as shown on the schematic (at least the first stop from ground) and they are all good. When I check resistance between Q1 source and ground the multimeter jumps all over the place, reading high and eventually settling to very low (1.2); same with Q3. Also, no continuity between r6 and Q3 source.

 

[giovanni]

Ok, I was wrong. Q1 source is not connected to ground. Also, it’s the lower pin of r6 that’s connected to ground, so that’s ok (I checked all the ground connections as shown on the schematic (at least the first stop from ground) and they are all good. When I check resistance between Q1 source and ground the multimeter jumps all over the place, reading high and eventually settling to very low (1.2); same with Q3. Also, no continuity between r6 and Q3 source.

All of that is suspicious. Q1 and Q3 sources are connected, can you check continuity? I wonder if R6 is bad or you have a short somewhere causing the reading. Either way it’s probably one of the reasons you don’t get signal at the drain of Q1.

 

[Jeff C]

No continuity between q1 and q3 sources. If I’m looking at it correctly, the trace looks pretty messy near q1.

 

[giovanni]

No continuity between q1 and q3 sources. If I’m looking at it correctly, the trace looks pretty messy near q1.View attachment 26752

If you have isopropyl alcohol, pour a bit on the board and really scrub the hell out of it with a toothbrush. All the see through gunk is flux and it can in some cases cause shorts. It’s never happened to me personally but it’s worth a shot. Also some of the joints don’t have enough solder.

 

[giovanni]

No continuity between q1 and q3 sources. If I’m looking at it correctly, the trace looks pretty messy near q1.View attachment 26752

Also those two wires next to the pot on the right look like they may be shorting? I can’t really tell from the pic.

 

[Jeff C]

You’re not gonna believe this but………..I soldered in a little bridge from q1 source to q3 source, see pic, and now I’m getting audio out to the amp!

 

[Mcknib]

Go that man

I was just gonna say did you take resistances with no power to the circuit

It'll jump around with caps discharging etc unless you let it drain for a few minutes once you disconnect power

Anyway good to hear you've got a bit of action

 

[BuddytheReow]

300+ posts and we finally got sound! It sounds like R6 is your problem area. Check your solder joints.