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]

You’re telling me! I think I’m just gonna pull that op amp out, put a socket in and a new op amp. What else to do?

 

[giovanni]

You could try but I don’t know that we have enough evidence that the opamp is busted. We don’t have evidence that it works either tho!

 

[Jeff C]

You could try but I don’t know that we have enough evidence that the opamp is busted. We don’t have evidence that it works either tho!

What would be a better next step?

 

[giovanni]

What would be a better next step?

Ideally identifying the cause of the ghosts you are seeing

 

[Jeff C]

 

[Mcknib]

You shouldn't get audio on anything coming directly from a voltage source ie power section you will sometimes get what I call bleed through audio, although I don't know what it is or why, it's usually very distorted

Pin 8 of your op amp for example I learnt this from another debug too my great surprise, regardless of whether you get audio it's still not part of the audio path

The audio path is more or less a straight line from in to out carrying and effecting your guitars small AC signal e.g. amplifying it filtering it boosting it etc mainly done by your active devices

Ground side you should get no audio

Your earlier op amp voltages looked ok

I think you should do your audio probe again with a printed schematic start at the ouputs of your active devices Q1 drain Q3 source (I think) Q5 collector Q2 drain Q4 source Q6 collector IC1.1 pin 6

Tick each one that has audio on the schematic

This will very quickly highlight problem areas I do it this way because first I'm lazy and second it's quicker if you get no output on an active device you'd then trace back to the input and so on till you find where audio stops or drops level

It's a lot less confusing and if you mark it on the schematic you'll see a clearer picture

Although feedback loops filters etc can stop audio I ignore them initially until I find an active device with no output then I'll go back to its input and everything in line before it until I find my problem area

I'm no expert on differential pairs so I'm not 100% on Q3 and 4 output

All I know is the difference between the inputs is multiplied and the actual amplifiers of the 2 are Q1 and 2 so you can teach us by letting us know if there's audio and if theres a difference between Q1 and 2 drain and Q3 and 4 source pins

 

[giovanni]

Q3 and 4 are part of the feedback loop of the amplifier stage (I consider Q1, 3 and 5 to be one stage; 2, 4, 6 another) together with 5/6. There should definitely be signal on the source and gate of both Q3 and 4 but not on the drain (grounded for signal). Basically Q1’s gate acts like the + input in an op amp, Q3’s gate as the - and it’s used for shunt feedback. Q5 (and 6) provides further amplification (common emitter stage) after which the voltage is fed back to the - input.

 

[Mcknib]

Q3 and 4 are part of the feedback loop of the amplifier stage (I consider Q1, 3 and 5 to be one stage; 2, 4, 6 another) together with 5/6. There should definitely be signal on the source and gate of both Q3 and 4 but not on the drain (grounded for signal). Basically Q1’s gate acts like the + input in an op amp, Q3’s gate as the - and it’s used for shunt feedback. Q5 (and 6) provide a buffer after which the voltage is fed back to the - input.

I knew it was a basic op amp set up could see the collector feeding the gates but just wasn't sure if source would output audio coz my head hurts! I've kinda lost track

That explains it nicely cheers @giovanni

 

[Jeff C]

Very helpful! I’ll work on it. Tks, guys!

 

[Jeff C]

Ok, been doing alot of probing and testing. Did continuity testing on all points consecutively across the schematic. All good there (unless the partial continuity noted below from Q3 matters). Then checked for audio with the probe on every point. While I get at least very faint audio almost everywhere, the good solid audio signal stops after the source pin on Q3. There is no audio on drain or gate at Q3. Could that be the source of my problem (pun intended)? If so, should I just reflow the Q3 solder and see if that works? If not, moving on: From my amateur reading of the schematic, from Q3 should I not get audio on one or more of r8, c4, r10 and C5? I get no audio on all but C5 (but as noted, continuity is there), and very faint audio on C5. Some audio on the gain pot. Everything beyond that is very low or in some cases none. So, if Q3 is not the problem, am I correct that the problem may lay somewhere between Q3 and those points?
There is some continuity between Q3 and those points but not from all 3 pins: Drain connects only to the bottom pin of C5 and both pins of R10. Source connects only to the top pins of C5 and C4. And Gain connects only to the bottom pins of C5 and R10, and the top pin of R8. So, in other words, it seems that all three Q5 pins connect to one of the C5 pins, only drain and gain connect to R10; only source connects to C4, and only gain connects to R8.
Whew! The beat goes on.

 

[BuddytheReow]

Q3 seems to be your problem area for now. Reflow your solder joints there and check for continuity between Q3 and each successive point. My tidbit of advice for troubleshooting is to start at the input and fix all your problem areas (working through the schematic) as you come across them until you reach the output. There's no use trying to check audio signal on your opamp (end of circuit) if one of the first JFETs (beginning of circuit) isn't passing signal correctly

 

[Jeff C]

Will do. I wonder if the bridge I put in between the source of Q1 and the source of Q3 may also be a problem (it‘s in a previous pic in this saga - around the time Caesar defeated Vercingetorix at the battle of Alesia and took him back to Rome to be quietly strangled in the tiny Mamertine Prison on the Capitoline Hill……)? I don’t think it bypasses the gate (the signal comes first to the source, right?), but just asking. I’ll resolder all three Q3 pins and leave the bridge off and see what happens……

 

[giovanni]

You should get no audio on the drain of Q3. It you don’t get audio on the gate, you won’t get audio on the source either. Do you have audio on the base of Q5? If you don’t, there can be no audio on the gate of Q3 either. So that could be the problem. There is a big network between the collector of Q5 and the gate of Q3 and all those components should have signal.

 

[Mcknib]

That would definitely lower the level as @giovanni pointed out it's part of the feedback network of a basic op amp set up

Probe from Q5 collector through each component to Q3 gate and see if level drops at any component or fails to get through

 

[Jeff C]

I’m on it…..

 

[Jeff C]

You should get no audio on the drain of Q3. It you don’t get audio on the gate, you won’t get audio on the source either. Do you have audio on the base of Q5? If you don’t, there can be no audio on the gate of Q3 either. So that could be the problem. There is a big network between the collector of Q5 and the gate of Q3 and all those components should have signal.

Well, I get audio on the Q3 source but not the gate, thus my question about the bridge. Should I have connected it to the gate instead?

 

[Jeff C]

View attachment 26975

Excellent

 

[giovanni]

Well, I get audio on the Q3 source but not the gate, thus my question about the bridge. Should I have connected it to the gate instead?

Nope. Check the schematic for reference. So I think you have a problem with Q5 or with the network that takes the signal from Q5 back to Q3. Did you check if you have audio at the base of Q5?

 

[Jeff C]

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?

 

[BuddytheReow]

Have you retested the voltages on Q3?