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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Chuck D. Bones said:
Sorry, I see the data now.

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.
Click to expand...
will do!
 
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Chuck D. Bones said:
Sorry, I see the data now.

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.
Click to expand...
I didn’t notice the EB voltage being so high! Chuck to the rescue!
 
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Either there's a bad connection or Q5 is broken. The voltages on Q5 should more-or-less match the voltages on Q6. I've had no problems with Tayda transistors.

Maybe there is a short from Q5-C to gnd. With the power off, measure the resistance across R12. Remove Q5 and measure R12 again.
 
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Chuck D. Bones said:
Either there's a bad connection or Q5 is broken. The voltages on Q5 should more-or-less match the voltages on Q6. I've had no problems with Tayda transistors.

Maybe there is a short from Q5-C to gnd. With the power off, measure the resistance across R12. Remove Q5 and measure R12 again.
Click to expand...
Will do!
 
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So, pardon the question, but I measured resistance on R12 and it is the correct 2.2k. Once I remove Q5, what should it read if it’s working properly? 2.2k or something else?
Also, Q6 readings are 9; 8.6; and 7v. Q3 today is 9; 6.7 and 0.4. Clearly that collector pin is way different.
 
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Ok, Q5 removed. R12 still measures 2.2k. The patient did not survive the operation, however, so I’ll be ordering more Q5’s (2N3906) transistors. In the meantime, is that result what you expected, meaning Q5 was the problem?
 
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Chuck can correct me here, 2.2K is probably not what we expect? Since we have a bunch of other components in parallel between the collector and ground?
 
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I was trying to figure out why Q5-C was at ground potential. Either it was shorted to ground or the transistor was blown.

When R12 measured 2.2K, that was evidence that Q5-C was NOT shorted to ground. That leaves the only other possibility: Q5 is broken. We don't know how Q5 got that way.

The previous measurement of Q6-C was 4V, now it's 7V. What gives?
 
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Most likely operator error. I will test again a few times. I can only imagine I broke Q5 during installation or it arrived faulty. We’ll find out when replacement arrives (already ordered). Tks, Chuck!
 
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Recheck Q6-C while you're at it, I don't know which measurement to believe: 4V or 7V.

Very unlikely you were sold a bad part. If you don't have a transistor tester, you can do a simple aliveness test with you DMM's Diode function. To test a PNP transistor like the 2N3906, connect the meter's black lead to B, red lead to E. Silicon transistors should read between 0.6V and 0.7V. Now measure with black on B, red on C. Again, you should get between 0.6V and 0.7V. For checking NPN transistors, reverse red & black. Germanium transistors will read between 50mV and 150mV.
 
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Chuck D. Bones said:
Recheck Q6-C while you're at it, I don't know which measurement to believe: 4V or 7V.

Very unlikely you were sold a bad part. If you don't have a transistor tester, you can do a simple aliveness test with you DMM's Diode function. To test a PNP transistor like the 2N3906, connect the meter's black lead to B, red lead to E. Silicon transistors should read between 0.6V and 0.7V. Now measure with black on B, red on C. Again, you should get between 0.6V and 0.7V. For checking NPN transistors, reverse red & black. Germanium transistors will read between 50mV and 150mV.
Click to expand...
Keeping in mind Q5 is removed, Q6 now reads 9, 8.6 and 2v. Q5 pads read 9, 7, 0.
aliveness test on Q6: 0.63v on both B-E and B-C.
 
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Q6 aliveness check is good. I should have warned you that making that measurement in-circuit can give unpredictable results, but looks like we're ok this time. I want to know why you get a different reading on Q6-C every time you measure it. It's not because of Q5.

One more test: With the power on, set DRIVE to zero and measure Q6-C. Not set DRIVE to 10 and measure Q6-C again. The change should be infinitessimal. When you make these measurements, you can't touching any solder pads or component leads with your fingers. If you do, the conductance thru your skin can throw everything off.
 
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Great question. I try to test the same way every time, but there must be some operator error in there. Sorry to ask, but how do you ”set DRIVE”?
 
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grounding on the power jack. 1.8v with gain on 0 or 10. off to bed here on the east coast. Tomorrow is our 46th anniversary so I better be coherent in the morning………….
 
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