Am working on a Cherrybomb, and the sound is fizzy and only comes out when strings of guitar are plucked hard. Wiring, values, and orientation of components seem correct. Only substitution was on C4 I used a 220pF cap instead of the 200pF as listed.
Voltage readings with 9.12 volts in are
Q1 E .63 B 1.19 C 1.73
Q2 E 1.26 B 1.73 C 9.10
Q3 E 5.29 B 1.60 C 9.01
My guess is q3 is the problem because it is misbiased, is this right? But shouldn't the collector voltages also be around 5 volts also?
Where should I be focusing on to get this working correctly? Thanks for any help.
Derek
I'm not the best troubleshoot guy here but I'll try.
Out of curiosity what transistors did you use?
I agree the voltages for your collectors on both q2 and q3 are high, I'd say by about 3 to 4 volts. Maybe re-check your resistor color codes, and if they still look right you could try increasing the value of r6 and r13 in order to reduce those collector voltages to around 5 or 6v.
I've actually read that increasing c4 even more (like to 470p or 560p) can reduce splattiness and will also add some bass, although I haven't gotten around to trying it myself.
But your main problem is probably those high voltages. IIRC mine has right around 6v on those 2 collectors.
good luck
I rechecked the resistors again with DMM for R6 and R13 and they both were at 1.78K. What value resistors would be good to try next to get the collector voltage down?
BTW the transistors I am using are 2n2222a for Q1 and Q2 and a BC109 for Q3.
Derek
What are you powering the pedal with? Are you sure it's 9v? We'd probably better know that before we proceed.
I honestly don't know what resistor values you should try next, maybe a couple of 3k3's. It's going to be a trial & error thing for you. If it was me, I'd solder some sockets in and change resistors until I got 5 or 6v, or maybe just tune it by ear til the splat was minimized. If you had a couple of 5 or 10k trimmers you could solder them into those spots & that would be a good way to go as well.
I would also try the c4 modification.
Power I measured at 9.12 volts, I will see what larger transistors will do to get the collector voltage down firtst, then maybe look at the C4 mod.
Derek
I meant larger resistors, that was typed before I had any caffeine. :)
Derek
I would replace the 2n2222a with something else and see if that fixes the problem. I've found that they are kinda finicky. I was able to use them in my build, but I've used the same transistors in other fuzz type builds (after doing the CB) with exactly the result you are describing. If you have some 2n3904 or 2n5088 try swapping those in and see what happens. It might be good to socket all three or use an audio probe to see at what point the splattiness is introduced.
Changed Q1 and Q2 to 2n3904 transistors, Q3 is still the BC109. Increased R6 to 470K and increased R13 to 150K.
With 9.12 volts in I am getting
Q1 C 1.58 B 1.04 E 0.5
Q2 C 5.30 B 1.58 E 1.11
Q3 C 5.33 B 1.60 E 4.10
R6 and R13 seemed like big increases from the 1k8, does that indicate a problem?
Q3 emitter is still high, how do I bring that down?
Otherwise it is somewhat improved, less fizzy, more volume (but still not much more than unity when maxed).
Thanks Haberdasher and Bean for your help so far, what do I do next?
Derek
Yeah, I would guess something is off. I wasn't expecting you would have to make that big a change in those 2 resistors to reduce the voltage. Also, my cherrybomb is a really loud pedal so if your unity is almost maxed there must be a problem.
I think you should take some nice hi res pics and post them if you can.
I'll get some voltages for you. I need to do that for the Retrograde as well. Just been swamped trying to get the last of the sale orders out.
Here are my voltages:
Q1:
C - 1.09
B - 1.73
E - 0.57
Q2:
C - 5.20
B - 1.74
E - 1.12
Q3:
C - 4.90
B - 1.5
E - 0.91
I think the next part I need to focus on is the q3 emitter, it is way higher than the base, which I think means it is saturated if I read the RG Keen debugging page correctly.
My voltages are:
Q1 C 1.58 B 1.04 E 0.5
Q2 C 5.30 B 1.58 E 1.11
Q3 C 5.33 B 1.60 E 4.10
Does that seem like the next logical place to look? Not sure how to decrease the emitter voltage though.
Derek
After re-reading the RG Keen debugging page, I thought q3 could possibly be a bad transistor, so I changed some things, Q1 stayed 2n3904, Q2 is now a 2n222 and Q3 is 2n3904.
Voltages are now
Q1 E 1.68 B 1.12 E .51
Q2 E 5.20 B 1.68 E 1.20
Q3 E 5.88 B 1.60 E 1.10
Sound is still crackly/splatty, volume when maxed is still only slightly greater than unity. Have I at least eliminated the transistors as the problem for now? Aargh.
Derek
One correction to my last post, Q1 C and B should be switched. Q1 C is the same as Q2 B.
Unity for mine is with the volume around 9 o'clock. I would bias Q3 Emitter to 5V. This pedal is very sensitive to biasing.