Pepper Spray hum issues - UPDATED WITH VOLTAGES

[Bret608]

Hi everyone,

This is for a stock specs Pepper Spray. The only part subs were 390pf for C1, and a 2n1305 (pnp) for Q1.

I attached this to my test box last night--all I am getting is massive hum/ground noise, but strumming the guitar doesn't cause any changes. The harmonics and balance knobs do cause change, and when I flip the diode switch I can hear the expected volume level changes.

So far, I've searched for bridges, cleaned some flux out from between joints, and done some continuity checking. Strangely, I seem to be getting continuity between the +9v and ground pads where power enters the board. Also, when I probe those pads and the ground pad between the in and out, it's the same deal. What could cause that in the absence of a bridge? And is this normal?

I'll check voltages tonight when I get home. Also, I have not yet ruled out my test box being the issue. The input clip keeps getting squished under the input jack when I put in a cable.

Thanks,

Bret

[Bret608]

Okay, unceremonious bump...I only get the power and ground continuity if I have the black lead of my DMM on power connections and the red lead on ground. Let me know if this is normal and I'm just barking up the wrong troubleshooting tree. I double-checked all my part values also.

[midwayfair]

Okay, unceremonious bump...I only get the power and ground continuity if I have the black lead of my DMM on power connections and the red lead on ground. Let me know if this is normal and I'm just barking up the wrong troubleshooting tree. I double-checked all my part values also.

This is normal.

Voltages?

Gains on the transistors?

Are you getting any guitar signal to go with your hum, or just hum?

Are both jacks grounded? (Are you suuuuure?)

[Bret608]

Transistor gains are 49 hfe on the germanium (leakage about 150 mA), and I haven't checked the 2n3565. It's one of the old ones, so I don't expect it's out of whack--I've had good luck with them so far. I'll check the voltages tonight. Can I use the red DMM lead on both trannies? Thought I should check as one is PNP.

No guitar signal, it's just buzz and hum central. This is the weirdest de-bug I've ever had to do, especially as I've built another one of these successfully. Is there a way to check and see if a cap is bad while it's on the board? I've been using my "jkokura style" test rig for about a year with no issues, but I feel like I should check it over as those alligator clips get handled quite a bit. So no, I'm not 100% sure both jacks are still grounded! 

[Bret608]

My test rig passed with flying colors, so back to the board. I think C1 might be bad. I will try swapping it if I can find a silver mica and report back.

[Bret608]

I went down to the electronics lab where I work and got one of the instructors to go after this thing with a capacitance meter. Surprisingly, most of the caps read where they are supposed to even in the circuit. Just C1 and C7 read that far off. I swapped C1 for a 150pf silver mica to see what would happen. No dice.

So, here are those promised voltages:

Q1:
E: 1.12v
B: 1.08v
C: 0.81v

Q2: 1.16v
B: 1.52v
C: 4.62v

Let me know if this sheds any light! I am getting a clean 8.96v at the board input, by the way.

Thanks,

Bret

[Bret608]

Quick update in case anyone else has similar problems in the future--just took this down to the electronics lab where I work and the instructor who knows a lot about audio in their area hit this with an oscilloscope and did some other tests. It felt like getting a master class yet again.

Anyway, after looking at the schematic and running these tests, he opened my working build and ran the same tests. Conclusion--the transistor I'm using for Q1 does not want to bias up enough to work with the "stock" specs values for R1, R2 and possibly R3.

So I grabbed the right parts to convert this to Albini specs, but I think first I'll try out a high-gain 2n404 I have just to see how it biases with the circuit as-is. I will be curious if any sound gets through. If it still doesn't, I'll swap some parts out for Albini and see how it all works with the 2n1305.

Just wanted to share! 

[das234]

Nice to have friends with an electronics lab.  I built mine to Albini specs so let me know what you find.  I'm still thinking my problem is just a Q1 with the wrong specs but I'm reluctant to cough up ten bucks for a new one just to see if it works.

[Bret608]

Still no dice--I swapped out R1, R2 and R3 to make those Albini values and it still just sounds like a theremin run wild. There has to be a bad cap somewhere for the circuit to just be feeding back on itself like this; I just have to track it down.

This thing looked nuts on an oscilloscope by the way--my working Albini-specs version had this smooth, round waveform. This non-working board had this odd, jagged waveform--quite square but with these weird notches up and down the curve.

I understand about not wanting to spring for a new tranny just in case!

[Stomptown]

I wonder if there is possibly a manufacturers error in the PCB. I've seen it happen. If you can compare the traces to the traces in the pic in the build doc to see if there are any duscrepencies.

Or maybe Q1. How do the voltages on Q1/2 compare to your working build? I can pull mine out when I get home and give you my voltages if you'd like.

[Bret608]

Thanks Jon!

I had wondered about a manufacturer's error too. I'll take a look at that tonight. Something is allowing way too much feedback into Q1's base. Wait a second--I just looked at the build doc and the picture doesn't show the traces. Oh well, I can look at the schem and my working build.

My working build has slightly higher voltages all around, but they follow the same pattern. I'll check the voltages again tonight now that I've swapped resistors and see if they're closer.

EDIT: I'd love to see what you have for voltages, Jon. What set of specs was your build?

[Stomptown]

So here are the voltages on my build:

Q1: 2N404A
E: 3.74v
B: 3.61
C:3.61

Q2: PN3565 (TO-82 version of 2N3565)
E: 3.73
B: 1.59
C: 4.94

It does look like your voltage on Q1 is quite low compared to mine. How does Q1 on your working build compare to mine? If it's similar maybe Q1 is the issue. If you want to try a 2N404A let me know. I have a few of them and would be willing to send one your way if you cover shipping cost...

I would like to try a different transistor in Q1 and see if I can get closer to the sound I'm looking for, but it seems to sound fairly similar to the version in Albini's youtube demo. Oh, and yeah I did build mine to the Albini specs...

Hope this helps.

Jon

[Bret608]

I really appreciate your posting this, Jon. The voltages on my working Albini version are almost the same as yours, with the exception of the Q2 base being around 3.8 volts. It's an old 2n3565 though, so maybe that could account for the difference. The voltages on my non-working stock build are similar to what I've seen posted at DIYSB for the Chuck Collins reproduction.

I might like to try out one of your 2n404As--maybe when you get in some more of your Percolator PCBs? Let me try swapping out a couple of more caps on this board and see if I can't locate a potential manufacturing bug while I'm at it. Then, if I don't have any luck, I'll order one of those boards straight away. If you wanted to try something different in yours, I could send a 2n1305 back your way. Let me know if you were looking for something higher or lower gain than what you have now. I've actually got what I think is a 2n404 at hand (really smudged-out marking), but it's way higher gain than this circuit would normally like.

This thing has been through some intense de-bugging! My electronics instructor friend even ran the whole thing by a friend of his who does amp and pedal design work for Sovtek and others. The guy said the circuit seems to draw concepts from the "cascode amplifier", which is supposed to be good at keeping the circuit input and output isolated, i.e. preventing this unending motorboating my build has going on.

[Bret608]

Just following up on a suggestion pinkjimiphoton made to me over at DIYSB. Has anyone who has built the fabbed Pepper Spray PCB had problems testing it on a breakout box or breadboard while it's unboxed? Jimi said some fuzzes will oscillate without the pot casings grounded. I've got an idea that will let me test this theory out; just wondering if such a thing is possible with this particular board.

[lincolnic]

Just following up on a suggestion pinkjimiphoton made to me over at DIYSB. Has anyone who has built the fabbed Pepper Spray PCB had problems testing it on a breakout box or breadboard while it's unboxed? Jimi said some fuzzes will oscillate without the pot casings grounded. I've got an idea that will let me test this theory out; just wondering if such a thing is possible with this particular board.

I put my fabbed Albini-spec Pepper Spray on my test rig without any trouble.