Univox Uni-Fuzz Hum Issue

[theseacowexists]

I'm looking at an old Univox Uni-Fuzz for a friend of mine. It's actually rebadged as the Sam Ash Fuzzola, but it's the same thing.

Schematic: https://sites.google.com/a/davidmorrin.com/www/home/trouble/troubleeffects/uni-fuzz

Anyway, he was having trouble with it randomly cutting out. Apparently kicking or tapping the case would bring the tone back. So I've replaced all the e-caps and the the two resistors in the power supply, reflowed all the solder joints, and now it works consistently - but with a very loud hum when the effect is engaged. The fuzz effect sounds fine, and the bypass works without any extra hum. All the ground points seem solid. Could I maybe have a bad transistor somewhere?

[PRR]

Welcome!

> replaced all the e-caps ..., reflowed all the solder joints... a very loud hum ..... Could I maybe have a bad transistor somewhere?

A transistor doesn't know what note to hum. Hum is from the power supply. You had intermittent, you shot-gunned the power supply, and now you have solid hum. I suspect a mistake in that area.

The gang here loves pictures.

[theseacowexists]

Here's some pictures of the boards:











I upsized the voltage rating on the two filter caps. They are still the original 200uf value.


These are the transistor voltage measurements, just in case it helps:

    E   B   C
Q1   .09   6.25   .6
Q2    5.6    9.5   5.9
Q3   2.4    7.3    3
Q4    1.14    2.5   1.77
Q5  1.15    2.5    1.77
Q6   .57    2.43   1.2

I numbered them Q1-Q6 based on their location left to right on the schematic.

[antonis]

Your measurements make no sense..

You've probably wrongly located transistors pin-out..

[theseacowexists]

Ah, you're right! Thanks for catching that. C and B should be switched in that table.

[willienillie]

I can't say how much it would help, but it would be a good idea to twist the AC wire pairs (i.e. PT secondary) and route them as far away from the amplifying circuitry as possible.

You haven't changed the power cord to a 3-prong, have you?  That could lead to a ground loop with the amplifier you connect to.

Unrelated, but are all the transistors marked "(P)"?

[willienillie]

Looking again at the schematic, is that an error on the PT primary wiring?  They've got both ends shorted to each other by a fuse.

[theseacowexists]

The AC wires are not twisted. I can't remember if they originally were or not, but there was a bad splice in one of the wires that was barely covered with electrical tape    so I redid that with a proper heatshrink connection.

The pedal still has its original two prong cord.

Yes, the transistors are all marked (P)! What is the significance of this?

[willienillie]

The AC wires are not twisted. I can't remember if they originally were or not

I saw some other Uni-Fuzz pictures online, and those wires were not twisted, and were routed similarly to yours, so probably not an issue.

Yes, the transistors are all marked (P)! What is the significance of this?

The letter indicates the gain range ("gain bucket").  I did as much online research as I could when I built my Uni-Vibe clone, and every clear picture I could find (not many!) had 2SC828-Qs, higher gain range than the Ps.  I thought the Super-Fuzzes used Qs also, but again, not too many pictures where you can read the markings.

Maybe they used both, whatever they had the most of that week.  Maybe they used the Ps for the 12V Uni-Fuzz, and the Qs for the 9V Super-Fuzz.  Maybe they measured them, and only used the higher-gain Ps, as there is some overlap of consecutive buckets.  And some Super-Fuzzes used a different transistor type altogether, I have no idea what those were.

Anyway, I bought 20 original Matsushita 2SC828s for my Uni-Vibe project, no specific gain bucket offered.  I received three Qs and 17 Ps.  I ended up using the three Qs and two highest-gain Ps in my 'Vibe, and similar-gain 2N1304s for the rest.  So now I have 15 2SC828-Ps sitting in a bag doing nothing, and I'm wondering if they might make a good Super-Fuzz clone.  Some of them are really low-gain, down to around 60 IIRC.  I've never played a Super-Fuzz of any kind, but I love Live at Leeds, so I wouldn't mind having one.

[willienillie]

Okay, I just noticed something with your voltages.  The collector of Q2 should have the full ~12VDC rail, but you are reporting 9.5VDC.  So your operating voltage is lower than it should be.  Do you get ~9VAC (relative to ground, or CT) from each leg of the PT secondary?  If not, you might have a bad PT.  If you do, maybe one of your rectifier diodes is shot.  Half-wave would have more hum.  Is the hum 50/60Hz, or 100/120Hz?  (I don't know what country you're in).  There's also the possibility that one or both of your brand new filter caps is bad.

[theseacowexists]

So both legs of the PT are giving me 10.7VAC. The output to the fuzz board gets 9VDC, and the output to the relay is getting 12VDC. Could those two wires have been switched at some point?

[willienillie]

The relay should be getting higher voltage than the audio circuit.  But all of the DC seems low.  I don't know exactly what it should be, I got "12VDC" from the document you linked in your first post.  Can you check both rectifier diodes?  Something else could be the cause, like a leaky filter cap, but it's easier to test diodes.

[theseacowexists]

Both diodes tested OK. I'll see if I have a couple 200uf caps and try replacing those next.

That page I linked mentions a 9V adapter being a popular mod. Would that be as simple as unhooking the circuit from the original power supply and hooking it up to a standard 9V wall wart? Would that be worth trying just to confirm that it is the power supply causing the hum?

Not sure if it helps or not, but the way the hum behaves is that with the volume ("balance") turned all the way down, the hum is there, and gets louder (and fuzzier) as the volume is increased.

[willienillie]

Both diodes tested OK.

Well dang, I was hoping it would be that simple.  Your reported voltages would have made sense with half-wave rectification.

Would that be as simple as unhooking the circuit from the original power supply and hooking it up to a standard 9V wall wart? Would that be worth trying just to confirm that it is the power supply causing the hum?

Yeah sure.  Just bear in mind that wall warts can be noisy also, especially general purpose ones that aren't intended specifically for audio/pedals.  If you can hook up a 9V battery instead, those have no noise.

the way the hum behaves is that with the volume ("balance") turned all the way down, the hum is there, and gets louder (and fuzzier) as the volume is increased.

That would make sense if it is power supply hum, as there is a transistor stage after the volume control.

I asked before about the frequency of the hum.  You may have no way to measure that, neither do I, but you can compare to notes on a standard tuned guitar.  120Hz is between the 1st and 2nd frets on the A string, 100Hz (IIRC) is near the 3rd fret on the low E string.  60Hz and 50Hz are an octave lower than those, so well below the range of a standard guitar.  But the point is, the only way you get 120Hz or 100Hz hum is from full-wave rectification of 60Hz/50Hz wall juice (depends on your country which one it will be), and that really helps narrow down the source.

[theseacowexists]

I'm pretty sure it's 100-120hz hum. We're in the US btw.

[willienillie]

the only way you get 120Hz or 100Hz hum is from full-wave rectification of 60Hz/50Hz wall juice

I failed to take into account that this is an OCTAVE fuzz, and has full-wave rectification built into the circuit for the audio!  So, if you're dealing with bad filter caps causing your hum, you'll get 120Hz hum when the volume is all the way down, and progressively more 240Hz mixed in as you turn the volume up.

That was literally the first thought I had when I woke up today, I must have been dreaming about the Super-Fuzz circuit. 

[theseacowexists]

I figured that I should at least try swapping out the filter caps for different ones. The first leg I went to remove was the positive leg of the first cap, and I found that the trace was broken right at that spot - so that cap was never completely in circuit! It was only noticeable when I took the leg out. One jumper wire later and the hum was gone. All is good now. Thank you for helping me narrow it down to the filter caps!

[willienillie]

Great news!  How does it sound?  What is the operating voltage now?

[theseacowexists]

12V to the board and 15V to the relay. Sorry it's been a minute, I had it closed back up!