Dream Buzz: a fuzz mystery! (boutique pedal arrives mysteriously DOA)

[ovnifx]

OK, so I let my trigger finger go wild and bought a fuzz pedal based on looks (and rarity) alone!  The Dream Buzz from TecAmp of Germany.  It arrived DOA.  I bought it from a third party, not direct from TecAmp, so it's not a warranteed deal.  OK, so I do stupid things sometimes.

I have hope that you guys can help, even with no schematic, because this thing looks like a very simple (and probably familiar to many of you) fuzz circuit.

Here's the rundown: it is true bypass, and the bypassed signal is perfectly normal.  But switching in on results in only the tiniest of output signals, so quiet that at first I thought there was no signal at all.  There is an LED, which does not light when switched.  I have tried using both a fresh batttery and a power supply with correct tip/sleeve polarity.  All of the connections in the box look clean and solid.  There are no obscure epoxied-over areas or hidden clusters of parts.  I'm actually kind of shocked how few parts it has on the circuitboard.  Check it out:







I did not remove the board from the housing (for a better shot of the bottom of the board) because the bolts have glue or loctite seals, and I don't want to break the seals before I finish negotiating with the seller.

I audio-probed the singal path, and found that at the blue square 47uF cap the signal is strong (and clean); it connects to a 1k resistor (with an upward-pointing solder joint), and is strong on that side of the resistor, but completely dead on the other side (closest side from camera perspective).  I could think the resistor was dead, but (a) how often do resistors die, and (b) the dead end of the resistor connects to the center pin of the leftmost transistor, which makes me wonder if the problem is in the transistor instead.  At each of the other legs of the transistor, there is a very weak original signal.  The same is true at the other transistor on the right.  Both xtrs are stamped "T8810" and "AC151", and they have hand-written numbers (I'm assuming beta) "104" on one and "73" on the other.  The trim pot on the side of the board is a bias for the xtrs.  The same solder joint where the "dead" resistor connects to the leg of the xtr also connects via a grey wire to the pot marked "buzz".

I'm wondering if somehow the power is not getting where it needs to go?  The non-lighting LED put that idea in my head, and I wonder if the xtrs aren't getting the juice they need to turn on?

OK, here is where I don't know how to proceed.  Can you recommend specific points or parts to test?  I have a scope, DMM, audio probe, etc.  Please feel free to ask for more detailed info or for photos of specific connections.  Any ideas at all will be appreciated.  Cheers!

[Joe]

Could be wrong battery polarity, or some other conflict/short related to the power supply. Try putting another LED in parallel with the first, but facing the opposite direction and see if it lights.

[cd]

I will bet that the schematic for the pedal is here:

http://www.generalguitargadgets.com/diagrams/fuzzface_b_sc_pp.gif

Or this one if it's negative ground:

http://www.generalguitargadgets.com/diagrams/fuzzface_b_sc_pn.gif

[petemoore]

  2 transistors...probably a Fuzz Face...
  Can you find a feedback resistor [around 47k - 150k probably] that's a 100k resistor between Q2E and Q1B...
  Contour and Bass are probably like what you can see at GGG's FF pages.
  If you're not 'used to debugging FF's, I'd start with clipping a ground wire from the DMM to a ground, then check all grounds [shown on schematic and both jacks..]are tied in, then take transistor voltage readings.

[Paul Perry (Frostwave)]

Petemoore is right, getting the voltages will probably let somebody work out what is (not) happening. Those trimpots are notoriously flaky, maybe the wiper isn't making contact? In a circuit this simple, it's usually mechanical (shorts or cracks).

[ovnifx]

Thanks for the tips so far!  I'll get those xtr voltages shortly.  Do any of you have a datasheet for these xtrs?  I couldn't find anything on alldatasheet.com.  I ask because I don't know C/B/E placement for these.  There is a little tab off one side- does that indicate Base?

[Gus]

It could be the switch..

Operate the switch a number of times and check it with your DMM

[ovnifx]

>>EDITS!<< On closer inspection I was VERY mistaken about which wires went from the jack to ground.  Corrections have been made to the following text:

OK, progress has been made!  I tested the xtrs and got a whopping 0V at all legs.  In fact, I got 0V throughout the circuit.  The only place I got DC was at the battery (reading a healthy -9.29V) and at both ends of the blue wire running from the battery to the AC adapter jack.

The red wire (at the top of the jack in the photos here) connects to the center pin of the "contour" pot, and also to the longer leg of the LED, and also to another red wire which travels to the cktboard and meets what appears to be a 20k resistor (at the "back" of the board from camera perspective).

The two red wires at the bottom of the jack, which join at the center pin (rivited-looking larger tab): one goes to a middle pin of the guitar input jack; the other goes to the pos. terminal of the battery.

I ran a jumper from the neg. terminal of the battery to the exposed part of the top (non-center pin) red wire, and voila!  The LED came on, and the effect works and sounds exactly like a fuzz face.

So I am guessing that the switch connection between the battery tab and the AC adapter "hot" tab (internal to the DC jack) is broken.  What is a good way to test or repair this?  Also, I am hella confused about the pos/neg/gnd connections here.  Any guidance about the pos/neg/gnd relationships here will help greatly, as well as suggestions about diagnosing the DC jack connections.  I'll do what I can with the DMM and continuity tester, and post the results shortly.

[ovnifx]

OK, here are the continuity test results, with a battery installed:

With a plug inserted into the DC jack, there is continuity between the sleeve and the top red wire but not to the two bottom red wires.  Continuity from the sleeve to the blue wire was intermittant.  There is continuity between the the jack tip, the two lower red wires, and the pos. terminal of the battery.

With no plug inserted, it's exactly the same!

In both cases, there was an intermittant state: either the neg terminal of the battery (and blue wire) did or did not have continuity with the jack sleeve and/or top red wire.  Additionally, testing continuity from the jack sleeve to the blue wire would intermittantly switch the unit on and light the LED.  But testing from the sleeve to the top red wire did not switch the unit on at any point.

So, with all that said, is it likely that an internal part of the jack is busted and I'll need to replace the whole jack?  Or is it worth re-heating the solder of the blue wire to try forming a better connection at the tab there?  (The solder joint doesn't look or feel weak as far as I can tell.)  Any other thoughts?

[ovnifx]

Here is a crude schematic of this pedal, for those interested:

The nonpolarized caps are polyester film "squares", and the polarized ones are aluminum electrolytic.  /Edit/: pot values added and xtr corrections made.

It sounds absolutely KILLER when I have it working.  It was designed especially for bass by TecAmp, and it really is powerful on the low end, with a wide variety of tones.

So, it's pretty clear by now that I just need to replace the DC jack.  But there's still a mystery about it, which I'm hoping somebody here can figure out:  I am able to get the unit working, with a battery, by bridging the top red and blue wires of the DC jack, or by jumpering the external sleeve of the jack to the blue wire.  But I have not succeeded in getting the unit to switch on while powering with an AC adapter, no matter what I jumper to what.  I do get full normal voltage at the "tip" pin and associated wires, including at the LED leg, but nothing switches on.  I am baffled.  Both the pedal and the AC adapter (a Voodoo Pedal Power) are marked +sleeve, -tip.  The pedal is also marked +=ground.  What is going wrong?  Is it just that the jack is faulty, or am I misunderstanding something about the pos/neg/ground?

[burnt fingers]

What's the deal with the seller.  Have you contacted them yet.  It probably is just a bad jack or a miswired jack but I think it should be the seller's resposibility. 

Just my .02

Scott

[Paul Perry (Frostwave)]

Sometimes what looks like an intermittent power fault, is caused by using a plug with a 2.1mm inner conductor, in a socket that expects a 2.5mm inner conductor. The plug feels as though it 'fits', the switch closes, but... the inner conductor makes intermittent connection.
Yeah, like everything, I learnt this the hard way.. more than once...

[lowstar]

the ac151´s have the little tip at the emitter side: seen from the bottom with the tip pointing to the left, they have e b c pinout. they are pnp germs (25V, 0.2A, 0.9W)
as far as i can tell now, it is a pnp positive ground fuzz face.
maybe that´s where the problem lies with your adaptor.
stay tuned, i´m working on it right now !

cheers,
lowstar

[lowstar]

normally, the center pin and the "upper pin" (in your description) of the dc jack have continuity if no power plug is present. for some reason, it seems that your plug has some fault, and by either bridging the two pins or jumpering to the jack sleeve, you put power to the circuit.

cheers,
lowstar

btw, "bass" is what is normally the bias-pot on ff´s, "milk" and "cream" change between a big and small input cap

[lowstar]

i checked the voodoo labs pedal power on the net and found out that the cables that come with it are center negative.
from what i understand, you have a dc connector on your pedal that is center positive. (since you mention that the positive terminal of the battery connects to the large pin on the dc jack)

now that would explain why it doesn´t work with your adaptor. (btw, when you say ac adaptor, i understand you mean the voodoo labs pedal power 2, not the voodoo labs pedal power ac adaptor, right ? cause with the ac pedalpower, it could not work at all).
cheers,
lowstar

[ovnifx]

Thanks for all the advice and ideas!  I have added the pot values to the schematic above.  See, that whole pos/neg thing is driving me nuts!  The back of the pedal is clearly labeled center -, sleeve +, ground +.  Yet, as you say, the + battery terminal connects to the center pin, and the pedal works that way.  So the pedal is labeled wrong?  Maybe they printed the housing for one circuit design, and then changed the circuit?  This may be, because I notice the "milk/cream" switch was originally labeled "HB/SC" (for humbucker or single coil) in their promo photos.  I was using a Pedal Power II; unfortunately I don't have a 2.5 center pin plug to test whether that's the issue.

I'm going to send this pedal back to the seller, and build my own.

[ovnifx]

the ac151´s have the little tip at the emitter side: seen from the bottom with the tip pointing to the left, they have e b c pinout.

So is this correct?
//Edit: I removed the image because it was incorrect//
I have corrected the schematic above to fit this pinout, and it looks plausible.  Any opinions?
//Edit: here is the correct pinout:

[lowstar]

yes, except that the base is not on the bottom, but on the upper side (like 3/4 of a clockwise circle), and the little tip (or tab) is at 8 o´clock, not 11.
the transistor connexions in your updated schemo are still not right, i believe.
let´s call q2 the left upper one and q1 the right lower one (that is how the ff schems usually look).

the two trannies are connected together from collector (q1) to base (q2)
10K trim goes to collector q2
input caps (47 and 0.047) go to base q1
100K to emitter q2 and ground to emitter q1 are right.
47µ (input cap) + to the bass pot
47µ (buzz pot) minus to wiper
33µ + to the vol pot
contour pot should have one leg in the air, not tied to ground, bass (=bias) pot is normally wired the same, but i´m no expert.

cheers,
lowstar

[ovnifx]

contour pot should have one leg in the air, not tied to ground, bass (=bias) pot is normally wired the same...

Ah, you're right!  The legs I marked as grounded on those pots are "in the air", not connected to anything.  The housings of the pots are grounded.  I was confused by the legs in the air, so I assumed they must be grounded internally.  I will correct the schem.
Thanks!!!

[ovnifx]

All right, I have made the recommended changes to the xtr arrangement and the pot legs.  Readers may need to hit "refresh" to update their view of the schemo above.  Thanks for all your help lowstar!  Now I am gathering some more AC151's and related parts to build one.
Cheers!

Edit: Oh, and I am not sure that the "bass" pot here is the same as the "bias" pot in a normal FF.  Turning it up to 10 passes the full bass range, and turning it down rolls off the bass freqs.  I am not sure if any other tonal changes occur- if they do, it's subtle.  When I build my copy, I'm just going to leave that pot out, as I preferred the tone of the full bass range (pot measures 0 ohms in that setting).