Neucleon Super Fuzz help please

[murdog47]

Populaed a Nucleon Super Fuzz and I get no signal. I probed it and found the signal loss is between lugs 2 and 3 of the 50kb Expander Pot. Bad Pot? Nope, switched it out and same results. Re-Flowed solder joints, same result. Here are my voltages

Q1: C 144.2mv / B 0.684v / E 5.33v
Q2: C 4.79v / B 5.37v / E 9.15v
Q3: C .3mv / B .1mv / E 9.08v
Q4: C .1mv / B .629v / E 15.8mv
Q5: Unstable

 

[murdog47]

The Schematic is here

http://rutgerockhorst.com/nucleon01.zip

[timbo_93631]

I'd pull out the pot and make sure that lug 2 isn't connected to ground (lug 1) with a trace on the PCB (a manufacturing error), just use your continuity setting on your DMM and find where the signal is being shunted to ground in that area.

[Rockhorst]

Hey Murdog, do Timbo's check and let me know. It sounds like a weird point for the signal to cut out. Just shows how valuable an audioprobe can be. I'll be happy to help of course, so please update

[Rockhorst]

What happens if you short the pot out, so directly connect pins 2 and 3 or bypass it all together by going from electrolytic to electrolytic cap? Another thing to watch out for is a possible solder bridge between the Expander pot and D2, the pads are quite close together.

This should help with the debugging:

[murdog47]

Hmmm....so C5 negative to C6 positive did not work. I checked and the pad for lug 2 is grounded. I attempted to bypass this by connecting the positive pad of C6 to Lug 2 of the pot and pulling the lug 2 leg completely from the pcb. Still no luck. It appears that the positive pad for C6 is also connected to ground somehow. Sounds like a similar problem as the other guy who posted in a separate thread has. Checked all of my solder points and no bridges. With it fully populated this is going to be a bear to find where ground is tied in. Rutger, any chance you can probe around an unpopulated board to check for this? Thanks!

[Stomptown]

I fired my super fuzz up a couple weeks back with no problems. This is sounding more and more like a fabrication error on some of the PCBs. It doesn't happen often but this wouldn't be the first time I've seen it.

[Rockhorst]

Though the design files used where all the same, I did order them in separate batches. I'll probe around a few different boards. Thanks for the detailed description, I'll get back to you in a few hours!

[Rockhorst]

Murdog, could you try one more thing for me? Bypass both the pot and C6. So connect negative of C5 to the node that's connected to negative C6. If that doesn't work, go from the last transistor that outputs sound and connect it to a node further upstream until you get sound coming through again (for instance, connect the output of C5 to the input of Q6 and work back). That would make it easier to isolate which sections are malfunctioning.

I just probed around the five remaining board and I can't find a problem with those. Let's do a few more tests and if we can't resplve the issue, I'll send you a fresh board to compare and rebuild.

[murdog47]

So I had a minute to play around and found that c5 output to the balance pot I get signal at lugs 1 and 2, not 3. From the schematic it looks like lug 1 should be going to ground and not lug 3. Right?

[pietro_moog]

i'm following this thread. did you guys find a solution?

[Rockhorst]

@Murdog: yes, lug 1 should be ground, lugs 2 and 3 should carry a signal. Notice the '1' printed on the pcb silkscreen and the orientation of the pot on the other side. It sounds like you might've put in the pot the wrong way round? (Just to be sure: the pots should not stick out but be entirely covered by the PCB. Lug 1 is the square pad. Remember Brian's mnemonic: 3, 2, 1 pot shaft faces down)

@Pietro: working on it

[murdog47]

Yes I have it in correct 

[Rockhorst]

Ok, I just needed to check...weird, cuz like I said: the 5 boards I have left don't have this issue. I don't want to post the entire trace layout, but if I send a picture of it to you, could you compare the top and bottom traces of your board against that? Check your PM in a few minites.

[murdog47]

As far as I can tell, it looks correct. That being said, it's difficult to say for sure with it fully populated. Here's something else that's strange, if I turn the expander pot fully counterclockwise it turns the LED off.