Yet another Big Muff issue--Q2 not biased right

[cleverbeefalo]

Hi folks,

I have spent 12hrs+ debugging a Big Muff that I'm building for a friend, based off of this layout and schematic:

http://tagboardeffects.blogspot.com/2014/10/ehx-nyc-big-muff.html

Here are my most current transistor voltages:

Q1: C--4.36   B--.66  E--.06
Q2: C--9       B--.42   E--0
Q3: C--4.25   B--.66  E--.06
Q4: C--4.95   B--1.45  E--.96

The final sound is a muffled one, with no indication that the signal is clipping. It basically sounds like a tiny bit of fuzz, with almost no top end.

It's obvious to me that Q2 is not biased right. When I searched through other threads there was a post from RG about why a transistors Collector would be reading so high--there could be a solder bridge causing the transistor to short, the emitter could be missing a resistor before it reaches ground, etc. I've double quadruple checked this area and cannot seem to find a problem with it.

One thing that I've noticed (this may be a dumb observation, I'm not entirely sure): as I've been tracing voltages from Q2, back to Q1, the voltage at the collector of Q1 reads (as you can see above) 4.36V. The voltage on the other side of C4 that's connected to Q1's collector reads 0V. This might be stupid--and I don't know enough about how electronics works to really know--but is that a DC voltage that's being filtered out by C4? Also, the only thing connected to Q2's emitter is a 120 Ohm resistor, which is connected to ground.

Everything after the first clipping stage, in terms of voltages, seems to be operating normally (based on my limited understanding of how everything should be working). So does anyone have any ideas outside of what I've already mentioned? This has been driving me crazy for two days. I hope my girlfriend isn't getting too worried about it 

Thanks ahead for any time/help you fine folks put into considering this issue.

Cheers,
ZB

[thermionix]

The voltage on the other side of C4 that's connected to Q1's collector reads 0V.

Capacitors block DC, but allow signal (AC) to pass.  That's their main job in audio electronics.

Looks like you have a dead transistor at Q2, or at least one that's not connected properly.  It's drawing zero current, so no voltage drop across the collector resistor, and 0V at the emitter.  Double-check connections there, and if they look good sub in another transistor.

[cleverbeefalo]

That's what I thought too! I swapped the previous Q2 (had identical readings) and still have the same problem. So it's sounding like I likely have something connected wrong. Knowing that is even more infuriating considering the amount of time I've spent checking connections.

Time to check again... 

[thermionix]

If you can post pics, both sides of the board, maybe a fresh set of eyes will spot something out of sorts.  You posted voltages, so I guess you probably have a multimeter or access to one.  I would check continuity and resistance around the collector and emitter of Q2, and their resistors.  Best if you can remove Q2 first.

You'll get there!  I'm sure it's something extremely simple!

[antonis]

Check for Q2 collector resistor value (9I - 9L, 10k)..

It could easily be mismatched with another of 10R, or so...

[duck_arse]

are you using any tantalum caps in your build? pull Q2 out, (power off for these measures) and measure the resistances from ground to base, base to collector, collector to supply line. something won't tally. also, photos, please.

[cleverbeefalo]

Alright guys,

Warning ahead: it's pretty messy. I hope the pictures are helpful, though.

https://zackbassage.files.wordpress.com/2017/02/p1010095.jpg
https://zackbassage.files.wordpress.com/2017/02/p1010096.jpg
https://zackbassage.files.wordpress.com/2017/02/p1010097.jpg

The area with the cap underneath is where the problematic transistor lies. In my troubleshooting since my last post I have switched out the 10k resistor from the power rail to Q2's collector. Still reading at 9v on the collector.

I moved the cap underneath because I had thought that maybe this way would work better. Pretty much just taking shots in the dark.

In comparing the first clipping stage to the second, I tested the voltage of the 10k resistor from the power rail to Q3's collector. At the power rail it reads 9v, but at the collector it reads 4.3v. Compare this to Q2, the 10k resistor from power (reads at 9v) to Q2's collector maintains 9v. This makes me think that there's some short between the Collector of Q2 and the power rail... which I cannot find evidence of. I assumed having two bad transistors in a row is highly unlikely, but I'm beginning to think it's unavoidable that I'll have to change out Q2 again.

Thanks again to everyone who has helped so far. And apologies again for the mess that is this board!

[LightSoundGeometry]

youre not getting voltage on the base, look around your voltage divider that puts the tranny into operation

[thermionix]

pull Q2 out, (power off for these measures) and measure the resistances from ground to base, base to collector, collector to supply line. something won't tally. also, photos, please.

I'm with this guy.  And add in "emitter to ground".  Not just measuring the 120 ohm resistor, but measure from the emitter hole in the board to offboard ground (jack?), and make sure that's 120 ohms also.  Likewise for the other resistors and electrodes.

[antonis]

the 10k resistor from power (reads at 9v) to Q2's collector maintains 9v. This makes me think that there's some short between the Collector of Q2 and the power rail...

Or no current flows through 10k resistor (no voltage drop - see what LightSoundGeometry above wrote..) 

Or 10k ISN'T actually 10k (but much much much lower..)

[antonis]

I'm with this guy.

You'll be soon awfully sorry for your preference...

[thermionix]

I'm with this guy.

You'll be soon awfully sorry for your preference...

I'm aware of the risks, but those are mitigated by the molten planetary core conveniently located directly between us.

[cleverbeefalo]

Resistances surrounding Q2:

emitter to ground - 94.4 Ohms
base to ground - 48.5 kOhms
collector to supply line - 6.2kOhms

Something weird happens when i measure resistance from base to collector--it starts at about 60 kOhms, and then the DMM continues to count up. And it doesn't stop.

[thermionix]

This is with Q2 removed?  All those resistances seem kinda low.

Something weird happens when i measure resistance from base to collector--it starts at about 60 kOhms, and then the DMM continues to count up. And it doesn't stop.

That's likely due to the caps in parallel (C12 and more or less C6) charging up with the tiny voltage output by your meter.

[cleverbeefalo]

This is with Q2 removed?  All those resistances seem kinda low.

Something weird happens when i measure resistance from base to collector--it starts at about 60 kOhms, and then the DMM continues to count up. And it doesn't stop.

That's likely due to the caps in parallel (C12 and more or less C6) charging up with the tiny voltage output by your meter.

Q2 is completely removed.

[antonis]

If you use carbon resistors, R_E should be considered OK..

R_C seems to be dominated by something with 15k resistance..

Same for R_B by 100k resistance..

I smell something like a forgotten cut on the 7 cut column or/and an inproperly placed jumper next to Q2..

[duck_arse]

I was looking at your board, and I can honestly say "I can't read that". tri-pad, who needs it? anyway, looking at 1% resistors, I notice a funny thing; if you put a 120R 1% resistor (brn-red-blk-blk-brn) into a board backwards (upside down if north of the equater), it becomes brn-blk-blk-red-brn, which I think is a different value.

that resistor near the top with its guts hanging out is a prime candidate for replacement [impeach!] too. only other thing I can think of is to pull the transistor out and then measure the voltages where it came out. that would then show if the resistor string supply-to-ground is good.

[cleverbeefalo]

If you use carbon resistors, R_E should be considered OK..

R_C seems to be dominated by something with 15k resistance..

Same for R_B by 100k resistance..

I smell something like a forgotten cut on the 7 cut column or/and an inproperly placed jumper next to Q2..

Bing bing bing! I erroneously jumpered the ground rail to where the diodes and the 1uF cap cap are. If I understand this correctly, this would explain why there's no clipping--there's nothing flowing through the diodes. Again, in my limited understanding, this would also prevent electricity--not sure if it's voltage or current or both--to flow through the base of Q2.

I was looking at your board, and I can honestly say "I can't read that". tri-pad, who needs it?

I feel you on that! I originally thought this would be a good candidate to help keep everything clean because the holes are plated and solder a bit nicer than regular perf. Now the whole thing is just a disaster!

Thanks for everyone's help! Soldering Q2 in place. Will report back with results!

[cleverbeefalo]

IT WORKS!!!!

Thank you so much to everyone who has helped with this! I haven't built THAT many pedals, but I've spent so much time working on this one trying to figure it out. This is super rewarding and I couldn't have done it with everyone else's help.

So thank you guys, again! Much appreciated!!!

[thermionix]

Nice!  Knew it had to be something simple.