Problem with Chasm Reverb Build

[undercurrent77]

Hi everyone,
  I'm attempting to build the chasm reverb by dead astronaut and am using his schematic in the 1st post here (http://www.diystompboxes.com/smfforum/index.php?topic=105109.0).  I'm not getting any output signal, and using an audio probe I tracked down the problem to the very first capacitor after the input on the pcb.  It's labeled C1 on the schematic and is 10nF.  I've got clear audio on the input side of the cap, but as soon as I switch the probe over to the output leg, it makes a popping sound and I get a hum.  I'm using a film capacitor and I double checked that it's a 10nF.  It seems to me that it's just a bad component, but I swapped it out for another and am getting the same results.  I'm confused what else I could try.  I'm definitely more of a beginner at pedal building so I wanted to ask you guys and learn from your experience.  Is it worth replacing it with a third capacitor and seeing if that fixes it?  Or could it be a problem somewhere else in the circuit?  The traces near it are not touching, so that makes me wonder what could be causing it if it's not a bad component.  I'd be more than happy to post voltages on the IC and FET's in the circuit if that would be helpful.

Thanks so much.  I appreciate any ideas of things I can try to get this pedal working!

-Chris

[samhay]

>I'd be more than happy to post voltages on the IC and FET's in the circuit if that would be helpful.

It would.

p.s. Welcome to the forum.

[undercurrent77]

Sorry for the delay.  Here's what I've got.  I measured all of these with the switch in bypass.  Thanks so much!

IC1
1 4.26v
2: 4.26v
3: 2.09v
4: 8.59v
5: 4.23v
6: 4.26v
7: 4.26v
8: 4.26v
9: 4.26v
10: 4.24v
11: 0v
12: 4.24v
13: 4.25v
14: 4.25v

FET 1
1: 4.25v
2: 4.01v
3: 0v

FET 1
1: 4.26v
2: 4.26v
3: 0v

Voltage Regulator
1: 8.59v
2: 0v
3: 7.92v

[samhay]

Your voltage regulator should be be giving a 5V output, so you should get this working properly first.
Unfortunately, the BTDR brick may be dead if you have been feeding it 7.9V.

[bluebunny]

You've not got the regulator back-to-front, have you?   

[deadastronaut]

78l05 or 7805?

[undercurrent77]

Thanks for the replies everyone.  It's an LM7805 (the one with the large heat sink).  I think that the output voltage may actually my have been a typo.  I have a bad habit of putting components on backwards, so the orientation and output voltage were the first things I checked and I thought it was working correctly.  I'll double check when I get home tonight and post what I find.  Sorry about that. I hope I didn't inadvertently blow out the brick! 

[bluebunny]

You may have picked the wrong forum name.  Overvoltage may turn out to be more fitting.   

[undercurrent77]

hahaha, very true bluebunny! 

It turned out it was just a typo thankfully.  I rechecked my voltages on the regulator and I've got
1: 8.59v
2: 0v
3: 4.92v

I went ahead and checked the voltages across everything else with the switch on and came up with pretty much the same as with the switch off.  Figured it may help troubleshoot things. I made sure all the potentiometers were turned fully clockwise before I took the readings.

IC1
1 4.24v
2: 4.24v
3: 2.00v
4: 8.55v
5: 4.22v
6: 4.24v
7: 4.24v
8: 4.25v
9: 4.25v
10: 4.22v
11: 0v
12: 4.22v
13: 4.24v
14: 4.24v

FET 1
1: 4.24v
2: 4.24v
3: 0v

FET 2
1: 4.24v
2: 3.52v (this value continually decreased the longer I held the probe on the lead)
3: -0.04v

Mosfet
1: 0v
2: 4.23v
3: 2.05v

Thanks guys!

[samhay]

Excellent - your brick is probably still alive.

I am guessing you have a cheap multimeter, which has a relatively low input impedance. This is loading down the voltage on pin 3 of IC1 and probably causing problems when you measure the JFET voltages too.

However, the MOSFET voltages look very wrong. This is set up as a common drain (buffer) stage, so the drain should be tied to the positive rail  (8.5 V in your case).  It's not clear which way round you have put the MOSFET, but please check the source is up (and connected to 2k2 resistor) and the drain down, when oriented as per the layout here:
https://dl.dropboxusercontent.com/u/7464107/CHASMREVERBLAYOUT.pdf

On a positive note, the gate voltage looks ok - this should be at about half supply - so once you fix the drain voltage, there's a pretty good chance the rest of the circuit will work.

[undercurrent77]

Thanks Samhay. Yeah, it's more of a 'jack of all trades' type multimeter.  I remember reading somewhere that ones you would use in more electrical applications aren't necessarily the best for small electronics like this.  So that's good to know! 

I'll focus on looking at the mosfet and post back what I discover.  Just for my own education, I did a little research on a common drain circuit and it sounds like it alters the impedance of the signal.  Does it lower it in this case?  Is this used to prevent signal loss through the circuit? 

[samhay]

The buffer is there to have high input impedance and low output impedance.
IIRC, it was added to prevent switching noise.

If the MOSFET has not survived whatever you have done to it and you don't have another, you should be able to sub it for a JFET (if you can find one with a DGS pin out) or reasonably high gain NPN BJT transistor (collector where drain would be).

[undercurrent77]

Ah, thanks for the explanation.

I checked the orientation of the mosfet; the source is tied into the 2.2k resistor (up) and the drain (down) has continuity back to the diode immediately after the 8.5v coming into the board.  The gate connects to the 1k resistor and 56k resistor.  I traced it out through the schematic below just to follow the circuit a little better and everything looks okay.  No broken or jumped traces or anything like that.  All of the resistances seemed to check out okay too.  The 56k was reading just above 48k, but that's about it.

http://www.coda-effects.com/2016/02/dead-astronaut-fx-chasm-reverb.html

....so typing all of that out, I realized that if the drain has continuity, then it should definitely have 8.5 volts.  It's a straight shot after all.  Checking the voltages again showed me with 8.55v on the drain (down), 2.05v on the source (up), and 4.24v on the gate.  I'm really confused now.  I was definitely getting different readings before.

So back to the audio probe.  I'm now getting audio through the circuit at bypass.  although it's maybe only at half volume.  With the circuit switched on, I get audio but it's very, very faint.  Almost imperceptible.  I haven't done anything really up to this point beyond read voltages, so I'm stumped   

I think I'm going to start looking for a loose connection somewhere around the volume pot and see where that gets me.  I'd gladly welcome any other ideas of things to investigate though!

[duck_arse]

under - can you post some photos of what you have built? the staring sometimes find things.

[undercurrent77]

Sure thing. I'll snap a few pictures when I get home and post them.

[samhay]

>I'm really confused now.  I was definitely getting different readings before.

This suggests you may have bad solder joint(s). Posting pictures is potentially useful, but look very closely at the solder work too and re-flow any joints that look suspect. If your FETs are in sockets, make sure they are seated properly too.

As you now have some sound, one possibility is that the JFETs are not working properly. If you socketed these, what happens if you remove them and then temporarily add a jumper to FET1 (connect source to drain with a piece of wire)?

[undercurrent77]

duck - here is a link to the pics of my board. It's not the clearest shot but maybe it'll help. The pot off to the side is a dual gang 10k. Everywhere I checked was out of the btdr-2h blocks,  so after looking into it I just went ahead with the model 3 and figured a little more fine tuning control couldn't hurt.

https://imgur.com/a/hD6ic

Samhay - I'm checking over all of it for cold solder joints and reflowing where needed. I etched the board myself so it could very well be a trace that pulled up around the hole when I soldered it. Unfortunately I foolishly didn't socket the jfets, but will pull fet 1 out and jump it as you described if I don't get anywhere with the solder joints.

Thanks again guys!

[samhay]

Don't pull the FETS, there is too much chance you will destroy the board.
You can use a pair of tweezers or similar to temporarily connect drain to source.

Why had you left several long leads on the solder side of the board?

[undercurrent77]

Okay good to know. The two long leads are from the capacitor that I replaced before I posted here. That was the one that I initially thought was bad. Just haven't cut them yet. I had left them uncut in case I needed to pull that capacitor again. But there's no reason to have them like that now. They're always facing up when I test it so I make sure they're not shorting against anything.

I did find that two of my wires had gotten mixed up going to the mix pot,  so I fixed them while I was going over the board reflow in solder where necessary. It didn't make any noticeable difference though so I'll use the tweezers to jump the few like you suggested.

[undercurrent77]

Here's a few updates. Made some progress tonight!  I realized as soon as I sat down that the volume pot was turned all the way down from when I reflowed the solder  ...man did I feel stupid.  I swear, I'm my own worst enemy sometimes.  I did jump the FETs and they effectively just bypassed the switch depending on which one I jumped.  That means they're working correctly, I'd assume? 

So the long and short of it is that I've got sound!  The next issue I'm facing is that the reverb is very tinny sounding and has a strange predelay to it that I can't get rid of.  The output is decently delayed behind what I play regardless of the potentiometer settings.  The signal also sounds overdriven as well.  I set out to make a reverb pedal and so far I've got a funky overdrive/delay combo.  I'm going to check over my components to make sure I didn't mess up a gain resistor value on one of the opamps.  It feels good to make some progress though!

Just wanted to give you all an update and thanks again for all the help