Fuzzdog Spectre Verb issue

[idiotwind]

Hello

I recently built a Spectre Verb from Fuzzdog (based on the EQD Ghost Echo).  The info pdf with the schematic etc is at http://pedalparts.co.uk/docs/SpectreVerb.pdf - I built it with the Dwell mod (R17 = 12K, Dwell pot = 25KB), and R5 is 18K for unity volume.  It uses a long decay BTDR-2H Belton Brick.

All was working fine, but whilst playing with the Dwell control right up, it got a bit out of control with the self-oscillation, there was a pop and the output became faint.  It then worked on and off for a couple of weeks, sometimes working fine all day, then other times flickering back and forth.  At the moment though it has not worked for a week.

I built an audio probe and started investigating.  I can tell for example that the Belton Brick is working as I can hear the clean signal going in and the wet signal coming out as expected.  The output from the circuit has reverb and also has the pre-delay from the PT2399, but is very faint.  Turning up the volume shows it is quite distorted.  If the Depth control is turned right down there is no output - this should be the dry output.

Here are my voltage readings:

Board V in = 9.12V

IC1 TL074
P1 = 6.72V
P2 = 6.08V
P3 = 6.08V
P4 = 9.12V
P5 = 3.04V
P6 = 6.08V
P7 = 6.08V
P8 = 8.53V
P9 = 6.01V
P10 = 6.03V
P11 = 0V
P12 = 6.04V
P13 = 6.08V
P14 = 6.08V

IC2 PT2399
P1 = 4.97V
P2 = 2.47V
P3 = 0V
P4 = 0.05V
P5 = 2.00V
P6 = 0V
P7 = 0V
P8 = 0V
P9 = 2.48V
P10 = 2.48V
P11 = 2.48V
P12 = 2.48V
P13 = 2.48V
P14 = 2.48V
P15 = 2.48V
P16 = 2.48V

IC3 78L05
P1 = 9.13V
P2 = 0V
P3 = 4.97V

IC4 78L05
P1 = 9.13V
P2 = 0V
P3 = 4.98V

D1 1N4001
A = 0V
K = 9.13V

From what I've read, IC1 pin 8 looks off, and using the audio probe I have a dry signal at 10, wet signal at 9, then 8 is about 3 times the volume and very distorted.  I replaced the TL074 but it does the same thing.  I was kind of hoping this would have been the fix, but perhaps there is something else that has gone wrong.  I guess the pop suggests some component got damaged and has been slowly failing to the point where it is now dead?  The way it was working perfectly for a couple of weeks makes me think it is unlikely to be a short or bad solder joint, though I could of course be wrong.

Happy to provide images/sound clips/readings - this is only my second pedal build and the first one went fine, so I've not got much experience of debugging circuits.  There is a bit of an issue as the back of the board is pretty inaccessible due to the pots and Belton Brick covering it, but I'll do my best.

Thanks

[bluebunny]

Welcome!   

Yeah, pictures would be useful.  "Pop" is not good (I'm assuming you didn't mean a pop on the audio output).  Anything obviously blown-up, singed, cracked?

[idiotwind]

Welcome!   

Yeah, pictures would be useful.  "Pop" is not good (I'm assuming you didn't mean a pop on the audio output).  Anything obviously blown-up, singed, cracked?

If you check out the last 30 seconds or so of this demo video you can hear what I was getting, with it feeding back and building up.  But that went a bit too far and then yes, there was a "pop" sound in the output and then it went faint.

I can't see any obviously bulging capacitors or scorching, and have tried replacing both the TL074 and the PT2399 chips.

For photos, front and back are here: https://imgur.com/a/OYT631F - as I said, it is pretty difficult to get a clear view of the back of the board because of the pots and brick

[PRR]

IC1 p12 p13 p14 come from two equal resistors R20 R21 across the 9V supply. We expect 4.5V here. You have 6V here and all Vref points?

< worked on and off for a couple of weeks, sometimes working fine all day, then other times flickering back and forth.  At the moment though it has not worked for a week.

The PT reverb chip has funky modes. But "usually" a build which works and not-works unpredictably, you should suspect Solder Joints.

[idiotwind]

IC1 p12 p13 p14 come from two equal resistors R20 R21 across the 9V supply. We expect 4.5V here. You have 6V here and all Vref points?

< worked on and off for a couple of weeks, sometimes working fine all day, then other times flickering back and forth.  At the moment though it has not worked for a week.

The PT reverb chip has funky modes. But "usually" a build which works and not-works unpredictably, you should suspect Solder Joints.

Thanks very much for taking a look.

Yes, I'm definitely getting 6V at those pins.  Investigating that part of the circuit, I can see R20 is 9.13V on one side, 6.04V on the other.  R21 is 6.04V on one side and 0.001V on the other.  However, looking more closely, I can see that I actually made a mistake on R21 - that brown first stripe is actually red.  So it is 20K not 10K (they really do look very similar in normal light)



That explains the voltage only dropping to 6 instead of 4.5, correct?  I'll get that swapped for a 10K and recheck the voltages.

[antonis]

looking more closely, I can see that I actually made a mistake on R21 - that brown first stripe is actually red.  So it is 20K not 10K..
That explains the voltage only dropping to 6 instead of 4.5, correct?

(actuall resistors values aren't critical as far as they're equal..)

[bluebunny]

That explains the voltage only dropping to 6 instead of 4.5, correct?

Correct.

The voltage at the "middle" of a potential divider such as this is directly related to the ratio of the resistances.  Ohms law tells us V=IR.  The I (current) must be the same though both resistors since electrons can't appear or disappear spontaneously en route (the current entering one end has to emerge at the other - all of it).  Therefore the voltage V is directly proportional to the resistance.  Since your resistors were in the ratio 20000:10000 (or 2:1), so are the respective voltages across each resistor.  Hence 6V as your V_REF.

Phew!  That was long-winded!  Just eaten lunch, so some sugar-rush I guess...   

[idiotwind]

Ok, thanks, I've replaced that resistor and have new values for IC1:

P1 = 4.94V
P2 = 4.56V
P3 = 4.56V
P4 = 9.12V
P5 = 2.28V
P6 = 4.56V
P7 = 4.56V
P8 = 8.52V
P9 = 4.51V
P10 = 4.53V
P11 = 0V
P12 = 4.54V
P13 = 4.56V
P14 = 4.56V

So, that is hopefully a bit better.

Still the same output.  Pin 8 of IC1 still confuses me.  If I'm looking at it right, is IC1C supposed to take the dry signal in at pin 10, the wet signal in at 9 and output a blend of the two at 8.  This then goes through R18 and C15 on it's way to the output.  I have a good dry and wet output at 10 and 9, but 8 is just massively loud and blown out.  Checking now, the sound at R18 seems like a slightly poorer quality version of pin 9.  As I mentioned in my first post, if Depth is right down, there is no output, as if the dry signal is not getting to the output.

[PRR]

I suggest R5 is not really connected.

[idiotwind]

I suggest R5 is not really connected.

Well, it's now working.

I redid R5 as you suggested and there was a definite improvement - I got a faint dry signal when Depth was turned off, whereas before it was silent.  But it was still not 100% there, so I pressed on.  I redid R6 as well, and then because it was bothering me that the loud signal from pin 8 of IC1 seemed to be weak by the time it got to R18, I redid pin 8 as well.

Plugged it in and put my probe on the output and got a solid noise - just a BRRR.  Probed around and found it all over the place.  I was just starting to panic when I remembered the Dwell control was right up and I was just listening to a wall of reverby feedback.  Turned that down and it was perfect.

Thanks so much for your help, it really showed me where to focus my attention.

[ElectricDruid]

Good work! It's nice when you fix one, isn't it?!