Poison apple. help me think through what I am seeing

[Diynot]

So my Poison Apple “works”, but I am getting serious popping/thumping at the envelope opening and even a little distortion on lower strings/frets. I have been pouring over this thing doing all the troubleshooting basics and the thing I keep coming back to is that my 5v keeps disappearing. So long story shorter, a continuity test between the + terminal of the power jack and the pads of the regulator beeps on both the left and middle pads, but not the far right.
This would seem to indicate that the left pad is meant for the Vin. Great except the pinout for the regulator has the left pin as Vout.


And I do get continuity between the anode side of D4 and the left regulator pad. So should I just ignore the silkscreen and flip the regulator? then my question becomes why does the middle pad show continuity to the positive terminal, shouldn’t that be connected to ground (which it actually does show connection to both)? Also get continuity between the left pad and the collectors of the bc550. One other oddity is that I do not get Vcc on ANY of the pads Help a brain dead brother out please.

 

[jwin615]

L78Lxx and L78xx have reversed pinout. Power the board up without the regulator and see which pin is hot.
If you have 9v on the center pin, you have a bridge/short somewhere

 

[Diynot]

L78Lxx and L78xx have reversed pinout. Power the board up without the regulator and see which pin is hot.
If you have 9v on the center pin, you have a bridge/short somewhere

The regulator I am using is the one pictured so I’m going with there may be a short. In desperation I have off boarded the regulator on a vero and fed it into the left pad of the regulator and every thing seems to be behaving. It’s what I get for taking a build break.

 

[Diynot]

Well, the popping/thumping is resolved but I am still getting light distortion on the E string and some of the higher frets of the A. Sounds a bit like fret buzz, but it happens with multiple guitars (Humbucker and single coil) and is gone when I turn the pedal off. Going to audio probe some other day since I am tired of looking at this. Posting pics in case someone else wants to put eyes on it.
As per my previous post, the regulator has been off boarded so disregard that in the pics

 

[falco_femoralis]

Try different op amps for that light distortion. The TL072's you have look sus

The middle pin of the L78L05 is supposed to be ground. If you are getting continuity between pin 1 and 2, then your 5v supply is shorting to ground somewhere. Check anywhere that 5v is present, like Q2, Q3, and the bias pot, and downstream of these, for any stray solder or broken/bridged trace, leads not trimmed short enough etc that could cause this

 

[Diynot]

Try different op amps for that light distortion. The TL072's you have look

Thanks for taking a peak at my problem child. So I have tried 2 different sets of lm358 as well as the TL072 which came from Mouser so would trust to be reliable. All with the same issue. I somehow must have borked the regulator section of the board. Off boarding the regulator solved the thumping issue. Entirely possible I’ve F’d something else up somehow causing the distortion. Meh

 

[gtfields13]

I have one of these built and unhoused, so I am curious to see what I get for low frequencies. Was this happening at all control settings, or only some specific ranges?

 

[Diynot]

I have one of these built and unhoused, so I am curious to see what I get for low frequencies. Was this happening at all control settings, or only some specific ranges?

All settings. I did a brief audio probe and it seems like it’s being injected at R5 BUT I actually don’t hear it coming out of the LM13700. I thought maybe me releasing the magic smoke on C100 (which was rectified) might have caused some AC leakage, but strapping a cap across the power terminals didn’t change it any. It is better with single coils.

 

[gtfields13]

I definitely don't have this happening on mine, including testing with a humbucker as well as single coils.

What I have figured out is that I don't understand how to read/interpret signals through an OTA yet.

But your symptoms (clipping/buzz, stronger on hotter pickup) sound a lot like when an Op Amp is biased off center and starts running up against a rail. I have tested my board, and almost everything on the Op Amps and the LM13700 was sitting around 4.5 V:

LM13700:
3, 4, 13, 14: 4.5 V
10, 12: 5.7V
5, 7: 5.6V
15, 2 (NC)
5: 0 (Gnd),
11: 9V (V+)
16: 1.2V
1: (forgot to wwrite down)

IC2 (LM358)
1,2,3, 5,6,7: all at 4.5V

IC3 (LM358)
1: 0.1V
2, 4.8V
3, 4.5V
5,6,7: 4.5V

Also maybe check Q1
1: 9V
2: 5V
3: 0.5V (with a 1V signal input...)

Perhaps something else was damaged and your Vref isn't running at 4.5 V now.

 

[Diynot]

But your symptoms (clipping/buzz, stronger on hotter pickup) sound a lot like when an Op Amp is biased off center and starts running up against a rail. I have tested my board, and almost everything on the Op Amps and the LM13700 was sitting around 4.5v

Thanks for the info. Once I moved my regulator off board I am getting the correct voltages where expected. The 5457 has clean signal going through it. This is going to be a really dumb question, but I am second guessing myself after staring at the schematic for far too long, but does the audio signal actually pass through the LM358s?

 

[gtfields13]

The audio signal is not passing through the LM358s. That branch is just establishing the control current that is feeding the LM13700.

I found a post on GuitarPCB that may be helpful:

Small issue with my SWAH

Your Source for DIY Pedal PCBs and NostalgiTone! › GuitarPCB Forum › GuitarPCB Build Support › Small issue with my SWAH Tagged: SWAH issues This topic has 19 replies, 4 voices, and was last updated 9 months ago by Playsforfun. Viewing…

guitarpcb.com

scroll down and there is a moderator re-post of an earlier post (that I find inaccessible). This includes a colored diagram showing the functions of the different blocks. You might consider that what the post calls pink looks pale-orange, and lilac = magenta.

There are a handful of other good forum posts on people troubleshooting the GuitarPCB Snow White. I suspect the circuit is basically the same, so the information should still be relevant.