PT80 debug. Dry signal no effect

[dcjim]

Hello folks

Finished my PT80 and ... drum roll ... nada

Well actually first time round it was ... drum roll ... fizzing, squealing distorted mayhem! But I found a short somewhere between pins 1 and 2 of the TL072 (the 10K resistor beside it was reading 0 Ohm!) and that disappeared to be replaced by ...

... just guitar signal and no effect. There is some hum (hey, a self-build pedal just wouldn't be the same without some noise!) which varies a little in intensity if I tweak the pots but no r-r-r-r-r-r-r- ... peats

So other info

1. Using this layout (witch exceptions, see 5) http://www.generalguitargadgets.com/pdf/ggg_pt80_lo1.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a
2. It is a DIY etched board but I don't have the short that Solderman discovered
3. I'm using a charge pump but don't think it's a problem as my voltages are mostly good (see below)
4. I've used battery and wall wart
5. I have no switch in the circuit as the whole thing is ultimately part of a modular effects thing-a-me-whatsit but it's wired as link above just without the switch

Voltages - first are the values given in the WLS post which various people have used as the best reference point for this pedal, then there are my voltages. My input voltage is 8.62V

2N5088

C 10.85 11.34
B 5.22 4.92
E 4.73 4.95

TL072CP

1 5.37 5.74
2 5.36 5.74
3 4.77 5.73
4 0 0
5 5.31 2.86
6 5.32 5.74
7 5.32 5.74
8 10.67 11.46

SA571

1 .72 1.27
2 1.78 1.78
3 1.78 1.78
4 0 0
5 1.78 1.78
6 6.64 6.49
8 1.78 1.78
9 1.78 1.78
10 5.94 6.03
11 1.78 1.75
12 1.78 1.78
13 10.51 11.3
14 1.78 1.77
15 1.78 1.77
16 1.22 1.39

PT2399

1 5.03 5.01
2 2.51 2.52
3 0 0
4 0 0
5 4.67 0.4 to 3.8 (varies with turn of time pot)
6 2.51 2.3 to 2.5 (varies with turn of time pot)
7 .82 1.1 to 1.6 (varies with turn of time pot)
8 .82 1.1 to 1.6 (varies with turn of time pot)
9 2.52 2.52
10 2.52 2.52
11 2.52 2.52
12 2.52 2.52
13 2.52 2.52
14 2.52 2.52
15 2.52 2.52
16 2.52 2.52

And then the power bobbins (my values only, WLS didn't post any IIRC)

MAX 1044

1 8.62
2 4.14
3 0.02
4 0.16
5 -0.08
6 3.9
7 3.81
8 8.62

7805

1 12.47 (where pin 1 is the top of the board)
2 0
3 5.01

7812

1 12.5
2 0
3 11.4

So mostly the same with exceptions in bold. Pin 5 of the TL072 is low although somebody else posted a similar value for this and reported a successful build. Pin 5 (CLK_0) is low-ish and pins 7 and 8 (CC1 and CC0) of the PT2399 are high-ish but I'm getting no effect anyway and it's Vcc and GND are good so I think it's happy, no?

I've swapped TL072s, MAX1044s, tried a 7660 instead of a MAX1044 and I'm waiting for new PT2399 and SA571's to try out.

My thoughts are ...

Are my supply voltages Ok?
Is the low voltage on pin 5 of TL072 a problem?
Have I guffed up the panel wiring
Must I really quadruple check values, wiring, shorts, Etc?!

Thanks
Jim

[dcjim]

Ok ...

Audio probe in full effect ...

I've got signals (where applicable) as follows on these ICs/pins

X = nothing or inapplicable anyway G = dry guitar D = delayed guitar ! = very distorted

PT2399

1 X
2 X
3 X
4 X
5 X
6 X
7 D!
8 G!
9 G
10 X
11 X
12 D
13 X
14 D
15 G
16 X

SA571

1 D quieter than elsewhere and with what sounds like a slow attack
2 D
3 D
4 X
5 X
6 D quieter than elsewhere and with what sounds like a slow attack
7 D quieter than elsewhere and with what sounds like a slow attack
8 X
9 X
10 G
11 G
12 X
13 X
14 G
15 G
16 X

TL072

1 G
2 X
3 X
4 X
5 G
6 G
7 G
8 X

Where delayed signal is heard it does respond to the delay time pot but the repeats and level pots do nothing

Soooo ....

I'm thinking my PT2399 and SA571 are just fine and dandy. I'm assuming the signal with what sounds like an attack phase are the repeat signals? So it would seem something between the SA571 and the TL072 is letting the side down. Shameful.

So I guess I better go check everything between those two stages - as I'm getting a dry signal out of the TL072 I guess there must be a short - but presumably the repeats come from feeding the signal back into the delay? So do I have to trace all those paths back too? Or should I consider them as separate? I know somebody did post an issue where they had delay but no repeats.

Any other pointers folks?

[dcjim]

Ok well here's what happened on this one and it'll make you shiver!

I tried some debugging but didn't get much joy. I finely admitted to myself that I'd fab'd the PCB pretty badly so I made myself start all over again with a brand new PCB to eliminate that variable. I redo the lot and ...

... exactly the same behaviour.

There I was ready to throw the thing out the window when purely by chance I kneeled on one of the pots (I was testing on the floor beside my guitar amp) and voila! Luscious analog delay!

Turns out one of the 50k pots I'd bought of eBay was mechanically faulty. The pressure of my knee against it obviously pushed everything in contact and it worked. If I'd never have leant on it I'd never have found out!

I won't name names but needless to say I won't be leaving positive feedback. The moral of the tale? Avoid cheap crap on eBay or at least test everything to death!

[soohyun2222]

Very glad to hear your success

I am in the process of debugging my PT80 too, and come to your post.
You kept posting replies even though the reply was delayed. Finally you came to success!
Hopefully I can get mine to work too.