PT-80 Delay troubleshooting

[mathrawk]

Hi there. I'm very new at the whole building pedals game and was wondering if anyone could shed some wisdom on my situation. I recently finished wiring up my first pedal, a PT-80 delay from GGG. When I went to fire her up, the led turned on but I get about 95% dry signal. In the background you can the faintest hint of a delay when the pots are positioned a certain way. Bypass works fine. I've checked my voltages and everything matches up except pin 16 on the SA571 is 1.2 instead of 1.0 and on pin 5 of the TL072 is at 6.53 instead of 5.5. I still don't know much about the troubleshooting process so if anyone knows what this could mean, the help would be GREATLY appreciated. Also, not sure if this is a potential issue but I felt it worth mentioning, I found out a few days after wiring up that the SA571 I was supposed to install is actually labeled SA517NG on Mouser's order sheet. The part itself is labeled SA571N. Could this be my issue? Thanks to all who take the time to help me out, I really appreciate it. Hope to hear from you soon and happy building!

-Sean

[Govmnt_Lacky]

Did you install the MAX1044 charge pump? I just finished debugging a PT-80 build only to find that I got 3 bad PT2399 chips (they all had the same batch number so it must have been a bad batch) however, I had quite a few BAD voltages compared to JD's build instructions.

Another note is that JD has the Delay Time pot wired backwards. So it actually works kinda in reverse.

Try setting the pots to these positions and let us know what you hear:

Delay Level: 50%
Delay Repeats: 50%
Delay Time: 10%

[mathrawk]

Thanks for the quick reply! I set it to your instructions and I get 0 wet signal. No difference from my bypass really.

[mathrawk]

Ha sorry, yes I did install the 1044. Wow I can't believe that many were bad!

[Govmnt_Lacky]

At this point it would probably be good to measure the voltages again and post them here. Dont forget to include you power supply voltage. also include the voltages at your 5V and 12V regulators.

[mathrawk]

Will do! I'm currently without a multi meter but I'll have access to one in a few days. I'll post back soon. Thanks a ton.

[mathrawk]

Hey there, sorry for the wait. Just moved into the new dorm and been trying to get everything situated, finally got ahold of the multimeter once again. The voltages we're measured as follows:

TL072
1.= 5.91
2.= 5.91
3.= 5.91
4.= .001
5.= 5.38
6.= 5.91
7.= 5.91
8.= 11.92

PT2399
1.= 4.98
2.= 2.5
3.= .022
4.= .022
5.= 2.777
6.= 2.492
7.= .65
8.= .658
9.= 2.5
10.= 2.5
11.= 2.5
12.= 2.5
13.= 2.5
14.= 2.499
15.= 2.5
16.= 2.5

SA571
1.= .7
2.= 1.78
3.= 1.78
4.= .005
5.= 1.78
6.= 6.52
7.= 6.52
8.= 1.78
9.= 1.78
10.= 5.91
11.= 1.78
12.= 1.78
13.= 11.92
14.= 1.78
15.= 1.78
16.= .966

Max 1044
1.= 9.45
2.= 4.65
3.= .024
4.= .17
5.= .354
6.= 4.75
7.= 7.18
8.= 9.45

Power Supply= 9.45

5v reg
E= 16.17
B= .02
C= 4.98

12v reg
E= 16.18
B= .022
C= 13.55

There you have it. Thanks and hope to hear from you soon!

[Govmnt_Lacky]

All of your voltages look to be somewhat OK (your 12V regulator is kind of high at 13+ volts) but at the 2399 and 571 all looks pretty good. You may want to investigate the following:

1) ALL off-board wiring....especially pots. Are they grounded properly, wires on correct lugs, and are the lug jumpers correct?
2) Grounds. Make sure there are NO GROUND LOOPS. Ensure the box is wired AS PER JDs SPEC.
3) Pot values. Did you mistake a wrong value? (Log for Linear or Rev Log for Linear???)

Dont forget that the Delay level pot is kinda like a wet signal level control. The higher you crank it, the more delay you will hear with your dry signal.

Also, like I said before, the Delay time pot is backwards in the Layout.

Posting pictures could help too if you can. Pictures are worth..... well you know!

Good Luck and let us know what you come up with 

[mathrawk]

Thanks for the quick reply! I've gone through your list and everything seems ok. As far as the ground loop section however, I don't believe there are any but what exactly should I look for to find one? I'm pretty sure I have an idea, mostly just wanna clarify. All pot values are correct and I've looked over the wiring several times and can't find any issues. I'll post pics later this evening. Thanks for your patience!

[studiostud]

Did you make the PCB yourself?  I've done a few of these (from the GGG layout) and one thing I had problems with was that on this layout, many of the traces are small and close together.  On two of the boards, there was actually a bridge that from a copper pad to GND where the copper didn't dissolve completely in the etching process so I had to take a hobby knife and scrape a break in between.  That took me many frustrating hours to troubleshoot that little problem.  Just a head's up in case it could be a similar problem.  I didn't think about looking for that until someone else told me to so I thought I would be that guy for you.   

[mathrawk]

To Govmnt Lackey: Here're some images. I apologize for the really bad quality but its the best I can offer with what I got :/.

StudioStud: I did make mine by hand. I took your suggestion and scrutinized every bit of the board. Made a couple resolders where things looked sorta iffy and scratched off all areas that looked to even possibly be touching. Still nothin :/. I'm about 99% sure I cleaned up any potential problems, but I'll keep searching around the board for any bad spots. Thanks for the tip though! I actually had to make a second board for this pedal because the first one had too many copper spots that looked like future problems.

I know it was mentioned earlier that my PT2399 might just be a bad chip? Is there any way to test it without simply replacing it?

[studiostud]

dunno if this will help, but if you look at the photo you posted of the bottom of the PCB, match the orientation of the photo with the actual PCB in your hand and then scan the top row of solder points and the 5th from the left (the only joint on the top row that's NOT directly under the ground path).... that's the joint that I had to scrape through a bridge.  You can even see on the PCB graphic that that solder point and the path directly above are extremely close together.  If you're unsure of a connection, just use your multi-meter to check continuity.

Do you have an audio probe?  That would be the best way to troubleshoot the circuit because you could follow the signal path and see where it ends.

http://experimentalistsanonymous.com/diy/Datasheets/PT2399.pdf   Here is the data sheet for the PT2399 as well.  It has a layout of the chips connections which will be helpful if you are probing it.

[Govmnt_Lacky]

+1 for thecopper traces being close. EVERY PT-80 board that I etched needed the SAME SPOT to be "trimmed" with my hobby knife. I did it every time since then just to be sure.
Follow me here.... I am going to try to explain which copper trace I am referring to:

According to JDs layout:

If you look at your 2N5088 transistor, directly below that is a .001uF (102) film capacitor. If you notice, one of the legs of that capacitor is VERY CLOSE to the trace of the 10K/333 cap/5088 junction. Check for continuity.

[mathrawk]

Bingo! The lead near the transistor did the trick! That would've taken me hours more to notice. Thank you so much everyone for your insight. This is a wonderful community and I'll definitely be coming back.

[Govmnt_Lacky]

Just remember to help out someone else when you can. A lot of people like to add their 2 cents but offer no "real" help. Most of us are not EEs and sometimes need things broken down in common terms. Glad you got it working and enjoy the Delay.................   

[studiostud]

Bingo! The lead near the transistor did the trick! That would've taken me hours more to notice. Thank you so much everyone for your insight. This is a wonderful community and I'll definitely be coming back.

DIYStompboxes needs a facebook-style "like" button.  How 'bout it, Aron?