Help troubleshooting/repairing PT-80 Delay

rm77 · March 29, 2014, 08:07:35 PM

Hello, first-time poster here. I'm hoping to get some advice about troubleshooting and repairing a damaged GGG pt-80 delay pedal I built. It worked great for a while, but today I could only get one faint clipped echo regardless of settings. I decided to try another power supply and-here's the fun part-I accidentally plugged in a 12V AC adaptor. BOOO!

Now signal gets through either engaged or in bypass, but no echo at all. I checked IC voltages referring to the GG instructions, and here's what I got. I know that plugging an AC into a DC can cause all kinds of problems, but I'm hoping it's still salveagable. If anybody can offer some advice, I would really appreciate it. If there's other things I should check please let me know. Thanks!

PT2399: All voltages match those listed very closely; the ones farthest off are pins 7 and 8 which measure .5 and should be .9.

NE571:
pin 1 should be 1.0; it starts at around .35v and then steadily drops down to 0.0
pins 6 and 7 are 5.5 rather than 6.5.
pin 10 is 1.8 rather than 6.0
pin 12 is 6.3 rather than 1.8
pin 13 is 1.6 rather than 12
pin 15 5.5 rather than 1.8
pin 1.8 rather than 1.0
the others are right on or very close.

First TL072:
pins 1&2 are 9.3, rather than 6.0
pin 3 is .007, rather than 6.0
pin 5 is 9.3, rather than 5.5
pin 6 is 8.8, rather than 6.0
pin 7 is 7.4, rather than 6.0
pin 8 is 0.1, rather than 12

Second TL072:
pins 1,2,3,6&7 are all 3.1, not 6
pin 4 is 0, should be 0
pin 5 is 6.3, not 5.5
pins 6&7 are 3.1, not 6
pin 8 is 2.7, not 12

Charge pump regulators: 1st top leg 7.7, rather than 16, bottom leg 5.0 (right I think). 2nd top leg 7.7 rather than 16, bottom leg 6.3 rather than 12. Middle legs both read .006 mV.

Govmnt_Lacky · March 29, 2014, 08:23:57 PM

Check the output voltages of BOTH the regulators (5V and 12V)

The AC may have fried your voltage regulators. If so, you may have gotten lucky and it stopped it before it fried ALL of your ICs

rm77 · March 29, 2014, 09:54:44 PM

Thanks very much for your reply. I went back and checked the output voltages on the regulators, but also caught a mistake I made in my post. I thought I had 2 TL072 ICs, but one is actually a Max1044 (the charge pump). This is what I mistakenly listed as "First TL072," and it has these voltages:

pins 1&2 are 9.3, rather than 6.0
pin 3 is .007, rather than 6.0
pin 5 is 9.3, rather than 5.5
pin 6 is 8.8, rather than 6.0
pin 7 is 7.4, rather than 6.0
pin 8 is 0.1, rather than 12

So the voltage coming out of the charge pump is close to nothing, instead of 12! This must be the beginning of the problem since it's right where the power enters the PCB and it's about as off as it can possibly be, right?. The regulators both show in voltages of 7.7 when the instructions say they should be showing 16v. The regulator that should have an output voltage of 5 does have 5, but the one that should have an output voltage of 12 shows only 6.3. So, maybe and hopefully, the problem might be fixed by replacing the Max1044? The instructions did say that the pedal can operate on 9v without a charge pump, albeit at a lower quality, so I'm hoping that I can take out the extra charge pump pieces and try it to see if that helps. If not, I guess I know that something more is wrong. What do you think?

rm77 · March 30, 2014, 12:32:29 AM

Update: since the charge pump seemed to be screwed, I took it out as well as the capacitors and diodes that were part of the charge pump chain according to the layout doc. I rewired the power to the spot on the pcb where the no-charge-pump version of the layout shows it entering. Unfortunately, same result-signal either in bypass or engaged but no delay. The two regulators now show:

1. 5v regulator: 9.3v in, 5.0 out
2. 12v regulator: 9.3v in, 7.8 out...

The TL072 now shows:
pin1: 3.9, should be 6.0
pin2: " "
pin3: " "
pin4: 0.0 as it should be
pin5: 7.9, should be 5.5
pin6: 3.9, should be 6.0
pin7: 3.9, should be 6.0
pin8: 3.4, should be 12.0 (don't know if this should actually be different in the non-charge-pump version)

The PT2399 still shows pretty much the voltages that it should according to the instructions.

Govmnt_Lacky · March 30, 2014, 10:44:44 AM

Quote from: rm77 on March 30, 2014, 12:32:29 AM
Update: since the charge pump seemed to be screwed, I took it out as well as the capacitors and diodes that were part of the charge pump chain according to the layout doc. I rewired the power to the spot on the pcb where the no-charge-pump version of the layout shows it entering. Unfortunately, same result-signal either in bypass or engaged but no delay. The two regulators now show:

1. 5v regulator: 9.3v in, 5.0 out
2. 12v regulator: 9.3v in, 7.8 out...

To get the proper voltages out of BOTH regulators, you need to feed BOTH of them at least 15VDC. You will not get 12V out of the 12VDC regulator unless you feed it AT LEAST 3V higher than the output.

Try feeding 15VDC+ (no more than 30VDC) to both regulators and then check the circuit. I suspect that you may have fried your MAX1044 with the AC adapter. In any case, you really should change that out for a TC1044 or ICL7660 charge pump. MUCH, MUCH more reliaable and better voltage tolerance.

rm77 · March 30, 2014, 06:48:49 PM

Ok I fed it 18v with a couple of 9v batteries and the 5 and 12v regulators now read about 5 and 12v respectively. I tested with a guitar signal and now there is an echo although very faint with the pedal volume control at Max; also the time control is working. So I guess I'll get a new charge pump IC like one of those you mentioned...but now I'm wondering about the low volume on the delay...maybe a bad connection with the volume pot? Now that I've circumvented the fried Max1044 for the time being, I'm back to my original issue, which is the barely audible delay.

Thanks again very much for taking the time to help!

Govmnt_Lacky · March 30, 2014, 07:02:09 PM

I recomend that you re-take ALL IC voltages while you have the proper 5V and 12V coming out of the regulators.

This will tell us where the problem may be.

rm77 · March 30, 2014, 07:53:27 PM

Sounds like a good idea

. Here we go:

TL072:
All voltages are just about right, except that pin 5 is 12v instead of 5.5v, and pin 8 is 5.5v instead of 12v...

PT2399:
Everything is within 0.3v of the voltages listed in the instructions

NE571N:
First 4 pins are right on, then:
pin 5 1.2 (1.8 in instructions)
pin 6 1.2 (6.5)
pin 7 1.2 (6.5)
pin 10 1.8 (6.0)
pin 12 12 (1.

pin 13 1.8 (12)
pin 15 6.3 (1.

pin 16 1.8 (1.0)

Govmnt_Lacky · March 30, 2014, 08:00:30 PM

Is this a fabbed PCB from GGG -OR- is it a self etched PCB?

Could you post some pics of your build?

rm77 · March 30, 2014, 09:11:37 PM

It's a prefab PCB from GGG. Here are a couple of pics

http://s189.photobucket.com/user/Bwv1048/media/DSC_0297.jpg.html

http://s189.photobucket.com/user/Bwv1048/media/DSC_0296.jpg.html

bluebunny · March 31, 2014, 03:22:59 AM

Quote from: rm77 on March 30, 2014, 07:53:27 PM
. . . except that pin 5 is 12v instead of 5.5v, and pin 8 is 5.5v instead of 12v...

Are you counting pins correctly? Go counter-clockwise from wherever you decided pin 1 was. (Don't count down one side, then down the other.)

rm77 · March 31, 2014, 10:19:47 AM

No I was not counting them correctly...another rookie mistake. So that explains the strange voltages for the TL072, since I was calling pin 8 pin 5 and vice versa...

I'll go back and re-measure the NE571N tonight since I didn't list every voltage and don't have the paper I wrote it all down on with me. I can see that this explains most of the strange voltages on that one too. I'll just measure it all again tonight and see what I get. Thanks for pointing that out, I completely overlooked it!

rm77 · March 31, 2014, 09:25:09 PM

Measurements along with ggg instructions listed voltages in (). Now that I have these listed right, it seems like maybe the problem is with the NE571N IC, pins 6 and 7? I looked at the data sheet and it says pin #6 is "Res. R3 1, " and pin #7 is "Output 1." Looking at the location of the NE571N on the parts layout diagram, it looks like pins 6 and 7 connect to each other, then connect to one leg of a cap, the other leg of which connects to the spot on the pcb where a wire runs to lug 3 of the volume pot, which in turn is wired to lug 3 of the repeats pot. Since my problem is hearing only 1 repeat which is very faint, almost inaudible, i wonder if this is the source of the problem? If so, would I start by replacing the NE571N to see if the right voltages from a new one fix the problem?

NE571
1. 0.4 (1.0)
2. 1.85 (1.

3. 1.85 (1.

4. .005 (.003)
5. 1.15 (1.

6. 1.15 (6.5) <-
7. 1.17 (6.5) <-
8. 1.85 (1.

9. 1.85 (1.

10. 6.3 (6.0)
11. 1.85 (1.

12. 1.85 (1.

13. 12.4 (12)
14. 1.85 (1.

15. 1.85 (1.

16. 0.9 (1.0)

TL072
1. 6.2 (6.0)
2. 6.2 (6.0)
3. 6.2 (6.0)
4. 0 (0)
5. 5.4 (5.5)
6. 6.2 (6.0)
7. 6.2 (6.0)
8. 12.4 (12)

PT2399
1. 5.0 (5.0)
2. 2.5 (2.5)
3. .006 (.015)
4. .006 (.015)
5. 2.9 (2.

6. 2.5 (2.5)
7. 1.2 (0.9)
8. 1.0 (0.9)
9 through16 2.5 (2.5)

Govmnt_Lacky · April 01, 2014, 10:56:39 AM

Try checking Pins 6 & 7 of the NE571 with the Delay and Repeats pots at full CCW and full CW. MAybe JD's reading were taken at different points in the pots rotation.

If there is still no change, you should verify ALL of the connection between the NE571, to the 1uF cap, then to both pots, etc.

Definitely rule out everything you can before calling the Compander chip dead. They really aren't that cheap $:-\$

duck_arse · April 01, 2014, 11:44:00 AM

it should be possible to "listen" at pin 3 and 2 on the ne571, to see if you have full sized signal going in, no? the companders are usually only doing filtering/shaping in these circuits.

rm77 · April 01, 2014, 05:03:56 PM

Got it working

!

I failed to notice, since it wasn't obvious visually or even when I was poking at the pins repeatedly with a multimeter for a few days, that the NE571N was just a tiny bit out of its socket on the side with pins 6 and 7. But when I gave it a firm press to see what would happen, it snapped in just a little further. Voltages are now correct on 6&7, plugged it in to guitar and amp and it works fine. Somehow the IC must have gotten a bit loose in the socket; I suspect a certain 3-year old who lives in my house, in combination with a bit of shoddy construction on my part

.

I tried it on 9v, since the instructions said this would be ok if not allow the effect to function at highest quality, and since that works well enough I'll stick with it until i have gotten a new charge pump per Govmnt_Lacky's suggestion.

Thanks very much for the help guys...I wouldn't have found the simple solution if I hadn't been following all of the suggestions. Glad I didn't just give up and write it off as "fried." I learned a lot about troubleshooting and I'll be in better shape to do some diagnosing on my own next time I have a problem!

By the way, for future reference, how would I go about "listening" to the IC pins? I saw something somewhere about a way to rig a TRS cable to insert an audio signal at different points in a circuit, is it something like that?

duck_arse · April 02, 2014, 10:27:49 AM

search up "audio probe" or "signal injector". there is one hereabouts people seem to like. I have a cro, so I've never used an audio probe.

and well done. now you can join the opinions on IC sockets when they come around again.

Govmnt_Lacky · April 02, 2014, 11:04:51 AM

Quote from: rm77 on April 01, 2014, 05:03:56 PM
...that the NE571N was just a tiny bit out of its socket on the side with pins 6 and 7. But when I gave it a firm press to see what would happen, it snapped in just a little further.

I'm gonna take a not-so-wild guess here and say that you were using the cheaper IC sockets from Tayda? Am I right?

I have sworn off those things! I have had several pedal builds have issues because the ICs will work themselves out of those sockets.

I use ONLY machined IC sockets now. Only a few pennies more and a LOT better reliability.

rm77 · April 02, 2014, 04:09:12 PM

Not sure about the socket brand, they're the ones that came with the kit...I'm curious now though so I'll see if I can find an identifying mark of some kind. And yeah, I think for future builds, especially when I try one without kit, I'll spring for some higher quality sockets, or maybe avoid them altogether depending.

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Help troubleshooting/repairing PT-80 Delay