More PT-80 problems...please help!

[DaveTV]

I had my PT-80 up and running but now I can only get a clean signal through it, no delay.

Checking around with my multimeter while the circuit has power, I can measure a small DC voltage (1-2 volts) on most of the pins of the IC chips. This is a bad thing, right? Still, the V+ pins on each chip show the correct voltage.

Is it possible that I've somehow shorted out a capacitor and the voltage is bleeding to the IC's through ground? How might I test for this? Would a shorted out capacitor show continuity?

[Erich F]

Boy..have I ever been there! Scroll down the page and check the posts “Help with 571 Compander”. It turns out a bad capacitor was the culprit with my PT-80, but it works now. There’s a lot a good info there. 1.8 V or so on most of the NE 571 pins is OK but that would be too low for the opamp.  Check the voltages on the tranny, that was the clue that finally pointed to the area where the bad cap was.

I suggest you use an audio probe and see if anything delay is coming out of the delay chip. Also, make sure the 50K delay pot is good. I could go on and on…let us know what you find

[DaveTV]

Thanks for the tip, Erich. I just read through your posts. Lots of excellent information there. Can you tell me how you figured out which cap was the culprit?

[Erich F]

Rob Strand walked me thru this. The voltages on the 5088 tranny were off..C-12.4V, B -0.45V and E-0.45V. I  measured the Reference voltage before the 100K near the tranny and it was correct at half the supply voltage (approx. 6V), once I removed the capacitors before the base (the 0.001, 0.02, and 0.033), the voltages at the tranny measured correctly. As I reconnected each cap back into the circuit, I measured the tranny voltages until they were screwed up again..this way I could the identify the cap that caused the problem.

I blew this cap when I tried to cut the board back with a Dremel and I slipped. The cap looked fine but, in the end, it was broken. Try to remember what you may have done prior to the effect failing, there might be clues there. Also, check that your pots are functioning right, especially the delay pot. I had an earlier problem that was resolved this way.

[Ed G.]

DaveTV, I hope you don't mind me 'piggybacking' on your post, but I just finished the pt80 and I also get no delayed sound, only regular signal.
I don't have a signal generator or audio probe (yet) but I took some voltages. Most look allright so far from what I could see, but pins 9 to 16 of the PT2399 were all exactly 2.51 volts. I thought that a bit funny.

[Ed G.]

One more thing I noticed, the 'active' signal is duller sounding than the bypassed signal. I don't know what that has to do with anything, but it shows that something isn't right. I'm still wondering about half of the chip reading right at 2.51 volts.
I'm going to throw together an audio probe this afternoon, and poke around, i'm pretty confident that i'll figure this out even though it's the most complex pedal i've built so far.

[DaveTV]

No problem joining in here, Ed.

After reading through all the information on Erich's thread, I've done some poking around of my own. On the SA571 chip, all of the voltages appear correct, except at pin 1, where my multi-meter reads 0.01 V. Pretty low, huh?

It would make sense that this might be the culprit because all of my troubles started after removing and re-soldering the two 0.47 uF caps for cosmetic reasons (couldn't get the box closed, those caps are BIG!). It's highly possible that I might have cooked the cap at pin 1 with the soldering iron and destroyed it.

Would the loss of that cap result in the problem I'm having? Again, I get no delay signal, just clean.

[Scott Swartz]

You guys are the right track, use an audio probe and a signal generator to find where the signal stops.  The signal generator can be built with a single transistor, do that first.

2.51 on all pins 9-16 is definitely wrong.  For pins 9-16 the 36k resistors are specifically chosen to set the DC output voltage at pin 10 about 6 volts, and for pins 1-8, the 10K resistor sets the DC output voltage on pin 7 at about 6 VDC also.

On the "duller" issue, that is probably bad or incorrect value parts around the opamp.  The dry signal goes through a a high frequency emphasis (at the input stage opamp section) and a de-emphasis (at the output opamp section), this is a noise reduction technique.  The emphasis/de-emphasis are exactly opposite but equal in amount, so the dry signal should sound exactly like bypass.  Pins 1,2,3,5,6,7 on the TL072 should be 6 VDC also.

I will check the pin voltages for all the chips on my original unit, and post them, but won't be till the weekend.

[Aharon]

Just a heads up.I know building a signal generator to use as a tracer is not dificult but I found that even some $10 multimetres have a square wave generator that puts out a signal that could be used to trace stuff like that.
Aharon

[Ed G.]

Scott,
I was getting 2.51v on the PT2399, not the SA751.

[DaveTV]

I'm thinking that I might start over again with this one. Debugging has certainly been a lot of fun and very educational. but after weighing my options, I think I might be more satisfied with a pedal that I know was put together right the first time. I think I did a real sloppy job laying out the parts and soldering, and I think I could improve upon these things based on what I've learned so far. I'm pretty sure I must have burned out a component with all the soldering and re-soldering I did, and I'm concerned that I may have damaged other parts as well.

Cost-wise, all of the expensive parts are reusable, and the box is drilled out and everything, so it wouldn't be a great loss. I was so impressed with how this pedal sounded when I first got it up and running that I'm committed to seeing this through. Perhaps I can chalk this first attempt up to experience.

[Ed G.]

I put together a handy-dandy little audio probe and did some probin'
Traced the signal down past the delay chip into the input of the expander part of the compander chip (just like Erich) and that's where it stops. The signal coming out of the circuit is much quieter at pins 6 and 7.
Erich had some problems with the 2n5088 set up as an emitter follower, right? I tested that, it looks like i've got about 12 volts on C, about half of that on B and E. That should be right, correct?
I remember on the other thread it was said that the expander only works if the signal is at least unity gain. I don't know too much, but the delayed signal is at least as hot as the signal is at the input of the circuit, that's unity gain, right?
Only other thing, early on I accidentally used a 10K resistor in place of one of the 36k resistors on that chip. I don't have 36k resistors, so I used 33K's, thinking that would probably fall within the normal tolerance of a 10% resistor anyhow, right?
Any help at this point is appreciated. I'm up against a roadblock at this point, although my debugging chops have improved somewhat due to this little exercise. I'm ready to hear this thing.

BTW, here's the SA751 voltages:

1 - .678   /                16 - .875
2-  1.824/                 15 - 1.82
3 - 1.824 /                14 - 1.82
4 - .4 /                     13 - 12.05
5 - 1.825 /                12 - 1.82
6 - 6.71/                   11 - 1.82
7 - 6.71  /                 10 - 5.96
8 - 1.824 /                 9 - 1.824