PT-80 almost working...HELP PLEASE!

[gguru3]

Ok, I built the PT-80 from GGG.

I accidentally wired the power backwords and fried both my regulators (5V and 12V). I replaced those.

Now, when the pedal is switched OFF, I get the dry signal from the guitar. When I switch it on, I get NO signal from the guitar, but instead I can hear a FAINT wierd hissing squeal sound.  

I am using a 24VDC power supply (wired correctly now).

The LED lights up correctly, and the 3 pots used to adjust the delay and such are working correctly, when I turn them up and down I can adjust the sound of the wierd squeelling sound, but no signal from the guitar when the box is ON.

Should I replace the 100 uF caps right after the regulators? Or what should be the voltage readings there?

P.S. - I am using one of those super clear bright blue LEDs from Small Bear, does that affect the circuit differently than a regular LED? It says something in the schematic about using a J201 fet instead of a 2N5485 with a super bright LED.

Hopefully someone who has had these types of problems with the PT-80 can help me...I REALLY want to hear this pedal work so I know that all my hard work isn't for nothing!!!

[R.G.]

Ok, I built the PT-80 from GGG.
I accidentally wired the power backwords and fried both my regulators (5V and 12V). I replaced those.
Now, when the pedal is switched OFF, I get the dry signal from the guitar. When I switch it on, I get NO signal from the guitar, but instead I can hear a FAINT wierd hissing squeal sound.

...oops...

Odds are, the regulators are not all that got fried. It's likely that the opamps are dead; the compander is quite possible, and it's entirely possible that the delay chip is as well, and possibly a number of electro caps. It's also likely that the 1N4002 is dead too, if you put it in.

Best advice? Audio probe it. But be prepared to replace all the ICs and the electros. Possibly the transistors as well.

Do the obvious: measure all the IC pin voltages. Are the power supplies now sitting at their proper levels? Bias and operational voltages?

Protection from reversed power supplies is not really a nicety - it's mandatory.

[gguru3]

Thanks for the help...

I didn't try the audio probe yet, but I tested the circuit and it seems as though the ICs are working fine, I measured voltages and compared them to correct values found in some other threads, and most look good except for a few.

I suppose I will just list ALL my readings:

PT2399:
Pin1: 5.06V
Pin2: 2.50V
Pin3: 0V
Pin4: 0V
Pin5: 2.0V
Pin6: 2.5V
Pin7-8: 4.6V
Pin9-16: 2.52V

SA571:
Pin1: 2.84V
Pin2: 1.8V
Pin3: 1.8V
Pin4: 0V
Pin5: 1.8V
Pin6: 6.76V
Pin7: 6.76V
Pin8: 1.8V
Pin9: 1.8V
Pin10: 6.4V
Pin11: 1.8V
Pin12: 1.8V
Pin13: 23.4V
Pin14: 1.8V
Pin15: 1.8V
Pin16: 2.85V

TL072:
Pin1: 11.7V
Pin2: 11.7V
Pin3: 11.7V
Pin4: 0V
Pin5: 5.9V
Pin6: 11.68V
Pin7: 11.67V
Pin8: 23.4V

Like I said earlier, I replaced to two regulators.

I really think it could be one of those electrolytics, I may have burnt one up when soldering...

I plan on replacing all the electrolytics as we speak...let me know what you think of these readings, since I'm not really sure if they are good or bad.

[gguru3]

Oh yeah, I got my 24VDC adapter powering it now...with the right polarity!!!!

:oops:

[gguru3]

I just noticed that on the SA571 datasheet it says MAXIMUM Vcc is 18VDC...and I measured 23.4V!

What does this mean?

[R.G.]

I just noticed that on the SA571 datasheet it says MAXIMUM Vcc is 18VDC...and I measured 23.4V!

What does this mean?

It means - cross your fingers. The manufacturer is telling you that they don't guarantee that the chip will survive voltages over 18.

However, they don't look all that bad. Luck may be on your side.

Pin 5 on the 072 looks odd. I'll have to look at the circuit to see whether it's supposed to be a linear amp. If it is, then it should be sitting at 11.7 like pin 6 and 7.

I'll have a go at the circuit, and get back.

[gguru3]

Ok, I checked out the 5088 and it seems fine. Didn't replace the elctrolytics yet tho...

From looking at some other threads I see that my problem lies possibly in pin 11 of the SA571...if both sides of the SA571 are supposed to be the same values.

Plus, pin 5 on the TL072 definately seems wrong, half of what it should be???

Also, is 24VDC too much power??? The tech info on the PT-80 calls for 18-24, but my regulators (regular 7805A and 7812A) are getting MEGA hot...

[Scott Swartz]

If you are getting 24V on some pins of the chips, the 78L12 12V regulator is not working right.

Pins 1,2,3 and 5,6,7 of the opamp should be 1/2 of the 12 V or about 6 volts.

Remove all the chips and get the power supply section working first, the 5 V regulated, the 12 V regulated and the 6 VDC "Vref" (shown as R on the schem) that is made by the two 1K resistors and the cap.

When that is all good plug in the chips.

[gguru3]

Ok Scott, I've done what you suggested.

Turns out my 12V regulator was bad. Replaced it.

Now I am getting the correct voltages on the TL072 EXCEPT for Pin 5, which sits at 3V instead of 6.

What should I do from here???

[Scott Swartz]

Pin 5 of the opamp need to be at the 6V Vref.  IS the Vref correct, did you replace the 100 uF cap after the reverse power polarity?  

Pin 5 connects to Vref via a 1 meg resistor, check the voltage on both ends of the resistor, both should be 6 volts.

Its possible that your meter is loading the circuit and giving a faulty reading when probing pin 5 due the high 1 M impedance.  The opamp will always try to make to voltage on pins 5,6 the same due to the differential action.  If pin 5 is really 3 volts, and since pin 6 is not DC coupled to ground (there is the .015 cap blocking the DC), then the output on pin 7 and 6 would have to be at 3 volts.  Obviously you are checking 1 pin at a time but if you had another meter and could check both pins 5 and 6 at the same time you might be able to see this loading, its hard to explain typing it.

Another possibility is the opamp is bad.  Try pulling the opamp and checking pin 5, it should be 6 volts.

[Paul Marossy]

I lucked out on my Phase 45 build that I just finished. Tried a new style of DC jack and got the polarity backwards. Since I have no polarity protection on that pedal, I was expecting my opamps and FETs to be fried. But they worked just fine after I got the polarity right...  :?

Hope you get your circuit working. Sounds like that opamp may have been damaged.

[gguru3]

Ok Scott, first off I just want to say I very much appreciate your help and everyone else's on this...

I did not replace the 100uF cap (should I ), but I did pull the TL072 out and check some voltages.

Pin 5 is sitting @ 3V just like before.

When I measure the voltage on the cap end of the 1M resistor I get 6V, so my Vref is correct...but when I measure the voltage on the TL072 side of the 1M resistor I get 3V.

The resistor is reading the correct resistance, 1M.

I think what you said about loading the ckt might be true here, hopefully I didn't blow my opamp...is there any way to show for sure that the opamp IS blown?

Other than this problem, how do my other voltages look? (scroll up, disregard the 24V on the SA571, change it to 1/2 that amt since I replaced the 12V reg)

[Paul Marossy]

Regarding the opamp, do you have a circuit built that uses the same type of opamp? You could pop it in there and see if it works like it is supposed to. It may be that your cap is bad and in need of replacement, too.

[gguru3]

Sorry, don't have another circuit...this PT-80 is actually my first build.

Everything would have went perfect if I hadn't wired the damn thing backwords  :oops:

I'll go ahead and replace the cap and see what happens.

Thanks for the help guys, any other suggestions are definately welcomed.

[Paul Marossy]

Wow, you're a brave individual doing that for your first build.  
You know, you could go to the Shack and buy a couple of new opamps to try, too. That's only a couple of bucks...
Anyhow, hope you get it working soon.

[Scott Swartz]

If its 3 volts with the TL072, it must mean the meter has a 1 meg input impedance which is low, most are 10M or 20M.

The only other possibility is a short or bridge on the PCB, but its probably just the meter.

The other voltages look good, so put all the chips in and see what you get.

You may want to temporarily wire your input and output jacks directly to the PCB, take the footswitch out of the mix for now.

[gguru3]

Yeah, I just have one of those "El-Cheapo" 10$ dmms, so it could definately be that.

Well, I did what you suggested Scott, and to my avail...no luck.

I am just going to replace ALL the chips with some new ones and see if that fixes the problem.

I have went over this thing for days looking for problems with the board/connections/components and everything seems fine. I must be one or all of the chips that is causing the problem. I mean, well, I DID run 18V through it for about 20 sec in the wrong polarity without the diode in... :oops:

Thanks for your help so far, I am determined to get this thing working.

Let me know if anyone thinks of anything else!

[Scott Swartz]

I agree time to try new chips.

The only other thing I would recommend before powering up with the new chips is to replace all the 1 uF coupling caps IF you used electrolytic caps for those.  

The film caps spec'd on the parts list will not be affected by reverse polarity, but electros certainly can be.

[Brian Marshall]

yes.... RS stocks tl082's basically the same.

[gguru3]

Well guys, with all your help, i finally got it working...

The problem was, like for so many of you, a faulty cap.

THANKS FOR ALL YOUR HELP! I really appreciate it. It is great to see that the support is there for new people to jump in and take part, get their questions answered. I'm hooked...

By the way...the PT-80 sounds absolutely beautiful, better than I expected. Although it is a slightly challenging build, I recommend it to anyone looking for a good delay/echo pedal.

BONUS - I took pictures of my ENTIRE build process so I will try to get them up for you asap! :wink: