Big mistake building PT-80!...What did I destroy?

[DaveTV]

I'm building a PT-80 delay and I mistakenly reversed the polarity of the DC jack and plugged in the 24V DC adapter. Ouch.

I had just just finished soldering the board and was just beginning to test the circuit. The first thing I did was connect the circuit to a 9V battery to see if it worked. I was able to get a clean guitar signal through the circuit, but the delay controls didn't seem to work at all. I figured I was in for a little debugging, but I thought I'd first try the circuit with the higher voltage.

I connected the DC jack and plugged in the 24V adapter without the 1N4002 diode across the positive and negative. As I said, the polarity was reversed, but I didn't know it at the time. Of course, there was no guitar signal at all, and when I unplugged the adapter, it was EXTREMELY hot. I tried to measure voltage from the adapter plug, but there was none. Is the adpater destroyed?

I tried the 9V battery one more time, but this time there was no guitar signal. Using an Audio Probe, I couldn't hear the guitar signal beyond the input of the opamp.

So, how much damage did I do to this circuit and how might I be able to salvage it?

[Ed G.]

Did you socket the ICs? I've accidentally fried out the dual opamp in a tube screamer in the same way you described. I would think that is what happened here.

[Aharon]

Are getting power at all?,doesn't the PT80 have a regulator?.Check that you have power first at the relevant pins.
Aharon

[Samuel]

Well, I'm def. a novice, but I think that 1n4002 is designed to protect the circuit from what you just did

Any other components get heated up/burnt? In particular when I screwed up the PT-80 (several times, ahem) those voltage regulators were the first thing to go kablooie. So check the 78Ls05 and 12 before you buy new ICs...

[Scott Swartz]

The reason the adapter was hot is due to a dead short of something, likely the 100 uF cap before the regulators.

Replace that for sure.

The adapter must be fried also if you can't measure any voltage.

Hopefully that is the only thing fried, the regulators may have stopped the backward power from flowing to the circuit.  If the regulators shorted and allowed power to flow to the chips, they could be fried also.  You may want to replace all the electros and the regulators as a precaution.

The good news is all the 1 uF poly caps will be fine they are not polarized.

So replace that first cap, remove all the chips, and power it up.  Check voltages on the +5, +12 and the +6 (Vref for the opamp) rails and power pins of the ICs.  If all looks good, plug in the TL072 ONLY, and check for dry signal.

If the dry signal passes and power pins of the ICs measure correctly, you can then plug in the PT2399 and SA571 and hopefully it works. Plug them in one at a time, and apply the power for a short time only while probing the power pin for voltage.  If the chips are shorted, the voltage will be wrong at that pin.

[Samuel]

OK So am I misunderstanding the purpose of the 1N4002? Isn't there to "short-circuit" if power is going the wrong way?

[SoundTech]

Samuel is right on both counts.  The diode was there to protect the circuit from reverse polarity voltage.  The power adapter might very well be toast.  The next thing to go would be the voltage regs.  Replace those, and check your circuit again.  You might find that your damage is limited to that.

[DaveTV]

Thanks for all the great help. Unfortunately I didn't have the 1N4002 connected when I did this. I had planned on soldering it directly to the DC jack in order to save space on the board. Of course this is the one thing that probably would have saved me all this trouble.

Scott, I'm not sure what you mean by the first cap. My schematic shows the DC jack connecting directly to the two regulators, with no caps between them. Both regulators do have 100 uF caps connected to their outputs however. Is this an older design?

[Samuel]

Sorry missed the *without the 1n4002* in your post... sorry.

[Scott Swartz]

You're right no cap on the input, so one or both regulators must have shorted.  Hopefully the damage stopped there.

[DaveTV]

Just wanted to say thanks to everyone for all the help. Believe it or not, I was actually able to get the circuit to work and it sounds great. Excellent design, Scott.

I replaced the two voltage regulators, as well as the two 100 uF caps connected to the regulator outputs. I also bought a new 18V DC adapter and plugged it in with the right polarity. Everything works great. I think the voltage regulators must have protected the circuit. It would seem as though the IC chips were undamaged. Or else they're damaged in a good way!

Thanks again.

[gguru3]

Hey DaveTV-

I just built the PT-80 and I think I made almost the same mistake as you.

I know for sure my 5V regulator was blown, but i replaced it and it still doesn't work.

What did you do to fix yours?