fried pastrami

ashcat_lt · December 17, 2006, 11:33:49 AM

bought 3 used pedals yesterday for 19.99 a piece. 1 of them actually works.

Of the other two, one is a Danelctro Pastrami that wouldn't pass sound at all. The led would light up when I first plug it in, but no sound, and then I step on the switch to turn it off and it won't light up again. I opened it up in what I thought would be a vain attempt at figuring out what's wrong.

Found a diode, labled D1 on the board, seems to be simply regulating the polarity of the DC jack voltage. It appears to have exploded. I'm pretty sure that for this particular application I can use about any diode, but wanted to check with y'all and see if anybody knows the "proper" part, or can tell me for sure should it matter?

What could have caused such a catastrophic failure of this component, and what other parts of the circuit might it have hurt before the diode failed?

Another curious thing. this pedal has a resistor that seems odd to me. It's the only component on the "solder-side" of the board, and it connects the tip of the input jack to its own sleeve. Is this a stock component or somebody's attempt at a mod? If it is meant to be there, what the heck does it do?

BTW - the other pedal that doesn't work is a DOD stereo phase that lights up, and passes sound just fine, but doesn't phase at all. I think it's something in the switching circuitry, and have a "fix" in mind for it.

Rafa · December 17, 2006, 11:42:29 AM

Are there any opamps or transistors in the circuit?
Rafa

markm · December 17, 2006, 11:52:22 AM

Quote from: Rafa on December 17, 2006, 11:42:29 AM
Are there any opamps or transistors in the circuit?
Rafa

I have one of these units......(cheap!)
It has at least 2 IC's in it on the main board......weird.
Anyway, the diode in question is a 1N4004.
A 1N4001 will probably work just as well in there.

bancika · December 17, 2006, 11:58:08 AM

yeah, any rectifier diode will work fine

Barcode80 · December 17, 2006, 12:01:00 PM

probably the original owner plugged in a 9v supply of the wrong polarity and blew the circuit protection diode. likely no damage was done before it blew as the purpose of it is to blow and cut off voltage the the circuit and protect it's components.

as for the phaser, you were right to check the switching, but also check the seating of the IC's, i've had the same issue with a chorus where the IC was just unseated a bit.

ashcat_lt · December 17, 2006, 04:25:22 PM

so, what about that resistor that seems strange?

On the phaser I clipped out what I thought was the transistor that should be switching the phased sound in and out of the circuit. wired a jumper in so it would be always on. Didn't help. None of the ICs are socketed.

Been poking around with an audio probe. I can follow the signal from input, through a capacitor, to the first resistor. On the other side of the resistor I get nothing. I hear nothing at the input of any of the opamps where I figure I should, but I hear it at the output. How does that work? Is the signal just that low that I can't hear it at the input?

I can also hear it very faintly at some of the poles of the first LM13600.

markm · December 17, 2006, 04:43:33 PM

On the Pastrami that I have there are no extra resistors on either board it seems.

Paul Perry (Frostwave) · December 17, 2006, 05:42:53 PM

Quote from: ashcat_lt on December 17, 2006, 04:25:22 PM
Been poking around with an audio probe. I can follow the signal from input, through a capacitor, to the first resistor. On the other side of the resistor I get nothing. I hear nothing at the input of any of the opamps where I figure I should, but I hear it at the output. How does that work? Is the signal just that low that I can't hear it at the input?
I can also hear it very faintly at some of the poles of the first LM13600.

Yeah, an op amp (when wired up with negative feedback, like almost always) is trying to make the difference between the + and - inputs as small as possible. It's a GOOD sign if you can't hear anything there.
The LM13600 and other OTAs work differently, you will see a low level signal at one of the inputs.

This might help: http://www.talkingelectronics.com/Projects/OP-AMP/OP-AMP-1.html

ashcat_lt · December 17, 2006, 06:28:53 PM

replaced the diode in the pastrami. the switch works a bit more consistently, but still no sound. Tried cutting the resistor that looked funny. No difference either way. On this one, I've got sound all the way to the input of the opamp (which goes against the above post), and then nothing at the output of the opamp. I don't have a schematic for this box. I'm assuming the opamp is a TL072 or similar. The signal is audible at pin 5, but not 7. there's 4.5 volts at pin 5 and 6, 9 volts at pin 8, but not enough for my ancient meter to read at pin 7. Is that a fried opamp? don't have a replacement for it without tearing apart another box.

markm · December 17, 2006, 06:30:29 PM

TL072

Mine has 2 Quad Op-Amps in it.

ashcat_lt · December 17, 2006, 06:32:11 PM

mines got those too, but it hits a smaller opamp first.

Edit, i really need a schematic

markm · December 17, 2006, 06:33:25 PM

Must be on the second board.....I only removed the first one.

ashcat_lt · December 17, 2006, 06:41:25 PM

yeah, it even actually says tl072 right on it.

MartyMart · December 17, 2006, 06:46:34 PM

I had one for a week about two years ago, sounded good but the switch was intermitant and
the jacks were cutting out too, it went right back and i got my money back !

MM

ashcat_lt · December 17, 2006, 07:46:19 PM

mark m, the two "quad opamps" are, in fact, a 4013 flip flop and a 4053 cmos logic chip. both are involved in the switching. From what I can tell, most of the audio "magic" comes from the other board.

markm · December 17, 2006, 07:57:54 PM

Ahh.
Okay.
I didn't go that deep into it.
Ineteresting though.

johngreene · December 18, 2006, 11:53:26 AM

Quote from: Barcode80 on December 17, 2006, 12:01:00 PM
probably the original owner plugged in a 9v supply of the wrong polarity and blew the circuit protection diode. likely no damage was done before it blew as the purpose of it is to blow and cut off voltage the the circuit and protect it's components.

If the protection diode is in series with the power, then a wrong polarity power supply will not blow the diode. The problem with this configuration is the diode lowers the supply voltage by its forward voltage drop. So you'll usually find a schottky diode in this configuration. If the diode is fried, then it is probably the type of protection diode that is intended to short the supply if it is the wrong polarity. If the power supply can supply more current than the diode is capable of conducting, the diode blows and the supply is no longer shorted potentially blowing every device in the pedal. The most common problem is someone plugs a Line 6 9V AC supply into a pedal.

--john

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fried pastrami