Rat mod help

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Mike.Stockb
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Post by Mike.Stockb »

Hey there,

Long time lurker. Intermediate pedal modder.

Haven’t had a rat in awhile and picked one up for short money. Decided to do a few mods:

MOSFET hard clipping(irf520), silicone diode asymmetric soft clipping, changed the 4.7 to 1uf in the clipping frequency response, added a 1k and 10k trimmer to the 2.2 and 1uf caps, Changed filter tone cap to .0068. That’s about it. Took maybe an hour altogether.

Plugged in the correct 9v adapter to the pedal and started to get burning smell. Resistor 1 which is 100ohms in my version fried. Looked all around for the reason and the only thing I could find is that the 1uf cap I put in had tiny writing on it and with my deteriorating eyesight it would appear I put it in backwards. This cap didn’t have a band but had uneven legs so I went with longer legs being positive. Had to get out magnifying glass to find the tiny + symbol on the opposite side of where I thought it was. Oops. So again it appears I put it in backwards.

So my question is: if that is what is off about the pedal, would that cause R1 to fry? And if so how much other stuff would expect to be fried due to that? I plan to replace the cap and resistor and test it out. But I figured I could get some help here first.

Thanks so much!

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Manfred
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Post by Manfred »

There are three possible causes of error.
-The polarity of the supply voltage is reversed.
-The anode and cathode of the 1N4007 protection diode are swapped.
-There is a short circuit to ground behind the 100 Ohm resistor.
RatModSupply.jpg

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The G
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Post by The G »

When you give part names, you should give a schematic too.

A reversed electrolytic capacitor can and often does act as a shortcircuit (What Happens When You Connect an Electrolytic Polarized Capacitor in The Reverse Polarity?). So it will close the circuit for the power supply through whatever components the capacitor has in front of it. Which means that other than the resistors already affected, no other part should have suffered.

OTOH, there is a 100k resistor between the 1μF capacitor and the 100Ω resistor. So it doesn't look like the 1μF capacitor was the cause of the resistor burning. Better check the 100k resistor too.

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Mike.Stockb
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Post by Mike.Stockb »

Sorry bout not including schematic. Here is a link

https://www.electrosmash.com/images/tec ... -parts.jpg

I replaced C6 with 1uf and put it in backwards it looks like.

The pedal worked as it normally should before attempting the mods and worked fine with the power supply wall wart. Thanks!

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Mike.Stockb
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Post by Mike.Stockb »

Replaced R10 with 100 ohm, D3, and C6 with 1uf. Still burning R10

Schematic:
https://www.electrosmash.com/images/tec ... -parts.jpg
Attachments
pro-co-rat-schematic-parts.jpg

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Manfred
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Post by Manfred »

What Voltage can you read on the cathode of D3 in the case of fault?

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The G
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Post by The G »

Obviously, the mod is the cause of the malfunction, but it's probably about a short on your PCB, not a bad value.
One thing I'd do in your place is looking through the PCB with a strong light on the other side. Maybe post here a photo of it.
Another thing I'd do is isolating the blocks by lifting a leg of R10, R11, R3, R6, R7 and lifting the drain of Q1, then I'd measure the resistance between supply and ground or signal traces and ground. Chances are there is a place on your PCB where that resistance is very low.
I'd also put the IC and in a socket, much easier to test/debug/replace.

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