Turbo Rat Help!

[colorbars]

Not a build, but i need some troubleshooting help...

Recently bought a Proco Turbo Rat and its EXTREMELY quiet, even with the volume AND distortion at full its not even close to the same level as the pedal being off... I would really appreciate any help I could get troubleshooting this.

Thanks!

[GGBB]

Follow the troubleshooting guide here: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0.

I would also recommend you use an audio probe (google it if necessary) and work your way backward through the circuit starting at the output until you find where there's normal volume in order to find where the problem is.  (You'll need a schematic for that.)

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I used an audio probe and found that the volume dropped significantly on the output side of the .022 cap in the bottom right hand corner of this diagram... http://www.diystompboxes.com/pedals/rstrand/rat/pcratrev.gif

I assumed this meant the cap was no good and replaced it but it had no effect :s

[petey twofinger]

did it ever work properly ?

could be miswiring , bad solder , maybe a part that was installed backwards

[GGBB]

I used an audio probe and found that the volume dropped significantly on the output side of the .022 cap in the bottom right hand corner of this diagram... http://www.diystompboxes.com/pedals/rstrand/rat/pcratrev.gif

I assumed this meant the cap was no good and replaced it but it had no effect :s

Good - you're getting closer.  Measure the resistance to ground at that point.  Check the resistors (measure) after it.  Check the voltages at the three pins of the JFET.  Note that if you have a 1995 or earlier version of the Turbo RAT you have, there would be another resistor on the back of the PCB between the drain and gate of the JFET.  Also, note that that schematic isn't accurate, it does not include that resistor, and the values of R1 and R3 are 2.2M not 100K in the Turbo and "LED" RATs.  It also is missing a 2.2M resistor to ground from the input.  There could be other problems.

[colorbars]

I used an audio probe and found that the volume dropped significantly on the output side of the .022 cap in the bottom right hand corner of this diagram... http://www.diystompboxes.com/pedals/rstrand/rat/pcratrev.gif

I assumed this meant the cap was no good and replaced it but it had no effect :s

Good - you're getting closer.  Measure the resistance to ground at that point.  Check the resistors (measure) after it.  Check the voltages at the three pins of the JFET.  Note that if you have a 1995 or earlier version of the Turbo RAT you have, there would be another resistor on the back of the PCB between the drain and gate of the JFET.  Also, note that that schematic isn't accurate, it does not include that resistor, and the values of R1 and R3 are 2.2M not 100K in the Turbo and "LED" RATs.  It also is missing a 2.2M resistor to ground from the input.  There could be other problems.

I opened up my regular rat which works as it should and compared voltages of the 2N5458 near the output (near the .022 cap mentioned earlier) and this what I found...

Rat:
D=13.6v
S=2.4v
G=0v

Turbo Rat (with volume drop issue):
D=13.5v
S=13.1v
G=13.5v


Could this be the issue?

Mine appears to be 1995 or prior as it has the resistor on the other side that you mentioned.

[GGBB]

I opened up my regular rat which works as it should and compared voltages of the 2N5458 near the output (near the .022 cap mentioned earlier) and this what I found...

Rat:
D=13.6v
S=2.4v
G=0v

Turbo Rat (with volume drop issue):
D=13.5v
S=13.1v
G=13.5v


Could this be the issue?

Mine appears to be 1995 or prior as it has the resistor on the other side that you mentioned.

What is your power supply?  With a 9V supply, the drain should be somewhere just below that, not 13+ volts.  The value should be different between the two pedals because of R16 (on the back).  But the fact that you have +0.4V between gate and source probably indicates a problem.  I would replace that FET, but before doing so, measure with a +9V supply.

Here are my values:

1990 Turbo RAT
DC In +8.50V
D +8.46V
S +5.87V
G +3.74V

1992 RAT2
DC In +8.53V
D +8.48V
S +1.18V
G +0.0V

1998 Vintage RAT
DC In +8.51V
S +1.69V
G +0.0V

[jymaze]

G should be at 0 volts. It is the cause of your problem. Check that the resistor that goes to G is tied to the ground and not V+, and check for any short-cut around it.

[GGBB]

G should be at 0 volts. It is the cause of your problem. Check that the resistor that goes to G is tied to the ground and not V+, and check for any short-cut around it.

That's not correct for the Turbo Rat which has TWO 2.2M resistors connected to G - one to ground and one to V+.  But yes - check that the resistor to ground is okay.

[jymaze]

Ok, I thought it was the same as a normal Rat, my mistake. In any event, Gate at 13.5v is the cause of the problem. Have it at 4.5V or at 0V would fix the problem in an equal manner.

[colorbars]

I was using a boss 9v adapter that apparently put out closer to 13v that's why the higher numbers, but it turns out the 2N5458 was the issue! I replaced it and it works great! Huge thanks to everyone!

[GGBB]

Excellent!  I have recently rediscovered my Turbo RAT and it has replaced my regular RAT on the pedal board.  Not sure why, there's just something about it that I like better right now - I think it's a little less compressed sounding and has more of a raw tone, but still very RAT-ish.  I hope you get as much pleasure out of yours as I have been getting from mine.