#1
Hey all.
I've been trying to but the ProCo Rat clone from tonepad over the last week.
As with most first drafts, I plugged it in, turned it on, and got nothing.

After de-bugging for a day or so, I'm pretty sure I fried my transistor on installation. The tonepad instructions call for a 2n5458.

I took some readings but I am not entirely sure what they tell me. I am also not sure if a 2n5458 is a PNP or NPN.

I made a table of my readings which can be seen at
http://


The flat side of the transistor was facing to the left when the readings were taken. I used the diode test function on my multimeter.

Since I'm not sure of the type of transistor I have, I don't know which lead is the base, collector and emitter.
I've read that if a transistor is functional, then I should get no reading from the collector to the emitter or vice-versa.

Is this transistor fried???

Any help is greatly appreciated.
#3
Check the connection of the transistor. Look for shorts, bad solder joints and etc.
The 2N5458 is a Jfet, so the pins are D, S and G.
The D pin should be reading around 9v or whatever your power supply is.

2n5458 pin layout

Set your readings in this format. Make sure you ground the black cable of the multimeter, then use the red to touch the pins or joints.

Q1 2n5458
D = X volts
S = Y volts
G = Z volts
Ibanez Rg570EX
-Nailbomb & Liquifire
ENGL screamer 50
Dynacomp-TS9/808-OCD-Phase 90-Small Clone.
Last edited by Milotic at May 20, 2011,
#4
I already desoldered the transistor from the board.
Is there any way for me to check it while it is not in the circuit?
#5
Try replacing the transistor, then test the IC, diodes and transistors again, with a multimeter. I don't think your transistor is getting enough power or it may be faulty or damaged.
Ibanez Rg570EX
-Nailbomb & Liquifire
ENGL screamer 50
Dynacomp-TS9/808-OCD-Phase 90-Small Clone.
Last edited by Milotic at May 20, 2011,
#6
Ok I will do that.
Do you think there's any harm in replacing the 2n5458 with a 2n5457?

It will probably change the tone a little, but it should still work, right?
#7
It 'should' work.
Ibanez Rg570EX
-Nailbomb & Liquifire
ENGL screamer 50
Dynacomp-TS9/808-OCD-Phase 90-Small Clone.
Last edited by Milotic at May 20, 2011,
#8
two things i have experienced with jfets that might help you.

different manufacturers of the same part number for jfets have different pinouts, that tripped my up once check the pinout five times to make sure you have it right; i get transistor dyslexia every time i make a pedal.

and you really should have your biasing resistors as trimpots. jfets have a lot of variation. i thought i had a dead transistor and it was just biased wrong then it sounded great.

good luck
Last edited by brentonlatour at May 20, 2011,
#10
Quote by Cheeto333
So what's the best way to check which transistor lead is which?

Look up its data sheet.
#12
So I finally got around to replacing the transistor.

I switched the possibly blown 2N5458 with a new 2N5457.

I plugged my guitar and amp into the circuit and measured the following voltages at the transistor:

D: 9.02 V
S: 1.92 V
G: 0.00V

With the new transistor, the pedal makes a sputtering sound if I hit the strings REALLY hard, otherwise I get no sound.

After searching online for a while, I found this website: http://www.geofex.com/fxdebug/bias_prob.htm

It seems like my transistor is biased incorrectly. Is this because I am using a 2N5457 instead of a 2N5458?

Is there a way to bias the 2N5457 so that my circuit works?

Thanks for any help.
Last edited by Cheeto333 at Jun 4, 2011,
#17
2N5458, 2N5457, MPF102, and J201 can be interchanged in many circuits. Dunno about the Rat though.
And listen to Jim's words for he is wise and knows stuff.
#18
Quote by Invader Jim
yes sir. just get a 100-ohm trimpot and you should be good to go.


i am almost positive we mean 100K trimpot here

EDIT: I am in error with this post. i posted before looking at the schematic. sorry. I thought we were talking about an amplifying jfet. we are talking about a source follower.
Last edited by brentonlatour at Jun 5, 2011,
#19
Quote by brentonlatour
i am almost positive we mean 100K trimpot here

I'm replacing a 47 ohm resistor, so an 100 ohm trimpot makes more sense than a 100K one.
Can anyone confirm this?
#20
I installed the 100 ohm trimpot and I still get only the sputtering noise.

Should I try the 100K trimpot?
#21
i just looked at the schematic. that 47 ohm resistor is for cleaning up hum in the power supply not biasing. sorry about the wild goose chase.

i think the problem is with the op amp input biasing. can you disconnect the .022 cap in front of the jfet and attach the output(amp) to the intersection of the .0033 cap and 1K5 and let us know it you still hear the sputtering please? i suspect it will sound the same with a little less treble.

if you connect your input(guitar) to the front of that .022 cap and the out put to your amp. this will check if your source follower JFET is working right. I think this is fine. if so it should sound clean and a little bit quieter than your guitar's usually output.

is the 1M attached to Vb in the front to the circuit properly attached to the Vb (this is my gut feeling)? is the voltage divider, supplying the Vb, wired properly? is pin 4 grounded?
#23
Quote by Cheeto333
Ok I will try that. Where is Vb located on the schematic?

Vb is at the junction of the two 100k resistors in the power supply (this point is actually labeled "Vb").

they divide the supply voltage in half to bias the op-amp into linearity. in this case, the signal at the output of the op-amp is riding on a DC component of half the supply voltage, so that the AC signal can swing positive and negative relative to it. if the signal wasn't riding on this DC voltage, you'd get a string of half-wave rectified pulses, since an op-amp can't output beyond the supply rails (can't go less than 0v in a single-supply, in other words).
Last edited by Invader Jim at Jun 7, 2011,
#24
Quote by brentonlatour
i just looked at the schematic. that 47 ohm resistor is for cleaning up hum in the power supply not biasing. sorry about the wild goose chase.

i think the problem is with the op amp input biasing. can you disconnect the .022 cap in front of the jfet and attach the output(amp) to the intersection of the .0033 cap and 1K5 and let us know it you still hear the sputtering please? i suspect it will sound the same with a little less treble.

if you connect your input(guitar) to the front of that .022 cap and the out put to your amp. this will check if your source follower JFET is working right. I think this is fine. if so it should sound clean and a little bit quieter than your guitar's usually output.

is the 1M attached to Vb in the front to the circuit properly attached to the Vb (this is my gut feeling)? is the voltage divider, supplying the Vb, wired properly? is pin 4 grounded?

I just triple-checked the wiring. Everything looks fine. I'm not sure if I understand brentonlatour's instructions.
So I take the hot wire from the output jack and connect it to the intersection of the .0033 and 1k5 after disconnecting the .022 cap?

I feel like I should just order the PCB from tonepad and forget using a perfboard...