ProCo Rat Clone Troubleshooting... Please help!

[jimladladlooklike]

Hi everybody!

So I've been building a clone of said pedal based on the schematic from Electrosmash:

(https://www.electrosmash.com/images/tech/pro-co-rat/pro-co-rat-schematic-parts.jpg)

I adapted the following layout (http://tagboardeffects.blogspot.com/2012/01/proco-rat.html) in order to space things out slightly and for my own general practice and here is what I came up with:



Took my time and finished the build only to find that no sound at all came from the pedal, except for when the effect was bypassed. I've looked over the schematic and compared it to the layout to spot any discrepancies but can find nothing. I've used all the correct parts as far as I'm aware (as in I haven't swapped out the op amp or transistor for similar alternatives - all pot, res and cap values are the same) ... Where should I start troubleshooting this thing!?

Many many thanks in advance.

Jim

[jimladladlooklike]

Really sorry, left out some vital info:

The voltages at each pin on the LM308N are as follows:
Pin 1: 9.13
Pin 2: 4.61
Pin 3: 2.26
Pin 4: 0
Pin 5: 0.01
Pin 6: 4.61
Pin 7: 9.25
Pin 8: 9.13

And for the 2n5458:
Gate: 0
Source: 1.19
Drain: 9.25

Any help is greatly appreciated. Sorry for leaving this stuff out originally.

Cheers everyone!

[ElectricDruid]

Pin 3 looks suspicious, doesn't it?

I'd expect that to look more like pin 2. After all, that's kind-of the idea of op-amps; they try to keep the two inputs equal.

HTH,
Tom

[duck_arse]

also welcome. might we see photos of your actual built thing?

[jimladladlooklike]

Pin 3 looks suspicious, doesn't it?

I'd expect that to look more like pin 2. After all, that's kind-of the idea of op-amps; they try to keep the two inputs equal.

HTH,
Tom

I see. That makes sense. Any idea what might be causing this?

also welcome. might we see photos of your actual built thing?

Here's a few photos of the innards. I couldn't get a decent photo of underneath as the wiring isn't the neatest, as you can see. If these are no help then I'll unwire and take a snap of underneath. I did check for any unwanted solder bridges and copper traces that may have strayed from when they were cut but if you think this may be an issue then let me know and I'll take a look.



Many thanks again!

Jim

[stringsthings]

Did you make the trace cuts under the IC?  (to separate pins 1-4 from pins 8-5)

[jimladladlooklike]

I did indeed.

[stringsthings]

pin 3 is low.  That's your Vref pin which should be close to 1/2 supply voltage - around 4.5V.

Take your IC out of the socket and test pin 3.  That will tell you if your IC has a problem.
If the reading is still low, you've got a problem in that area.

The source voltage on the 2N5458 also looks a bit low.  Double check your gate and source resistors.

[jimladladlooklike]

Thanks again for the guidance but, how do I test the IC when it's removed from the circuit?

One thing I have noticed when the LM308N is removed is that sockets for pins 1 and 8 aren't showing as connected when tested with the Ohms setting on a multimeter. There is the 30pf cap between the two however so I'm not sure how this would/should affect the reading. I have tested the bag of 30pf caps and when the leads of a multimeter are connected to each leg of each cap cap there is no change in reading, as in they are not connected.... is that weird? There is a reading of about 18 K Ohms between pins 1 + 8 when the IC is removed from the circuit. Is this normal?

The source voltage on the 2N5458 also looks a bit low.  Double check your gate and source resistors.

When you say gate and source resistors do you mean R7 and R9 on the schematic, both 1.5K and 10K respectively? If so, then they themselves are showing the right readings.

Jim

[ElectricDruid]

Thanks again for the guidance but, how do I test the IC when it's removed from the circuit?

You don't. Stringsthings means you should test pin 3 of the socket once you've taken the chip out. That'd tell you if the fault is on the PCB, or whether a faulty chip is what's giving you the low reading on pin 3.

HTH,
Tom

[jimladladlooklike]

Ah I get it! The reading was the same on pin 3 of the socket once I'd taken the IC out, so I guess the IC itself is okay and the problem is elsewhere. I've also noticed that the 100k voltage split resistors are reading at about 50k ohms each when part of the circuit... is that normal?

[stringsthings]

Ah I get it! The reading was the same on pin 3 of the socket once I'd taken the IC out, so I guess the IC itself is okay and the problem is elsewhere. I've also noticed that the 100k voltage split resistors are reading at about 50k ohms each when part of the circuit... is that normal?

That's normal for resistors to read low when they're a part of the circuit.  There's often parallel resistance, especially to a power rail or ground.

One thing I have noticed when the LM308N is removed is that sockets for pins 1 and 8 aren't showing as connected when tested with the Ohms setting on a multimeter. There is the 30pf cap between the two however so I'm not sure how this would/should affect the reading. I have tested the bag of 30pf caps and when the leads of a multimeter are connected to each leg of each cap cap there is no change in reading, as in they are not connected.... is that weird? There is a reading of about 18 K Ohms between pins 1 + 8 when the IC is removed from the circuit. Is this normal?

Yes.

When you say gate and source resistors do you mean R7 and R9 on the schematic, both 1.5K and 10K respectively? If so, then they themselves are showing the right readings.

R8 (1M) and R9.  These two resistors set the gate and source voltages for the FET.  Drain is connected to V+ ( 9V ).

You've isolated the problem to the Vref voltage.  In the Rat, Vref only goes to one place:  R2 (1M) which sets the reference voltage for the 308.
Now you can use your DMM to figure out where the problem is.  Lift one end of R2 and check voltages.  You want to see approx. 4.6V at the junction of R11,R12.

Logic tells you that the problem is where Vref is set ( R11, R12, C13 ) or where Vref is connected to the circuit ( R2, R3, C3 ). 

[jimladladlooklike]

I thought as much regarding the two 100k resistors. Thanks for clarifying. I will try and find any potential problems with Vref when I get chance and get back to you. I presume you meant to say C2 as opposed to C3 though right?

Cheers!

[stringsthings]

my mistake. C2.

[duck_arse]

this isn't going to help any, but, if the oppie output pin is following the bias voltage, as in is sitting at ~4V6, then the bias divider is ok and the opamp DC conditions are ok. and, we now have a demonstration of "meter loading" on the bias at the opamp input.

we always have to ask this question with rat builds - what is the part # on your jfet? also, have you an audio probe?

[PRR]

> 30pf caps and when the leads of a multimeter are connected to each leg of each cap cap there is no change in reading, as in they are not connected....

That is correct. A capacitor should show infinite resistance ("not connected") when tested with DC (how we normally measure resistance). The cap conducts more and more as frequency rises. A 30pFd does hardly anything until far above the audio range. (The need for the 30pFd is complicated; just assume the factory docs are correct.)

Bigger caps may show a short blip of "conductance" as they charge-up.

[jimladladlooklike]

this isn't going to help any, but, if the oppie output pin is following the bias voltage, as in is sitting at ~4V6, then the bias divider is ok and the opamp DC conditions are ok. and, we now have a demonstration of "meter loading" on the bias at the opamp input.

we always have to ask this question with rat builds - what is the part # on your jfet? also, have you an audio probe?

The Voltage at the output is 4V6 So, in this case, you're saying that the problem can't be with R11, R12 or C13? And that leaves R2, R3 and C2? What is meter loading exactly?

The jfet is a 2N5458.

I do not have an audio probe. Is that the next step in solving the issue?

[stringsthings]

this isn't going to help any, but, if the oppie output pin is following the bias voltage, as in is sitting at ~4V6, then the bias divider is ok and the opamp DC conditions are ok. and, we now have a demonstration of "meter loading" on the bias at the opamp input.

we always have to ask this question with rat builds - what is the part # on your jfet? also, have you an audio probe?

The Voltage at the output is 4V6 So, in this case, you're saying that the problem can't be with R11, R12 or C13? And that leaves R2, R3 and C2? What is meter loading exactly?

The jfet is a 2N5458.

I do not have an audio probe. Is that the next step in solving the issue?

He's saying that the problem might not be with Vref. 
You need to test for audio on pin 6 before concluding that.

You can make a simple audio probe.  Basically, you're just taking the output signal from different points in the circuit instead of at the end.
That way you can tell if your signal/guitar is making it to the right parts of the signal path.  For example, in the Rat, you want to test for signal
at pin 3, pin 6, any lug of the tone pot, the gate of the FET, the source of the FET, etc.  If you can't hear signal/guitar at any point,
you know that the problem is somewhere before where you're currently testing.

See Geofex for making a simple audio probe.  https://www.diystompboxes.com/pedals/debug.html
Just plug your probe into your amp with the volume fairly low and test away.
It's basically just a wire with a 0.1uf cap.

As for meter loading, google search is your friend.  They can explain it better than I can.

[jimladladlooklike]

You lot are the greatest!

So, using an audio probe I found that the tone pot wasn't working, switched it with another and then found the volume pot wasn't working either. Once I switched the volume pot with another the effect was working apart from the tone pot not having any effect on the sound whatsoever... What could be the problem here?

Thanks again. I'm learning loads and making real progress it seems!

[ElectricDruid]

Possible reasons why the tone pot might not work are (in no particular order):

1) Link from "pot in" to "pot wiper" is actually from "pot in" to "pot out"
2) Link is not connected properly (effectively there is no link)
3) Pot wiper is not making contact with the track (broken pot, or potentially bad solder joint on middle lug of pot)
4) C8 is not connected properly. This could be at either the top or bottom of the cap - either would lead to the filter having no effect.

Those are the ones that come immediately to mind. There may be others that other people can add, but it's late here already!

T.