PROCO Rat Build No Drive Problem

[aBitSleepy9989]

Hi! I am attempting to remake a proco Rat distortion according to this schematic:

http://beavisaudio.com/schematics/ProCo-Rat-II-Distortion-Schematic.htm

I have copied this schematic exactly to a pcb of my own design, the one change I have made is to use a OP07 ic rather than the original LM because that was what I could get ahold of. Everything else is the same.

As for capacitor type I followed this guide here:

https://www.taydakits.com/instructions/rat/pages/designators-and-components--25

The Rat seems to work. When I turn it on hooked up to an oscilloscope and tone generator, I have control of the Volume (VR3)  and the Filter (Tone / VR2). With the wave being converted from sine to square when turned on, the Volume increasing the size (Amplitude) of the wave and the filter cutting the front of the wave the same as when this person plays with it in this video:

https://www.youtube.com/watch?v=Xzwqwo26l-Y (At the end he tests it and shows the results on the oscilloscope)

However, I have no control over the Drive (Distortion / VR1) at all! Turning the Pot changes the resistance across the IC pins but has no effect on the outputted wave.

It appears to me that the Drive is "stuck" at full blast... My current theory is that the op07 has different tolerances / operating voltages from the LM (maybe c5 needs adjusting?) - I cant seem to find any information on that though... and people on here seem to swap them out pretty easily...


I have triple checked:
PCB continuity and connectivity,
resistor / Cap values,
Cap polarity,
Transistor Pinout,
IC Pinout,

It is probably worth mentioning that the outcome of this project is literally just to have a RAT in a different shaped box, No electronic /audio modifications!

Has anyone else had this problem? I have been working on this for a while and am truly stumped!

Thank You!
 

[GibsonGM]

Hi, welcome!  You said you have control over the gain and filter...I take 'gain' to be distortion (DRIVE)....or do you mean output VOLUME?  You're seeing the wave go from sine to clipped?  That means 'Drive' is working.

If it is really 'stuck at full volume', perhaps the pot is bad?  First, tho, I would check EXACTLY how it is wired before going further, with the wiper tied back to output and so on.  Review the schematic and be SURE
Get back to us!

[aBitSleepy9989]

Hi!

So I am going off the Beavis notation for the Pot Labels, in which Filter and Volume are working but Drive isnt.

Looking at other schematics it is the Distortion pot which isn't doing anything, with the pedal essentially stuck set to full distortion, but with control over tone and output volume.

I have triple checked the solder joints and resistance across the pins and the pot is working fine (sweeping happily between 0 and 100k ohms) and by removing the IC from its socket and probing pins 2 and 6  can happily measure the position of the pot, with the resistance changing as expected.


Thanks for the response! let me know if you have any thoughts!

[ElectricDruid]

I have triple checked the solder joints and resistance across the pins and the pot is working fine (sweeping happily between 0 and 100k ohms) and by removing the IC from its socket and probing pins 2 and 6  can happily measure the position of the pot, with the resistance changing as expected.

This is very interesting. I'm struggling to see way the gain can be stuck at max *and* this still be true. So then I'm looking at my usual assumptions and trying to see which one might be wrong.
It *could* be the chip, but given a working op-amp, this would be a very weird problem.

[antonis]

When I turn it on hooked up to an oscilloscope and tone generator, I have control of the Gain and the Filter. With the wave being converted from sine to square when turned on, the gain increasing the size of the wave and the filter cutting the front of the wave..
However, I have no control over the Drive at all! Turning the Pot changes the resistance across the IC pins but has no effect on the outputted wave.

Could you plz make yourself more clear about Gain/Drive/Distortion pots..??
('cause, as far as attached schematic concerns, all of them is VR1..)

P.S.1
Gain doesn't increase the size (amplitude) of "squared" wave but the slope of its vertical sides (from trapezium to square)
(but OK.. if by "size" you mean area/acreage, yes..)   

P.S.2
Welcome..

[ElectricDruid]

So I am going off the Beavis notation for the Pot Labels, in which Filter and Volume are working but Drive isnt.

This is pretty clear, isn't it?

Although I agree, later on they say that "the gain increasing the size of the wave" when they mean "volume". But we'll let that pass 'cos we're friendly and nice, right?

[antonis]

we're friendly and nice, right?

Could you speak strictly on behalf of yourself..??
(I know a bunch of hostile and unpleasant guys - myself included - around here..)

[ElectricDruid]

(I know a bunch of hostile and unpleasant guys - myself included - around here..)

ROFL, Mr. tough guy!

You're ok, Antonis, we know you better than that.

[aBitSleepy9989]

Firstly thank you for the welcomes and responses! I apologise for not being clear, here I go at trying to simplify everything!

The pot names:

VR1 - Drive / Distortion (the one that don't work)

VR2 - Filter / Tone

VR3 - Volume (a certain idiot called aBitSleepy9989 might have referred to this wrongly as gain... - sorry)

- I will try to go back and edit the original post to make it a bit clearer!


Secondly, by increasing the size of the wave I mean the amplitude increases. So when I said the gain makes the wave bigger I actually meant the volume increases the amplitude. (You were right ElectroDruid!) - apologies again for the confusing terminology.

But yeah, I'm finding it very strange that I am changing the inputs to the op amp with no difference in outputs, I'm assuming from the lack of instant solution that the OP07 should just be a drop in replacement for the LM, and there isn't heavy modifications to the rest of the circuitry needed!

Thanks again!

[idy]

If there was a short across C6 or C7, would that do it?

Test to see if pin 2 of opamp is shorted (or nearly shorted) to ground?

[aBitSleepy9989]

If there was a short across C6 or C7, would that do it?

Test to see if pin 2 of opamp is shorted (or nearly shorted) to ground?

I didn't notice one when I tested earlier but I will double check and get back to you!!!

Thanks for the suggestion!!!

[PRR]

If there was a short across C6 or C7, would that do it?

If we had DC Volt numbers, as the debugging thread/posting suggests/demands, we would know already.

But we don't. 

[MrStab]

What's the amplitude of your test signal?

[duck_arse]

I have copied this schematic exactly to a pcb of my own design, the one change I have made is to use a OP07 ic rather than the original LM because that was what I could get ahold of. Everything else is the same.

because of this, especially, we need to see what you have done, otherwise we won't be able to cross check your layout or parts fitted. and the wiring! we have no idea what you might have done.

photos. please.


and - is the OP07 a direct replacement? it shows offset adjust pins where the LM308 has freq compensation pins.

[aBitSleepy9989]

Just a little heads up I've been called away with my work last minute so can't do much probing and measuring until tomorrow! - I will update as soon as I can with all the info requested!!

So far this is the DC voltages of the ic pins (and other notable locations!), whether pin 2 is shorting to ground and the test signal amplitude - and photos! I'll attach some of my schematic, Gerber, board and of the assembled thing! 

Once I've don't that I'll respond here and edit the initial post with a second section as an addendum so all the info is together! (Sorry to not include it in the first place...)

Thank you all for the suggestions I really appreciate the help and don't want you to feel like this thread has been abandoned!

[aBitSleepy9989]

I'm Back!

So after a load more testing and fruitless troubleshooting I realised that because I had to get 5 pcbs made up and I had enough stock of caps and resistors... I had enough to populate up a second board.

This one works perfectly.


---------- Seeing as I now have a working RAT please don't waste any time or effort trying to help me troubleshoot this dud! I only need one, I am simply trying to work it out for future reference now, and updating this for anyone else who has my problem! (suggestions are still welcome but it's academic curiosity only from here on out!) -------------------------


Seeing as I used the same PCB, schematics and componants from the same reels and the second one works, to me that means that the failure possibilities are either a dry solder joint, or a dud new component.

So far I have swapped the ICs (yaay for socketing) and it made no difference, so I dont think its a dodgy opamp.

I have also successfully continuity probed the board, and any joints that were even remotely registering a resistance, I have reflowed. Still no dice.

I have tested all of the resistors, they are resisting what is written on them, and correct for the scematic.
 
To me this indicates that I have a dud cap. I have a cap tester and will test all of them and report back and see if I can find the culprit. If that comes back all good caps then... Witchcraft? Curse? I'll defiantely have to have a rethink! (More likely I'll do a load more DC probing now theres a working one to side by side.   
   

In answer to questions:

DC Voltages on pins:
1 9.1
2 4.6
3 4.8
4 0
5 0
6 4.5
7 9.2
8 9.1

There are no shorts across C6 or 7

The test signal is about 10mV in amplitude (it's a 1kHz Sine tone). I have tried varying this and it is not making a difference.

I will add some photos later! Although knowing an identical PCB works, I think it is unlikely that there is an issue there.


Finally in response to the op07 being a drop in replacement. It does work, it sounds like a RAT to my (admittedly fairly deaf) ears! The only thing I can see is that the Distortion Pot seems to hit maximum effect when the pot is at about 25%. I have full control from no effect to massive distortion, but the top end of the pot doesnt seem to do anything. I may play around with smaller resistance pots in the future to see if it gives a more fine tuned control but thats a problem for future me. I cannot attest to whether that is different with the LM. but it seems like something PROCO would have tuned in the original.... 

Thank you all for the help and pointers! I will try to keep you updated as I figure this mystery out!

[duck_arse]

I will add some photos later! Although knowing an identical PCB works, I think it is unlikely that there is an issue there.

as a great tech I used to work with would say - "Must be something, isn't it?" and he was Greek, so no arguing.

whatsisname found Pluto by flicking between identical pics of identical builds, too.

[antonis]

whatsisname found Pluto by flicking between identical pics of identical builds, too.

IMHO, the major difference should lie on "who pays the ferryman" and "who doesn't"..

[duck_arse]

Chris De Burgh discovered Pluto?





this is what happens when we don't get those photos on time.

[bluebunny]

Chris De Burgh discovered Pluto?

Clyde Tombaugh is most of the right letters.