Rat Clone low output

[lukeferg]

Hi all,

Sorry for the long post, I've done all the searching that I can and I've got a lot of information I want to double check before I do anything. No one seems to have had my exact problem but there were a few with similar things that I've stolen ideas from.

Firstly, I picked up a Rat clone (op07 chip) thinking that if anything wasn't right with it I would be able to fix it. The pedal works but even with the volume at max it is still lower output than when bypassed, it's probably about 70-80% of bypassed volume. This is using a range of guitars including a Jazzmaster so I know it's not pickup output.
I figured something was off so I measured voltages to find out. Here's what I got.

Battery - 9.58

IC1 - OP07
1 - 9.2
2 - 4.7
3 - 2.3
4 - 0
5 - 0
6 - 4.7
7 - 9.37
8 - 9.24

Q1 - 2n5458
D - 9.37
S - 4.82
G - 1.76

So this raised a few eyebrows for me, namely the FET gate should've been 0 and the source should've been a lot lower. Secondly, pins 1 and 8 on the IC were pretty much the same despite the compensation cap, I should've been getting about half the voltage on pin 8.

I then did a bit of a trace of the circuit, something I'm not overly familiar with, and noticed a few things that I wanted to check.
The FET gate doesn't have the 1M to ground. It instead follows the GGG schematic (http://www.generalguitargadgets.com/pdf/ggg_rodent_sc.pdf) with the voltage divider biasing the gate but the resistor values are way out. R13 (according to the GGG schem) is 100k and R12 is 62k. C14 is also 1uF not 47uF.

My first question is, does this explain the FET voltages? If I was to just get rid of R12 and 13 and connect the 1M resistor to ground to turn it to stock wiring would that correct my readings? I can always change the FET too, seems to have worked for other people in other posts.

My second question is, what the heck is going on with pin 8? Leaky cap?

Finally, a bit of a weird observation, the 47ohm resistor at the start of the power supply is a 1N4002 diode (or similar, I can't quite read the numbers). I'm going to change that but I'm not sure how much difference that will make.

I really appreciate any ideas you guys have. I've built quite a few pedals and have needed to troubleshoot almost all of them. I'm probably just after confirmation that I'm on the right track.

[antonis]

First question:
Your FET should be biased at 3.66V (9.58V -> 62/100 Voltage divider)
Actually, it should be at 3.4V because of diode voltage drop..

In ggg schematic, is biased at 2.8V (9V -> 10/22 Voltage divider)
(considering voltage drop on R14 negligible..)

The 1M gate resistor is connected to whatever voltage is at the junction of R12/R13 and NOT to GND..

You may substitute R12/R13 with a 50-100k trim pot and check for "ideal" gate bias..

Second question:
The cap between pin 1 & 8 is a high frequency copensating cap for Op amp slew rate so your DC measurement is quite right..

Weird observation:
The 47R resistor is a power line noise filter and forms with C13 & C11 low-pass filter which attenuates high frequency impulse noise..
It's substituded by diode for reverse polarity protection purpose and it doesn't deal with your problem..

edit:If OP07 pins are identical to LM308 pins , you have a major problem at pin 3..
(it should be biased at half of your power supply..)

[lukeferg]

Thank Antonis, cleared a few things up for me. good to see the comp cap isn't a problem. Why then do I see so many other voltage readings where pin 8 is about half the voltage? Even GGG have it that way with the OP07 chip (http://www.generalguitargadgets.com/pdf/ggg_rodent_instruct.pdf)

In regards to the 1M resistor, I wasn't as clear as I should've been. I meant, if I change the wiring from the GGG schem to, say, this one (http://www.electrosmash.com/images/tech/pro-co-rat/pro-co-rat-schematic-parts.jpg) where the 1M is going to ground then will that get my pedal back up to volume? On all the other forums I've read I've seen very different voltages for the FET than what I've got, makes me think the volume drop is there considering the gain and filter controls work fine.

edit: Just read your comment about OP07 and LM308 - That problem has come up before in another thread (http://www.diystompboxes.com/smfforum/index.php?topic=111505.0) his was fixed by changing the FET to J201 which I don't have. This wouldn't have fixed pin 3 though. There might be an issue with the type of multimeter I use when I measure it, something about a 1M impedance on the meter or something which can account for the drop. I don't have another one though.

[antonis]

My bad, Luke.. 
(I got tangled with pins orientation..)

Your FET acts as a Source follower which means that it doesn't provide any gain so I think that the cause of Volume drop isn't placed there..
(as long as your FET is approprietly biased..)

I still insist of checking pin 2 & 3 voltages because if your readings are correct then pin 6 should allways be "tremendous" negative..

[duck_arse]

further to antonis' insistences, please be measuring the voltages at the biasing resistors, so at R15//16 and at R12//13. if your meter is loading down the 1M resistors, measuring at low impedance points will tell.

and generally, if the opamp output pin measures where you expect it to, the inputs are also behaving themselves.

C14 is not likely your problem, but you can change it up value for peace (piece?) of mind.

[Kipper4]

What's going on with pin 3 it's voltage should be around the same as 2 and 6?

[lukeferg]

Thanks for the help guys, just did a few measurements to report back.

R12/13 divider is at 3.45v (according to GGG), just what it should be.
R15/16 divider is at 4.46v, also in the ball park.

Things I've learnt so far: my voltages are in the ball park, and I need to get a better multimeter.

If I get a chance I'll change Q1 tomorrow, I've got a heap of 2n5457s lying around, maybe some other 58s. Would an MPF102 work? I struggle to get some parts in Australia easily, the J201 is one of them and futurlec is out of stock. I think I'm out of sockets too, just to make it a bit harder.

Any other ideas? Can someone explain to me what change moving that 1M resistor before the FET to ground might have?

Thanks again, I'm hoping to be knowledgable enough to help out someone else one day instead of just asking for it.

[ashcat_lt]

What's going on with pin 3 it's voltage should be around the same as 2 and 6?

We covered that, dude.  Low impedance meter is loading down that node.  I bet if you also metered the output at the same time, you'd see it head toward a rail.

If you move the resistor to ground then the bias on the transistor should change, but I'm not sure it'll make much difference.  Right now it sits pretty much exactly half way between the rails, but the output from from the diodes can't be much more than a volt, so if the bias goes a couple volts either direction there's still plenty of room.  The transistor bias is probably not your issue.  Honestly, since you haven't given us any real numbers on the actual output level, I'm not completely convinced there is an issue.

Build yourself an audio probe and follow the audio path till it seems like it's gone wrong.

[Kipper4]

Ok Ashcat. I didnt understand it. I see what he meant now.
my apologies OP

[lukeferg]

Thanks ashcat, I've got an audio probe stashed somewhere, it's just hard for me to get everything out at the moment, was hoping I'd find a way around doing that.

One thing I thought I'd check, I'm not getting any voltage at the clipping diodes whatsoever. Am I supposed to?

[ashcat_lt]

One thing I thought I'd check, I'm not getting any voltage at the clipping diodes whatsoever. Am I supposed to?

Not DC voltage, no.  If there was DC voltage at that point, it might be an issue.

[lukeferg]

Not DC voltage, no.  If there was DC voltage at that point, it might be an issue.

I thought so, some other threads were talking about voltage there so I thought I'd check.

I just plugged my guitar in with the pedal open and it's got way more volume now without me doing a bloody thing! So frustrating!
I'm gonna make sure when I box it up there's not a short to ground or anything.

Thanks for your help everyone, sorry for wasting your time.

[nmtlrx]

Hey! I know this thread is quite old (should I start a new topic? this is my first post), but I built a Rat clone from Fuzz Dog with Ruetz Mode (http://pedalparts.co.uk/docs/Rodent-v2.pdf) and experience the same problem. Maybe I'm just being paranoid here, I don't know, you'll tell me. So I installed Ge clipping diodes and LEDs. The signal output is quite high when I use the pedal in Turbo mode. In YDR mode, it's another thing.

Two things make me think there's a problem with my setup:

1- When the Dist is set to 0 and Volume set to max, the resulting output is lower than the bypassed output. I would have expected a higher volume.
2- My voltages are not what I expected (or what I read), pretty much like lukeferg's:

Power supp - 9.72 V

IC1 - OP07
1 - 9.51 V
2 - 4.79 V
3 - 2.38 V (I expected 4.1-ish; maybe from bad multimeter?)
4 - 0
5 - 0
6 - 4.79 V
7 - 9.62 V
8 - 9.51 V (I expected 0; is my 22 pF shorted or something?)

Q1 - 2n5458 (https://www.onsemi.com/pub/Collateral/2N5457-D.PDF)
D - 9.62 V
S - 1.52 V ()
G - 0 V ()

Soooo.... is this below unity gain when the Dist is dialed all the way down normal? If not, what should I go after?

Ty, DIYStompbox community!

[PRR]

Welcome.

> When the Dist is set to 0 and Volume set to max, the resulting output is lower than the bypassed output.
> 3 - 2.38 V (I expected 4.1-ish; maybe from bad multimeter?)
> S - 1.52 V ()
> G - 0 V ()

Yes, the circuit is below unity gain when distortion is on zero. Don't turn it to zero.

The voltages look correct. The "low" reading on pin 3 is meter loading; meter make and model? The FET Gate *should* be at zero, that's what R7 does. FET Source rises on R8 to 1V to 3V, 1.5V is quite reasonable.

Don't even be measuring pins 1 and 8 on that chip!! They go to sensitive internal nodes. Careless poking will blow the internals. We do not know what those voltages "should be"; we can know all we need to know from the other pins. Does it FUZZ! when Dist is turned up? Then the chip is OK.

LM308
OP07

[thermionix]

Soooo.... is this below unity gain when the Dist is dialed all the way down normal?

Agree with Paul, mine is that way too.  Kinda like the volume knob on a tube amp, it doesn't sound quite right until it's turned up to 1 or so.  If it bugs you, you can add a small series resistor to Lug 1 of the gain pot, setting a higher minimum.

Also, welcome!

[nmtlrx]

Thank you guys for your quick replies. The circuit does sound right; I was just not sure about the volume drop with dist at 0.