Dr Swamp Twin Distortion pedal lost its gain!

[tom1970]

Hi, I'm trying to fix a Dr Swamp dual distortion guitar pedal made by a Danish Company called T-Rex.. The circuit doesn't seem to be online but I have attached a schematic for another model they make that is similar (see the Mudhoney schematic image below).
The switching IC and flip flop circuit seem to be functioning fine - both channels light up when the relevent footswitches are depressed and also it switches to bypass no probs. However, the thing has lost a load of gain on both channels and so is not really clipping as it should. Its strange because each channel has its own separate pair of diodes for clipping the signal to create the distortion. They share the same amplifying IC (an OPA 2277 dual op amp something like this... https://www.ti.com/lit/ds/symlink/o...l-mousermode-dsf-pf-null-wwe&ts=1650312565053 (page 4)) but each channel uses a separate op amp on this ic - one side using pins 1,2,3 and the other 5,6,7. However, as I say, the lack of gain is identical on both channels.
Could the 2277 chip somehow have lost its gain on both channels? Is that possible?
DC Voltages on all pins seem correct (9V supply (battery)) plus around midway - 4.5v - on the +ve -ve and output pins.
Any help would be much appreciated!
CHeers
Tom, UK

[The Dude]

What are the voltages on Ta1 transistor?

[tom1970]

Thanks Dude. The voltages look good. We have 9V, 3.3 and 3.9V or thereabouts. The clean channel is working fine so another indication that all is good on that particular transistor.

[tom1970]

In addition there is some wierd addition to the feedback loops on the amplifying op amps. There is a transistor on both channels that seem to be turned off and on by the switching system. I'll try and draw it out when I have a bit more time later. I think the idea is to silence that side when it is not in use.

[g1]

They are probably muting Fet type transistors.

[tom1970]

They are probably muting Fet type transistors.

Maybe the switching system isn't working then because I seem to have the same signal on the op amp pins regardless of whether the overdrive is off or on. Here is this part of the schematic. When it is in bypass mode the +ve side of the diode reads 8V. When the distortion/overdrive is on we have 0V here. At both times we seem to have 0V on the -ve side of the diode. Is that right?
Thanks so much for your help!

[g1]

Any time I've seen N-channel fets in use, they require a negative voltage at the gate to turn off.
It was not clear in your first post whether the midpoint voltage was 4.5 pos, or negative. Please don't use dash symbol with voltages unless you mean negative.

[tom1970]

Any time I've seen N-channel fets in use, they require a negative voltage at the gate to turn off.
It was not clear in your first post whether the midpoint voltage was 4.5 pos, or negative. Please don't use dash symbol with voltages unless you mean negative.

Good point. Apoloogies for that. The voltages are all positive, so around 4.5 on the positive and negative pin on the op amp and thus on two of the fet leads. The third fet lead is attached to a diode and we have +8V or 0V on the far side (positive side) of the that diode and wierdly it looks like 0V on the the negative side (the side that connects to the third lead of the fet) regardless of the voltage. The diagram showing the feedback loop on the op amp is correct although I don't know which is drain, gate or source. So how that turns it off, I don't really understand.
BTW I've also just noticed that the feedback is non-inverting as there is a small 47R resistor and then a 4.7uF cap to ground from the negative pin as well.

[J M Fahey]

Dr Swamp dual distortion guitar pedal ..... is similar (see the Mudhoney .... switching IC and flip flop circuit seem to be functioning fine - both channels light up when the relevent footswitches are depressed and also it switches to bypass no probs. ..... has lost a load of gain on both channels and so is not really clipping as it should. Its strange because each channel has its own separate pair of diodes for clipping the signal to create the distortion. They share the same amplifying IC (an OPA 2277 dual op amp ..... but each channel uses a separate op amp ....
Could the 2277 chip somehow have lost its gain on both channels? Is that possible?
DC Voltages on all pins seem correct (9V supply (battery)) plus around midway - 4.5v - on the +ve -ve and output pins.

IF itīs "two Mudhoneys in a single enclosure" then it "lost" nothing,it simply has not much gain to begin with, by design.

Using the word "design" is very generous, itīs actually a mishmash of cutpasted blocks from different well known pedals.

Basic block diagram is classic: input buffer > gain stage > diode clipping > bypassing or here "channel switching" > output buffer.

Vaguely similar to a RAT but quite less gain, even less gain than a Tube Screamer or an MXR Distortion+

Hear its demo, it barely clips, no sustain at all (chord tails disappear quickly) even being driven by a Les Paul and into a 65 Amps boutique tube amp.

Player uses it as a booster half the time so there we are actually listening to an overdriven tube amp, go figure.

An Op Amp will not lose gain, it either works or not.

In a nutshell, not sure itīs "broken".

Just in case, if you have a scope, and referring to known Mudhoney schematic, (find equivalent elements in the dual one), inject 00,V 1kHz at the input jack and follow 9it.
Input buffer and also output one if effect is bypassed, should show basically same 100mV , up to the output jack.
Op Amp should show about 5X to 10X gain at minimum and 3 X as much when on full (yes, it has THAT limited range, the variation between 15k and 47k, look at the gain network).

Boost switch (not sure yours has one) murders gain down to about 2X.

In my view, a "meh" design.
Being kind.

EDIT:

Almost forgot:

Any time I've seen N-channel fets in use, they require a negative voltage at the gate to turn off.

Very true, but on 9V single battery pedals where they do not have negative voltages anywhere, the trick is to "DC ground" the switching FET to the +4.5V line , I mean drain and source, check the schematic, and then gate either goes to ground which to the Fet means -4.5V so it gets OFF,or to +9V, but because of the diodes, they become reverse biased and pass no voltage to the gate, so itīs now floating ... in practice "receiving 0V" so itīs ON

So what you measured at either side of Gate diodes is fine.

[Helmholtz]

The diagram showing the feedback loop on the op amp is correct although I don't know which is drain, gate or source. So how that turns it off, I don't really understand.
BTW I've also just noticed that the feedback is non-inverting as there is a small 47R resistor and then a 4.7uF cap to ground from the negative pin as well.

The jfet in your drawing doesn't turn the opamp off.
Rather it shorts the output to the inverting input, turning the opamp into a unity gain buffer.

Circuit shown is a non-inverting amp. NFB always goes to the inverting input.
The 47R is the shunt part of the NFB voltage divider.

[g1]

Thanks JM for the clarification on the Fet switching.
I guess it's still possible there is a leaky fet though I don't see how both channels could be affected and it seems very unlikely that both fets would go leaky at the same time.
Assuming there was more gain previously, leaky Fet's would alter the feedback loop and thus the gain.
Disconnecting the drain or source of the Fet's in the loops, (or just removing them) should tell you whether they were leaky or not.

[tom1970]

Hi everyone. I've ditched it as I kind of ruined it when trying to remove some components for testing - melted the tracks. I decided to use the chassis for a stripboard build of a customised RAT and TBH it sounds way better than the DR SWAMP. The whole thing had turned into a bit of a headache and I wasn't getting anywhere. Thanks so much for all your help. I learned a lot by reading your posts! Cheers. Tom