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Faulty Op Amp...

tom1970

Aug 23, 2021
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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
 

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CircutScoper

Mar 29, 2022
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Hi, Tom

Yes. It's certainly possible for a dual opamp chip to develop a defect that would disable both amplifiers.

Since you've verified the presence of proper DC voltage on the chip's pins, the next logical step would be to replace it.
 

CircutScoper

Mar 29, 2022
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H... I have attached a schematic for another model they make that is similar...

PS: This might be a quick check of the amplifiers. If your model has something comparable to S1--BOOST, opening that switch would normally (i.e., if the amp were working) "Boost" gain by about 20x = 26dB. But if the amp is dead, the effect of the switch will reverse and opening BOOST will REDUCE gain by 26dB. So set it up with an input signal and see which state of the switch makes the output louder.

You mentioned checking DC voltages on the in and out pins. Did you also check for 0V on the ground pin at the chip?
 

tom1970

Aug 23, 2021
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PS: This might be a quick check of the amplifiers. If your model has something comparable to S1--BOOST, opening that switch would normally (i.e., if the amp were working) "Boost" gain by about 20x = 26dB. But if the amp is dead, the effect of the switch will reverse and opening BOOST will REDUCE gain by 26dB. So set it up with an input signal and see which state of the switch makes the output louder.

You mentioned checking DC voltages on the in and out pins. Did you also check for 0V on the ground pin at the chip?

Hi,
Thanks so much for your help. The feedback system on the op amp stage is rather strange - see attached picture.. There is no booster switch. The incoming signal or input is on the + pin and the feedback between output and - pin is 22k, a capacitor (no markings) and the 47k-A gain pot all in parallel BUT in addition there is some kind of transistor/JFET or something - maybe collector and emmitter with the base or gate or whatever attached to a diode. When it is in bypass mode the +ve side of the diode reads 8V. When the distortion/overdrive is on we have 0V here). I think the idea is to turn off the gain when it is in bypass (although the bypass signal has a different route). Hope that makes sense!

The gain is making it louder when you turn it up but it is weak overall and there is no clipping. I checked the signal on all pins with a simple tone probe thing that I made (just a probe with a cap connected to a guitar lead so I can plug it into an amp and listen to the signal) and it is there on all pins but like I say it is weak.

Cheers again

Tom
 

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CircutScoper

Mar 29, 2022
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Hi,

The gain is making it louder when you turn it up but it is weak overall and there is no clipping. I checked the signal on all pins with a simple tone probe thing that I made (just a probe with a cap connected to a guitar lead so I can plug it into an amp and listen to the signal) and it is there on all pins but like I say it is weak.

Cheers again

Tom

Because adjusting the gain pot changes the signal amplitude, it appears the amplifier is working. If it weren't, the pot would have no effect. This implies the cause of your problem may lie external to the pedal circuitry. Are you sure the level of the input signal is adequate? Is it possible there's a mismatch between the plug you're using and the input jack? The jack in the schematic you included with your original post is peculiar in how it connects the 9V battery when the plug is present. It may require a special plug.
 

tom1970

Aug 23, 2021
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Because adjusting the gain pot changes the signal amplitude, it appears the amplifier is working. If it weren't, the pot would have no effect. This implies the cause of your problem may lie external to the pedal circuitry. Are you sure the level of the input signal is adequate? Is it possible there's a mismatch between the plug you're using and the input jack? The jack in the schematic you included with your original post is peculiar in how it connects the 9V battery when the plug is present. It may require a special plug.

No the input signal is fine. This is a pedal I have used for years. Only recently lost the gain. It's wierd.
 

CircutScoper

Mar 29, 2022
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No the input signal is fine. This is a pedal I have used for years. Only recently lost the gain. It's wierd.

Well, not to sound skeptical, but can you verify that the input signal is adequate -- by substituting a different signal source or a different "pedal?" I'm not trying to be argumentative but you really have produced some pretty good evidence that this "pedal" is working.
 

tom1970

Aug 23, 2021
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Sure. Of course. I'll have to do it tomorrow as I need to get to bed (I'm in the UK so its pretty late here). Thanks again for your help and apologies if I sounded overly dismissive!
 

tom1970

Aug 23, 2021
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Well, not to sound skeptical, but can you verify that the input signal is adequate -- by substituting a different signal source or a different "pedal?" I'm not trying to be argumentative but you really have produced some pretty good evidence that this "pedal" is working.


Hi,
I tried it with a different guitar and amplifier and there wasn't much improvement. However, interestingly when I put a pedal which had a high trebly output between my guitar and the DR SWAMP pedal, it did seem to boost that quite well. COuld that indicate that one of the capacitors in the initial input stage in the DR SWAMP is losing some capacitance and allowing the higher frequencies through but attenuating the lower ones?
 

CircutScoper

Mar 29, 2022
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Hi,
I tried it with a different guitar and amplifier and there wasn't much improvement. However, interestingly when I put a pedal which had a high trebly output between my guitar and the DR SWAMP pedal, it did seem to boost that quite well. COuld that indicate that one of the capacitors in the initial input stage in the DR SWAMP is losing some capacitance and allowing the higher frequencies through but attenuating the lower ones?

Since you've said the loss of gain is in BOTH channels and it's unlikely that a failure of any single passive component could affect more than one channel, that seems implausible.

A random question: Has the Dr. Swamp pedal worked successfully in the past with EXACTLY the SAME instrument and amplifier you were using it (unsuccessfully) with yesterday?
 

CircutScoper

Mar 29, 2022
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Since you've said the loss of gain is in BOTH channels and it's unlikely that a failure of any single passive component could affect more than one channel, that seems implausible.

A random question: Has the Dr. Swamp pedal worked successfully in the past with EXACTLY the SAME instrument and amplifier you were using it (unsuccessfully) with yesterday?

Cables too.
 

CircutScoper

Mar 29, 2022
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Hi,
...a pedal which had a high trebly output between my guitar and the DR SWAMP pedal, it did seem to boost that quite well...

Ya know, if the (unusual) input jack on the DR SWAMP pedal has developed a broken connection, it's not impossible that the intrinsic capacitance of the guitar cable might be acting as a high-frequency filter.
 

tom1970

Aug 23, 2021
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Ya know, if the (unusual) input jack on the DR SWAMP pedal has developed a broken connection, it's not impossible that the intrinsic capacitance of the guitar cable might be acting as a high-frequency filter.

The input socket is quite standard for guitar pedals. Electro harmonix use them too. Something like this...
https://www.ebay.co.uk/itm/29359951.../wZfAzRMN93V5b9iaAs1NlsRfw==|tkp:BFBMyIqil4lg
I've tested continuity on it and also inspected for dry joints etc and it seems fine.
 

tom1970

Aug 23, 2021
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Since you've said the loss of gain is in BOTH channels and it's unlikely that a failure of any single passive component could affect more than one channel, that seems implausible.

A random question: Has the Dr. Swamp pedal worked successfully in the past with EXACTLY the SAME instrument and amplifier you were using it (unsuccessfully) with yesterday?

The two channels do share a number of components (a few caps/resistors and one transistor) before the signal is split and also at the end. Also the transistors that mute the gain on the non-inverting feedback when in bypass mode (see the above images) have a common source supplying the gate/base (not sure whether they are jfet or other type of transistor).
 

CircutScoper

Mar 29, 2022
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The two channels do share a number of components (a few caps/resistors and one transistor) before the signal is split and also at the end. Also the transistors that mute the gain on the non-inverting feedback when in bypass mode (see the above images) have a common source supplying the gate/base (not sure whether they are jfet or other type of transistor).

Okay. I'm confused. The schematic you provided contains only a single opamp while the circuit board pictured contains a dual. So my assumption was that the b'd implements two of the channels illustrated in the schematic, and that the gain loss occurred in both of those two independent channels. Did I go astray?
 
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tom1970

Aug 23, 2021
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Okay. I'm confused. The schematic you provided contains only a single opamp while the circuit board pictured contains a dual. So my assumption was that the b'd implements two of the channels illustrated in the schematic, and that the gain loss occurred in both of those two independent channels. Did I go astray?
Yes the schematic is not exactly the same as I couldn't find the exact model online (Dr Swamp). I'm trying to map out the exact schematic at the moment. So what happens is the signal comes in from the socket, goes through a cap and a transistor then splits bypass and distortion, then goes through another cap and a resistor and then splits into the two distortion channels which feed into either side of the dual op amp. when the bypass is in effect, the idea is that both feedbacks are switched off with a jfet. Hope that makes sense. I'll get that homespun schematic up here asap.
 

Harald Kapp

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I'll get that homespun schematic up here asap.
:)
Make sure to add reference designators (labels) to the components and where meaningful to signals and power supplies, too. Like R1, R2, U1, U2, C1, C2, Vcc, GND, sig_in, sig_out etc. This will greatly help in discussing what's going on since you and we will have the same notion.
 

CircutScoper

Mar 29, 2022
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Yes the schematic is not exactly the same as I couldn't find the exact model online (Dr Swamp). I'm trying to map out the exact schematic at the moment. So what happens is the signal comes in from the socket, goes through a cap and a transistor then splits bypass and distortion, then goes through another cap and a resistor and then splits into the two distortion channels which feed into either side of the dual op amp. when the bypass is in effect, the idea is that both feedbacks are switched off with a jfet. Hope that makes sense. I'll get that homespun schematic up here asap.

Ah so. Then of course any component(s) common to all signal paths is a plausible suspect.
 

CircutScoper

Mar 29, 2022
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Yes the schematic is not exactly the same as I couldn't find the exact model online (Dr Swamp). I'm trying to map out the exact schematic at the moment. So what happens is the signal comes in from the socket, goes through a cap and a transistor then splits bypass and distortion, then goes through another cap and a resistor and then splits into the two distortion channels which feed into either side of the dual op amp. when the bypass is in effect, the idea is that both feedbacks are switched off with a jfet. Hope that makes sense. I'll get that homespun schematic up here asap.

Right. I get it -- finally! This "pedal" provides two independent sets of controls -- LEVEL, GAIN, TONE -- that you can set up before beginning a performance, and then switch between while playing. The incoming guitar signal is split between these two channels as well as straight through, then selectively recombined according to which switch you stomped last.

Components lying in the signal path before the split and after the recombination are all potential suspects.

Sorry for being so slow on the uptake.
 
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