Help with Greer Amps Lightspeed Overdrive

[sonolink]

Thank you EVERYONE for your input. I will follow Usbdevice's indications and report back.

BTW, I do have a small scope, just in case....I just don't really know how to use it here

Thanks again for all the help guys!!
Cheers
Sono

 

[sonolink]

- Measure the voltage coming from your power supply/battery before you hook it up to the pedal

9.5vdc

- Carefully remove the opamp from the socket
- Apply power to the circuit (judging from the picture you'll to need to have something plugged into the input jack
- Using the DC-jack ground point as your reference for all voltage measurements, measure the voltage at both sides of the 47 ohm resistor (left of the black diode in your picture)

Left of 47ohms=9.5vdc
Right of 47 ohms (Vcc)=9.45vdc

- Measure the voltage at the junction of the two resistors (4k7 and 5k6) right next to the diode, that should be Vref, slightly higher than half of Vcc

5.1vdc

- Now measure voltages on all pins of the opamp socket. Check that you get Vcc on pin 8, 0V on pin 1, 2, 4 and 5 and Vref on pin 3, 6 and 7. This is going on the assumption that the schematic is correct apart from the power section

P1,2,4,5 = 0vdc
P3 = 4.9vdc
P6,7 = 5.1vdc
P8 = 9.45vdc

Repeat the measurements with the opamp inserted. As a precaution I would set gain to minimum, volume and tone to maximum, but that's just me.
Report your findings.

Supply 9.5vdc

Left of 47ohms=9.48vdc
Right of 47 ohms (Vcc)=4.89vdc

Junction 4k7/5k6 = 3vdc

P1,2,5,7 = 4vdc
P3 = 4.3vdc
P4 = 0vdc
P6 = 3.53vdc
P8 = 4.9vdc

Thanks a lot for your guidance!!
Cheers
Sono

 

[Khron]

Probably easiest to confirm by swapping in another opamp - even a "lowly" TL072 would be sufficient, to confirm the rest of the circuit is working as intended. The opamp itself looks like is sucking down a lot of current.

Is the opamp, and that 47 ohm resistor, getting warm / hot? I bet they are... Those numbers work out to about 97mA current draw, which would mean the resistor is burning up almost 1/2 a watt - it's gonna be noticeable, probably.

 

[CJ]

#1 problem is ft sw.

They have a bunch of Greer schematics at pedal pcb.

Here is one, not same name as yours but might help.

 

[capacitorless]

Going back to that perfboard layout from earlier: the diode is listed as 5817 Shottky, which might be for reverse voltage protection? Like Khron pointed out earlier, if the opamp is getting voltage, that diode is likely ok.

Like CJ already said, #1 is footswitch (mechanical and literally gets stepped on repeatedly). I usually rule out mechanical issues like that (edit: including wiring issues) then look at semiconductors.

 

[Michael Tibes]

Supply 9.5vdc

Left of 47ohms=9.48vdc
Right of 47 ohms (Vcc)=4.89vdc

Junction 4k7/5k6 = 3vdc

P1,2,5,7 = 4vdc
P3 = 4.3vdc
P4 = 0vdc
P6 = 3.53vdc
P8 = 4.9vdc

It seems the power supply gets pulled down, it should only be marginally lower compared to the value with the opamp not inserted. Chances are that the opamp is fried, did you exchange it? Otherwise the reverse protection diode or the 47R (is it is in line with the diode?) could be damaged and behave weirdly. Quite unlikely, but not impossible. Another unlikely possibility might be that the opamp oscillates at a high frequency beyond the hearing range, but I guess at least some kind faint sound should still pass then. You might need an oscilloscope to check this As mentioned, you can pretty much try any other pin-compatible opamp. It might have a small effect on the sound, but it should work.

One more thought: if the opamp pulls down the 9V line, the 47R resistor burns half a watt, so it should get warm. If the resistor or the diode don't pass current as expected, the resistor should stay cold.

MIchael

 

[Khron]

It seems the power supply gets pulled down, it should only be marginally lower compared to the value with the opamp not inserted. Chances are that the opamp is fried, did you exchange it? Otherwise the reverse protection diode or the 47R (is it is in line with the diode?) could be damaged and behave weirdly. Quite unlikely, but not impossible. Another unlikely possibility might be that the opamp oscillates at a high frequency beyond the hearing range, but I guess at least some kind faint sound should still pass then. You might need an oscilloscope to check this As mentioned, you can pretty much try any other pin-compatible opamp. It might have a small effect on the sound, but it should work.

One more thought: if the opamp pulls down the 9V line, the 47R resistor burns half a watt, so it should get warm. If the resistor or the diode don't pass current as expected, the resistor should stay cold.

MIchael

I'd rather suspect that diode being a zener - notice its anode connected to the ground plane? And what's that, a 47 ohm resistor going from the DC input to its cathode?

Probably easiest to confirm by swapping in another opamp - even a "lowly" TL072 would be sufficient, to confirm the rest of the circuit is working as intended. The opamp itself looks like is sucking down a lot of current.

Is the opamp, and that 47 ohm resistor, getting warm / hot? I bet they are... Those numbers work out to about 97mA current draw, which would mean the resistor is burning up almost 1/2 a watt - it's gonna be noticeable, probably.

Yeah...

 

[usbdevice]

I agree with what khron and others have said. There's half a watt of power being dissipated through the 47 ohm resistor and that's noticeable if you touch it (if not with a callused fingertip then for sure with the back of your finger )
At this point I'd also try and swap the opamp and see if that solves the problem - and indeed a TL062/072/082 would be fine for that purpose.
I'm thinking there might be a possibility that there's something on the outputs of the opamp that's shorted to ground but I'm not sure what and where. You could measure resistance to ground (using the same DC-jack ground point as before) from pin sockets 1 and 7 (without the opamp in it).
You can also visually inspect the pcb (both sides) for solder bridges, single strands of wire which may shorting out something somewhere.
And lastly, you could also test the, presumably damaged, opamp from the pedal but you'd need a couple of resistors and preferably a solderless breadboard.

Keep us updated.
Lenny

 

[sonolink]

It was the opamp!
I swapped it with a TL072 and alles gut!!

Do you guys reckon there'll be a noticeable difference in sound?

Thanks again for all your comments and help!!
Cheers
Sono

 

[Khron]

Do you guys reckon there'll be a noticeable difference in sound?

Maaaaaaaybe only if you end up clipping the opamp; otherwise, doubtful.

Then again, OPA2134's aren't exactly what one would call "esoteric", so should be easy enough to source a replacement from reputable distributors.

 

[AnalogPackrat]

Op-amps don't often spontaneously die. Something killed it. The circuit has polarity protection on the supply, so it probably wasn't that common problem. Maybe over voltage, but the 2134 is rated for 36VDC across its supplies and most pedal supplies are 9, 12, or 18VDC. Would be worth checking for some other fault on the board as was suggested earlier (solder bridge, loose strand of wire...).

 

[Khron]

The circuit has polarity protection on the supply, so it probably wasn't that common problem.

Does it really? Referencing the photo of the actual pedal in question, not any of the attached / linked schematics...

(Or if that suspected zener there also worked as reverse-polarity protection, i would've expected that 47 ohm resistor to be burnt to a crisp, with at least 1.5W coarsing through it with reversed 9v; more at a higher voltage.)

 

[Lampie519]

It is a “stompbox” so why shouldn’t it die spontaneously …

With only 10 components on the board it shouldn’t be difficult to trace the error.., and also create a schematic…

 

[AnalogPackrat]

Does it really? Referencing the photo of the actual pedal in question, not any of the attached / linked schematics...

(Or if that suspected zener there also worked as reverse-polarity protection, i would've expected that 47 ohm resistor to be burnt to a crisp, with at least 1.5W coarsing through it with reversed 9v; more at a higher voltage.)

It looks like a standard diode to me, maybe a Schottky for lower forward drop. The linked clone schematic likely has different power rail details (not uncommon in my experience with cloned stuff).

 

[Khron]

It looks like a standard diode to me, maybe a Schottky for lower forward drop.

My point was, take a (MUCH) closer look where / how it's connected on the actual circuit board...

 

[AnalogPackrat]

My point was, take a (MUCH) closer look where / how it's connected on the actual circuit board...

I see. Ignore my previous comment.

 

[AnalogPackrat]

I see. Ignore my previous comment.

Actually, no. It looks like 9VDC in goes to 47R then a reversed diode to ground. The VCC is tapped from the node between these components. If a positive voltage is supplied the diode is reverse biased and has no effect. If a negative voltage is supplied the diode conducts and keeps VCC node from getting negative enough to kill the chip (maybe).

I prefer a forward biased diode in series with the supply.

 

[Khron]

Actually, no. It looks like 9VDC in goes to 47R then a reversed diode to ground. The VCC is tapped from the node between these components. If a positive voltage is supplied the diode is reverse biased and has no effect. If a negative voltage is supplied the diode conducts and keeps VCC node from getting negative enough to kill the chip (maybe).

I prefer a forward biased diode in series with the supply.

(Or if that suspected zener there also worked as reverse-polarity protection, i would've expected that 47 ohm resistor to be burnt to a crisp, with at least 1.5W coarsing through it with reversed 9v; more at a higher voltage.)

Indeed...

 

[AnalogPackrat]

Indeed...

Yes, I did the math. It's a good reason to do it differently. With the given evidence it's hard to know what happened. Did the pedal work normally for a time and die? What was the owner doing when it died? Maybe while changing the battery he reversed it by accident and there was enough negative Vcc to damage the 2134.

 

[Lampie519]

1 Schottky diode in series with the 9v should do the trick….or a mosfet that will also work as a soft starter…