Help with Greer Amps Lightspeed Overdrive

[sonolink]

I have this Overdrive pedal on my bench and can't figure out what's wrong with it.
Symptoms are no sound when on. Sound only when in bypass. LED works.

I have measured voltages on the opamp and readings when on are around 4v on all pins except pin 4.

I checked continuity between the input jack and the circuit input and between the output jack and the circuit output and the switch board seems fine.

I have used an audio probe to send a signal from the input into an amp and I can hear the signal on my amp.

I have measured the diodes and the read 0.6 with the black lead on the cathode and OL the other way around.

I have measured the black diode near the DC input and this one reads OL both ways (I didn't take it off the board).

Any ideas please?
Cheers
Sono

 

[Khron]

You could have considered at least attaching a photo of the board, y'know... "The black diode near the DC input" means absolutely nothing to anyone, with text-only for context.

What "input" did you send the signal from, via the audio probe, into your monitoring amp?

 

[AnalogPackrat]

Schematic? Op-amp model (or at least description as in single, dual)?

 

[Lampie519]

Yes, a schematic could help us helping You...

 

[sonolink]

Ooooops, I'm terribly sorry, I got a phone call while I was posting and completely forgot to upload the pics. Please accept my apologies

 

[AnalogPackrat]

Standard debug checks:

1) Is the power rail (9V or 18V) at the proper level?

2) Does signal pass through the footswitch to the main pcb (maybe you tested this)?

3) Does any signal exit the pcb into the output section of the footswitch?

4) If no output signal at footswitch, trace backwards and see where it stops.

This is a relatively simple circuit. Probably some kind of Tube Screamer derivative. You might find a schematic on freestompboxes.org.

What do the red/black twisted wires lead to?

 

[capacitorless]

perfboard layout (no schematic though) at Greer Amps Lightspeed

edit: without more diagnostic detail, the obvious thing is to swap out the opamp. That's the easiest and it is socketed.

Has good reviews at the above link. I hadn't heard of it until now, but I'm out of that particular loop.

 

[sonolink]

Standard debug checks:

1) Is the power rail (9V or 18V) at the proper level?

It is. 9v

2) Does signal pass through the footswitch to the main pcb (maybe you tested this)?

It does

3) Does any signal exit the pcb into the output section of the footswitch?

If I check continuity between the output of the pcb and the output jack I get a signal so I think the switch section is working

What do the red/black twisted wires lead to?

Battery

 

[sonolink]

I found this schem which is supposed to be the same

 

[sonolink]

"The black diode near the DC input" means absolutely nothing to anyone, with text-only for context.

Those diode readings (OL both ways) mean it's blown doesn't it?

What "input" did you send the signal from, via the audio probe, into your monitoring amp?

I sent a signal into the input jack of the pedal and connected the output jack of the pedal into an amp

Thanks for your help

 

[AnalogPackrat]

I found this schem which is supposed to be the same

So start tracing. When the effect is switched on does any signal make it through the first op-amp stage? Does the gain control of that stage change the signal? What about the second stage? Volume pot?

 

[sonolink]

I've uploaded a video showing the troubleshooting in case I'm doing something wrong...
The thing is that with the audio probe everything looks fine but no sound when the FX is on.
Here's the vid:


Thanks for your help

 

[capacitorless]

I would consider probing for a signal on either side of C2 as well as on either side of R10. That should help figure out if the opamp is functional. If you are able to disconnect one side of C9, that would rule out a shorted cap that could be grounding the output of the first opamp stage (IC1.1).

I'm not sure if this is germane, but since it's a split supply, it might be worth putting a capacitor in series with your probe. That way the probe wouldn't be screwing with the biasing on the opamp. If it's a high impedance probe (eg it is plugged into a high impedance preamp) that's probably less likely.

Finally, you've probably already done this but try removing/reinserting the opamp. Worth a shot!

That's all I've got from here.

 

[AnalogPackrat]

I've never used just a signal snooper. How can you evaluate signal level at each test point with it? Polarity? Wave shape? DC offset?

Do the controls have any effect at all?

Did your friend provide any info about what was happening when the thing died?

 

[capacitorless]

@sonolink I spent hours helping my brother debug a reissue Fender reverb unit (remotely; Denver here, Pensacola there). Everything checked out but no output from the wet side of the circuit. We went back and forth a bunch of times via email, and did a phone call as well. He ordered an output transformer, in case that was somehow failing in a wierd way.

Come to find out he had the tone control turned down all the way, which in that design shorts the whole signal to ground. So, he's got a spare OT (which are actually hard to find, so that's cool) but we were both out a bunch of hours of our respective time. He learned as a result; I got to stare at the schematic, which is mostly Fender Champ.

Moral: check the simple stuff first.

 

[AnalogPackrat]

Could be a bad pot, too. Stepping on the knob can push the shaft back into the body and lift the wiper from the track. Cheap pots aren't very mechanically robust.

If you're serious about this hobby you should start investing in some basic test equipment. An audio signal generator with adjustable frequency and level (and waveshape), a meter that can measure at least low audio freq AC RMS voltage, a basic oscilloscope, a variable voltage bench power supply. Everything is much harder when you don't have adequate tools.

 

[capacitorless]

+1 to the above. In my little example, my brother only had a DMM, and I'm not at all sure it could do RMS AC work accurately. He didn't have (nor was he able to make) an AC coupled probe. I've been there though - no equipment other than some kind of a meter.

 

[capacitorless]

Hey I missed this from earlier...

Assuming the schematic matches: the black diode you mentioned (that might have tested completely open earlier) looks like it would be D100 on the schematic. If it's working properly, you should have around 9vdc (to ground) on one side and a little less on the other side. 8vdc-ish depending on how much voltage was coming from the power supply - I think that diode drops .7v-ish (I didn't look it up). It sounded like you were measuring 4v to ground on the op amp at various places, so I think it's ok, but worth verifying.

If it is in fact open, the LED will come on but there won't be power to the opamp - so no sound when not in bypass mode.

 

[Khron]

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?

What DMM setting was that "OL" diode reading made on?

Then again, it's sort of irrelevant *IF* the 9V can be measured between pins 8 and 4 of the opamp.

If anything, i'd probe the audio (through a DC-blocking capacitor) at pins 1 and 7 of the opamp (each half's output).

 

[usbdevice]

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?

I was about to note that too. I think the schematic linked to above is incorrect at least when it comes to the supply section. Judging from the picture, the supply is rc-filtered 47ohm-22uF, after that the diode, anode to ground. The junction of the resistor and diode should be the circuits Vcc.

I have measured voltages on the opamp and readings when on are around 4v on all pins except pin 4.

Taking you by your word here, if that includes pin 8 then something is drawing too much current, could be the opamp or something else.
The 2134 probably won't run on just 4V.
With the opamp removed from the socket, do you still get "around" 4V on all socket pins except pin 4?

My methodical approach would be this, from the start:
- Measure the voltage coming from your power supply/battery before you hook it up to the pedal
- 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) - expect a difference of around 100mV or less. The only thing drawing relevant current should be the Vref voltage divider. As noted above, the voltage at the junction of the 47 ohm resistor and the diode should the circuits Vcc
- 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
- 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

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.

Now excuse me while I rush over to the new members forum
Lenny