Help Troubleshooting Glass Blower Booster Pedal Please

sonolink · Dec 31, 2015

Hello everyone

I have built 2 Booster/buffer by The Valve Wizard and I’m running into a couple of little problems.

Pedal 1 has no audio output. It has power but no sound whether on or off.

When checking with an audio probe I can trace the signal up to pin 3 of the opamp but I loose it after that. Also I’m not sure where to trace it after pin 3 of the opamp or going backwards from the output jack. Any ideas?

Pedal 2 has output but something is wrong. I’ve attached an audio clip of what I can hear through the audio probe. You can hear output jack, before C1, after C1, Pin 3 opamp, opamp output. When battery is disconnected the noise disappears before opamp pin 3 sop I guess it’s something related to power. Any ideas?

https://soundcloud.com/thesoundkitchen/noise

Thanks a lot for your time and help
Sono

Link to the original project page:
http://www.valvewizard.co.uk/glassblower.html

Schematic
http://www.valvewizard.co.uk/glassblower.pdf

pucho812 · Dec 31, 2015

schematic shows the opamp to be a tlo72(a duel opamp) Check for signal at pin 1 which is the output of the first opamp stage. Could also check for signal at pin 5 which is the input to the second opamp stage...

midwayfair · Dec 31, 2015

Measure voltages on all pins of the active devices for both builds.

Did you use Merlin's layout or your own?

The clip you posted sounds like a song recording. What's the noise you're hearing? The clicking?

sonolink · Dec 31, 2015

Thanks for the replies. Yes the noise is the clicking and yes I'm using Merlin's layout.
I'll measure voltages and get back.

Thanks

sonolink · Jan 4, 2016

Hello

Sorry for the delay in replying. I measured voltages on both pedals:

Pedal 1 (no output at all)

Pin1: 0v
Pin2: 0v
Pin3: 2.3v
Pin4: 0v
Pin5: 0v
Pin6: 4.4v
Pin7: 4.4v
Pin8: 8:5v

Pedal 2 (output with clicking noise)

Pin1: 2.2v
Pin2: 2.2v
Pin3: 1.5v
Pin4: 0.7v
Pin5: 2v
Pin6: 2v
Pin7: 2v
Pin8: 7.6v

Thanks a lot for your time and help

Cheers
Sono

midwayfair · Jan 4, 2016

I see a few problems.

In pedal 1, you have a short to ground at pins 1, 2, and 5 (pin 5 is connected directly to pin 1). There are a couple links. Make sure you didn't mix up one of the links with one of the resistors and that you're using the schematic that's in the build document and NOT the one on the website. The numbering is different. It could also just be a solder blob shorting against the ground plane, so pull out your multimeter and start probing the connections to those three op amp pins and try to find where the short is.

2.3V is a little low for pin3 so also double check your Vb voltage divider resistors. I would expect it to be closer to half supply since the bias voltage is 5V. (Measure the voltage AT the bias point just to make sure that's not off.)

In pedal 2, one problem I see is that somehow you've got a different voltage on pin 1 from that of pin 5. Those pins are directly connected -- they should have identical voltages! Maybe your battery drained a little between measurements. In fact, your battery is so low on that one (your supply voltage is a little low in both but particularly in pedal 2) that it leads me to believe that you're drawing a lot of extra amperage. Sometimes clicking is caused by the power rails connecting, or a cap that's gone bad filling and discharging repeatedly. So verify that your electrolytic capacitors in that build are all still good, and re-check the orientation/pinout of Q1 and Q2, particularly if you substituted other transistors. They're a complimentary pair, so it's VERY easy to mix them up, and if you get them backwards, it could produce the result you're hearing.

Keep in mind that you can also get the basic op amp parts working by pulling all three transistors.

And if you don't already have them in a box hooked up to the switch and jacks, photos of both sides of the board will help. If you have already boxed them ... take them out of the box and get them working on a test rig before you resolder them