Tonepad TS 808 debugging help

[R.G.]

I just triple checked - all my components are correct, all polarities are correct. I'm beginning to suspect there's a faulty cap or resistor, or even a transistor, but I don't know how to test that. Also, the voltages all seem to be correct from GGG's site, and if that's the case doesn't that mean that the parts are fine? Pete, I checked all the lugs on the switch - everything beeped loud and clear. It's not the switch. I've also beep tested all cabling. No breaks or hidden problems there (I use solid core sometimes, and sometimes that's the problem - not this time).

As frustrating as this is going to sound, chances are that you have some kind of wiring error. The DC voltages do seem to be reasonable. That means that nothing is grossly wrong with the part placement/selection/polarity and so on on the board. That only leaves the interconnection of the various sections by coupling capacitors, wires and controls as potential issues.

I might be able to find a local place that can sell me the stuff to make an audio probe, but I might have to figure out some other kind of solution out.

Here's one "Audio probe" that's pretty simple:

At an even simpler level, you can make one by winding a ground wire and one lead of a capacitor onto the plug end of a guitar cord. I suspect you have the parts to do that already. All an "audio probe" consists of is a DC blocking capacitor leading to a shielded cable into a guitar amp. Everything else is window dressing.

I'll keep trying to make this work, but if anyone has a suggestion I'll be glad to hear it.

I think you're going to do a big forehead slap when you find out the issue. 

[jkokura]

So I took a closer look at this again last night - I was rehousing it to give it a proper dc jack - and I decided to take an audio probe to it. Found the problem soon after. In spite of my best checking, my sincere diligence and my honest triple check, I still managed to put the wrong component in. The Tonepad layout for the 808 calls for a '100' ohm resistor for Ra, and I had used a '100k ohm' resistor. changed it out for a 110 ohm resistor I had on hand, now it works fine.

Also, a suggestion for people who may read this one day - use your audio probe backwards. If you're struggling to get sound at the output, start tracing AT the output back towards the input. It's much easier to find the source of the problem this way than trying to go from the input. Despite trying to do this debug with an audio probe starting from the input in the past, going from the output to the input made it very apparent which resistor was the problem for me.

It works!

Jacob

[oldrocker]

As I always say test every component before soldering anything in.  Metering resistors and caps even when it seem obvious can prevent headaches at the debug stage.  I used to be the same way.  I'd be absolutely positive everything was right.  I would even get frustrated when members ask me to double or tripple check components.  But in the end I'd have to eat crow when my bad eyes mis-read a color code or cap value.  And don't get me started with the (I'm possitive there isn't a cold solder joint.)  LOL!  As for audio probing your right you can work backwards but if the problem is at the input it will be about the same amount of time to find the problem.  Whatever works for you.  Great job and glad you debugged your pedal.

[jkokura]

Oh I was sure that the resistor was right and checked it with a multimeter even. I thought that there was SUPPOSED to be a 100k resistor there. So all my checking and rechecking would have been for naught really, and even metering the resistor would have told me only that I was right. Using the Audio Probe however told me that that resistor was the problem. Then re-examining the BOM on the Tonepad project file made me wonder why that resistor was giving me a problem, which led me to look closer and see that there was not 'k' after the 100 there, and that I had assumed it was just missing as an error, and that actually a 100 ohm resistor was what I needed, not the 100k I thought it meant.

However, you are right that metering every component is a big help. If I didn't order from companies like smallbear and PPP who separate them all into baggies I'd have to check every single one.

Jacob

[oldrocker]

I see what your saying.  What I meant by double and triple checking components also applies to checking the schem too.  That's why making a list of components and comparing it to the schem would be the first check.  But if you've already convinced yourself that the wrong value resistor was correct I guess it wouldn't have helped.  No matter as long as it works.  That's the hazards of pedal building.

[Mugshot]

'grats for getting the bug fixed!  however, the worse part is when you realize that the tone the pedal gives does not really surprise you or anything after a while  we are all fickle-minded