Phase 100 Repair

[Tall Steve]

I got a Phase 100 on the bench right now that survived a minor flood. I've cleaned out all the mold I could reach, cleaned some of the calcium buildup off the board with rubbing alcohol, fixed a couple of broken solder joints, and sprayed all the jacks, pots, and switches with contact cleaner. Here's what I've got so far:

- As far as I can tell, there's no visible component damage (no blown caps, burn marks, etc).
- Passes signal, but no phasing.
- Rotary switch works.
- The trimmer seems to be stuck at ~6.5k on one side and ~11k on the other. I sprayed it with contact cleaner, but that didn't do anything. Also, despite those readings, I only get 16k when I measure across the pot.  There's nothing in parallel with it, either.
- Speed pot works, sort of. At full CCW, it measured 250k at first; then over the course of about 30 sec. it climbed up to spec @ 505k.

My question is where to continue debugging from here.

Schematic: http://www.6v6power.ru/inf/Phaser/MXR%20Phase%20100.gif

The images I took on my phone are too large to be viewed in full, so this is the link to the photo album with gutshots and closeups:
http://imgur.com/a/nSRXd

[mth5044]

Time for voltages! Check out the debugging thread.

[Electron Tornado]

- passes signal but no phasing      is it passing only a dry signal?

- speed pot - take it out and measure it on its own. Don't use any more contact cleaner on it before you measure it.

- looking at the foil side of the PCB, there are some traces that look black. Clean those and make sure they are still good.

- If you have an audio signal tracer, you can trace the signal from your guitar and see where you lose it.

[Tall Steve]

Time for voltages! Check out the debugging thread.

Where would be a good place to start? 

- passes signal but no phasing      is it passing only a dry signal?

Yes.  Dry signal only.  There was a wire that had come off the switch, but re-soldering it didn't fix the problem.

- speed pot - take it out and measure it on its own. Don't use any more contact cleaner on it before you measure it.

It's a 24mm pot wired as a variable resistor.  The lugs are pretty easy to access.  I just checked it again with clips this time instead of probes so I could move the knob while the meter was reading.  It seems to start having problems around 150k.  At settings below that, the meter responds instantly.

- looking at the foil side of the PCB, there are some traces that look black. Clean those and make sure they are still good.

That's what some of them looked like after cleaning with alcohol.  What can I use to clean them?  I fried my Blues Driver IC using a small brass brush, so I'm a little hesitant to mess with the PCB with the op amps soldered in.  I think I may have left the power on in the Blues Driver, but I'm still scarred after seeing all the sparks.

- If you have an audio signal tracer, you can trace the signal from your guitar and see where you lose it.

No signal tracer  .

I emailed MXR support, too.  I'll let you know what they say.

[jimilee]

Well the trimmers calibrate the phasing. If they won't turn, you can't dial it in. I'd replace those.

[Electron Tornado]

You can build a signal probe yourself. Here's a link: http://www.diystompboxes.com/pedals/debug.html

And here's a link that explains and demos how to use one: http://billmaudio.com/wp/?page_id=1254

The trim pot you will want to replace. The speed pot may also be bad, but should be easy to replace. You might find it interesting to take it apart and have a look at it and read this  http://www.geofex.com/article_folders/potsecrets/potscret.htm

From the photos, the legs on a couple of the ICs look questionable, but troubleshoot with the audio probe first.

By the way, if you have only dry signal out, then you really don't know if the speed control is working. If you have a spare pot of a reasonably close value, try substituting it in.

[mth5044]

Time for voltages! Check out the debugging thread.

Where would be a good place to start?  

Yes.  Dry signal only.  There was a wire that had come off the switch, but re-soldering it didn't fix the problem.

It's a 24mm pot wired as a variable resistor.  The lugs are pretty easy to access.  I just checked it again with clips this time instead of probes so I could move the knob while the meter was reading.  It seems to start having problems around 150k.  At settings below that, the meter responds instantly.

That's what some of them looked like after cleaning with alcohol.  What can I use to clean them?  I fried my Blues Driver IC using a small brass brush, so I'm a little hesitant to mess with the PCB with the op amps soldered in.  I think I may have left the power on in the Blues Driver, but I'm still scarred after seeing all the sparks.

No signal tracer  .

I emailed MXR support, too.  I'll let you know what they say.

All IC's and transistors would be great. Plus voltage and/or resistance on the LED/LDR vactrols to see if they are all functional. That wouldn't be likely to cause the signal to drop out, but as you said, it's been wet and there's a decent chance that more than one thing has gone wrong.

Measuring resistance when connected to the circuit is likely to give you incorrect values. The speed pot has a reverse logrithmic taper, so for the first half it will slowly change then start to change rapidly towards the later half. Could be something you are seeing?

Build the signal tracer. Very simple jack and cap and will get your first problem very quickly.

[R.G.]

My question is where to continue debugging from here.

There is a stick thread at the top of the forum that you have to ignore to get down to the more active forums. It's titled "Debugging: what to do when it doesn't work". There's a reason it's a sticky message at the top of the forum.

[mth5044]

It was ignored from the second post in this thread too 

[R.G.]

It was ignored from the second post in this thread too 

I was hoping that reinforcing it in an interesting way might help.

Probably not.

[Tall Steve]

I read over the debugging page.  I'll build the audio probe, do some tests with it, and report back.  I'm sorry I didn't do that sooner.  I'm just more than a little nervous poking around inside a vintage unit.  I'm working beyond my skill level here.  But that's how we learn, right?

[mth5044]

We've all been there. When you probe around inside, just make sure you have a steady hand and are only poking at what you want. Bridging two things together can cause some problems, which is what happened in your Blues Driver. You don't want to do that with power connected. Luckily, a phase100 doesn't have any super unobtainable stuff that if you fry will cost you some big $$$ to repair. Voltages and signal tracers are pretty standard in debugging and will definitely shed some light on the problems.

[bluebunny]

. . . make sure you have a steady hand and are only poking at what you want. Bridging two things together can cause some problems . . .

+1   

I briefly made myself a "light emitting triode" whilst prodding around in a sub-mini tube pedal.      Won't be doing that again...

[Electron Tornado]

I briefly made myself a "light emitting triode" whilst prodding around in a sub-mini tube pedal.      Won't be doing that again...

See avatar for photo of actual event.   

[jimilee]

I made magic smoke with mine, it was cool.....once....