MXR Distortion + reissue no power;

[beedoola]

I got this pedal and it doesn't power up. Can see why: R9 and R8 were totally burned up and it looks like the trace was burned up. Does anyone know the value of these resistors? And do they connect to each other other and somewhere else on the board edge - top/component side? From the schematics I've seen, they don't show these resistors.

[GibsonGM]

"Got" the pedal...can you return it for refund? Or did you get it cheap and knew it was screwed up?  Just asking.

Can you post a pic of the full board, top & bottom?  Can you use your meter to find some landmarks, such as "+ pin of power jack to HERE"?  Maybe go backwards from opamp pin 7, see if you get another landmark in that burned area?    There was nothing left of the R's at all?

[Dormammu]

Found one diagram where the values are 1M. But not sure.
Catch this.

I see different colour code.
Poking in online calculator gave R9=10 ohm, R8=2.9 kOhm.
Still not sure.

[GibsonGM]

That's what I was thinking - the bias network.  If one end goes to GND, and one to V+, mid-point to the cap...a take-off to another 1Meg, then that's likely what they are.

Looks like "Alien" puked all over there.

[beedoola]

I got the pedal in a lot of broken pedals. Was able to repair most of the others but just opened this one up and it saw this.

I can post more pics later when I get home.

[beedoola]

Found one diagram where the values are 1M. But not sure.
Catch this.

I see different colour code.
Poking in online calculator gave R9=10 ohm, R8=2.9 kOhm.
Still not sure.

Thanks. I can see r8's trace looks like it runs under the power jack.

[Dormammu]

Thanks. I can see r8's trace looks like it runs under the power jack.

Just a little more poking  gave
R8=390 Ohm
Grab the tester and check where the traces from R9 and R8 go.

[beedoola]

The trace burned up and separated on the component side on the edge of the PCB. On the trace side you can see where the traces run on one side.

[Dormammu]

The trace burned up and separated on the component side on the edge of the PCB. On the trace side you can see where the traces run on one side.

We still can see something from 2-nd foto.   
All that stuff must be checked.


I'm increasingly inclined to believe that R8=390 Ohm, used like a circuit breaker.

[beedoola]

If you read the post you're quoting, I said there is a trace burned on the component side. The trace as you show on the trace side - I should have said "solder side" for clarity - is intact.

[Dormammu]

If you read the post you're quoting, I said there is a trace burned on the component side. The trace as you show on the trace side - I should have said "solder side" for clarity - is intact.

So we have to follow the traces to understand the purpose of these resistors.
It's quite difficult to read codes from photo.
We need to compile the all available data.

[GibsonGM]

A pic of the whole board might help...

[anotherjim]

Seems like its old style bypass using a DPDT to add a LED. One resistor might be for the LED and one for DC input filter to the electrolytic cap nearby. Continuity test with the DMM should prove it. There doesn't appear to be any reverse power polarity protection so the cap and the LED could be toast (cap looks a bit bulged up on top). Unfortunately, the 741 opamp could also have died.

[ElectricDruid]

Unfortunately, the 741 opamp could also have died.

A nuisance but cheap to replace, at least.

[beedoola]

LED is good. I cleaned it up with some alcohol and the bottom right of R8 goes up to the top left of r9 via an intact trace. The other side of r9 runs to the sleeve of the power jack: there is a small bit of trace left that I could get my probe on to test it.

Now it's just getting the resistor values right.

[GibsonGM]

Looks better. I bet they are 2, 1Meg resistors.  But rather than betting...can you find the ground pins of the power jack, and likewise, identify the + input from it? Show that in a pic?  So we can be sure.

Next, remove the cap that is by the power jack and replace it.  Just because.  It has to be the Vr bypass cap.  Why else would Vr short and burn, except for that cap dying (ok, guessing).  Don't know why it died (age? Cheap?), but it's a pretty good guess. 

   

[anotherjim]

1M resistors burnt out? Has it been hit by lightning? 
I'm sticking to the LED and power filter theory for those resistors.
The original was battery only. RC filter added for pedal 9v supply.
The 1M+1M Vref capacitor was only 1uF, I see a larger cap nearby. See R2, 3 & 4 for the Vref network.
The resistor that burnt the PCB trace is probably the DC input filter. The other resistors' burn might only be proximity damage - it and the LED still functional.
If the 741 took the brunt of the reverse voltage, the filter cap could be ok.

I don't know how accurate this schematic is. The bypass arrangement is bizarre - you would expect the FX input to remain connected in bypass, not the output pot! You can though imagine DC power and LED added on top of this to make a modern-original.

[beedoola]

I'll try and replace the cap.

I tried putting a 1m for r9 and 4.7k for r8 as the latter goes to the LED and no luck. The 1m is too big as I only get 1.7v on the other side. Maybe that cap is shorted.

[anotherjim]

R9 looks like 10ohm to me in the pic of the good one and R6 looks like 4k7 which is a fit for a LED resistor. If R9 is the DC input feed, it won't be 1Meg.
If the 741 is caput (it looks ok), it may burn the resistor out again if you fit 10ohm. I'd temporarily fit 100R for R9. If there is a short somewhere, it will smoke but give you time to cut the power.

[beedoola]

I replaced the R9 with 10ohm. I had figured R8 was 4.7k so I used that. Nothing. What should the voltages be on the IC pins?