MXR Blow Torch LED not turning off.

[served]

Hi.

I have a broken MXR blowtorch. Its LED is not turning off, when the pedal is bypassed. It was caused by wrong polarity power supply. Effect chain is working.

It is not a classical Truebypass pedal, so the LED magig is done with transistors, but its damn hard to figure out what is going on inside there.

Does anyone know how it has been done?

I will post some pictures today.


The switch itself is wired like this. http://amest.edicypages.com/photos/BlowTorch_large.JPG

I have changed one transistor that was phisically broken.

[Mike Burgundy]

Does the swithing *work* aside from the LED staying on?
DO check if there was any polarity protection present, and if that has failed.
You got lucky there if it has, although it wouldn't be a bad idea to replace all polarised electrolytics, preferably all semiconductors as well - or alternatively you can wait for problems to arise (which is 50/50 or worse). DO repair the protection, if any.

It's almost certainly going to be a flipflop/FET switching setup like in a TS (and practically any BOSS pedal, and and - have a look at some schematics showing the switching, like http://www.hobby-hour.com/electronics/s/schematics/od1-overdrive-schematic.gif). I'm assuming switching still works. Have a look if you can find a transistor attached to the LED in series to ground. You should be able to find something like V+ to LED to limiting resistor (and/or zener) to transistor to ground. THAT transistor (Q5 in the linked schematic, as an example) is the one switching the LED (it's not the flipflop(Q3 and 4 in the example schem), it's controlled by it), that's probably failed shorted.

[served]

Hi Mike.

Well its not wired like that. If you can see from the pic I posted, then you can see that signal is bypassed mechanically, no Fet switching there.

LED istatus is somehow controlled by SW1.

Current status of this schematic is describing bypassed state.

The pedal itself is working well, no problems there. Bypass also works well. Actually its all working and fine only the LED is not turning off.

[served]

Hi.

So this is how its looking inside.



Q1 - 2F  (2n2907)
Q2 - P1WCB - unknown
Q3 - Unknown
Q4 - 6Y - MMBFJ177


Q1 Base gets a signal when pedal is bypassed.
Then Q1 is disconnecting the connection from Ground, leg gets higher voltage reading.
Then Q2 does something. it is marked P1WCB (don't know what it stands for as I could not find it)
Then Q3 should take the voltage away from LED and Q4 base. I dont know what transistor to use there as this was also phisically broken and its face was off.

If the LED is turned off Q4 switches C1 and "FX In" to ground, so it acts like a 1M resistor in traditional truebypass schematic, only its resistance varies, so its smaller when the pedal is off and its very high when the pedal is ON.

Could you help me to fill the gaps?

[served]

A good site that I found for SMD codes.

http://www.marsport.org.uk/smd/mainframe.htm

But P1WCB is still unknown and I need to recheck it.


Can anyone tell me why this resistor is there with Q3? What is this for?

[fuzzlab808]

Hi. Just fixed a blow torch with the same problem. Q3 is a NPN transistor. I just changed it for a non-SMD BC547 (the cheaper generic one I could find) and it works perfectly. All this circuit is a kind of complicated "millenium bypass" to save two cents in a 3PDT.    The resistor you see connected to the Q3 terminals (base and collector) is a biasing resistor, it's feeding a little current to the base of Q3 (Ib) that is multiplied by the "beta" factor to provide the Ic current that goes to the LED.

[flo]

I have fixed a MXR Blow Torch with the same problem. Perhaps this can help future repairs...

Removed Q5 (was shorted, functions as a pull-to-GND when FX is bypassed), the short gave +10V DC voltage on the input line.
Replacing the DPDT foot-switch with a 3DPT foot-switch and wiring it "true bypass" (I could not fix the milenium bypass circuit...).
Removed R33 10k Ohm from PCB, this is the original LED resistor used in the millenium bypass circuit.
Added a 1M Ohm anti-plop resistor between the FX input line and GND, soldered on the new 3PDT foot-switch.

PCB JP3 connections to original DPDT foot-switch:
1 Jack Input -> DPDT-B2
2 FX Input -> DPDT-B3
3 GND -> DPDT PCB
4 Q2 Millenium Bypass control line -> DPDT-A1 
5 FX Output -> DPDT-A2
6 Jack Output -> DPDT-A3 & DPDT-B1

PCB JP3 connections for a new 3PDT fooot-switch:
1 Jack Input -> 3PDT-A1 & 3PDT-B3
2 FX Input -> 3PDT-A2
3 GND -> 3PDT-A3
4 Q2 Millenium Bypass control lijn -> Not Connected
5 FX Output -> 3PDT-B1
6 Jack Output -> 3PDT-B2

LED connections to 3PDT foot-switch:
+18V power -> 3PDT-C1
LED on PCB -> 22K Ohm resistor -> 3PDT-C2

[Rob Strand]

The footswitch circuit on a lot of the later era SMD MXR pedals use P-channel JFETs it's possible one of those has gone bad.
There's few threads on modern MXR pedals on the free stomp boxes forum (eg. MXR - Custom Badass '78 Distortion ).  There is also a schematic for the switching circuit.  The P-channels are marked 6Y K.

[flo]

Thanks for the tip!

I found a MXR "millenium"-style bypass schematic on page 5 of the "MXR - Custom Badass '78 Distortion" topic on the free stomp boxes forum.

[Stevenrb718]

I know this topic hasn't been commented on for going on 2 years, but I have the same issue where the LED doesn't go out... difference being the effect doesn't work either.  Some crackling comes through when hitting the guitar occasionally, but that's it other than the bypassed signal.  Which transistors should I look at to get the effect back?

[PRR]

> doesn't work

Did it ever work? Any voltages in there? Any pictures?

[Stevenrb718]

TP voltages (in parentheses are when its a different voltage after pressing switch):
TP1 - 0.65V
TP2 - 0.65V
TP3 - 0.65V
TP4 - 0.8V (42mV)
TP5 - 3.29V (3.5V)
TP6 - 2V (0.8V)
TP7 - 0.8V (0.7V)
TP8 - 4.6mV (1mV)
TP9 - 11mV
TP10 - 8.1mV (2mV)
TP11 - 400mV & falling

Since I don't have a schematic, I don't know pinouts and signal paths, but I'm getting 1.28V at the single leg top side of Q6, and audio signal at the left leg along with 0.7V.  The right leg is producing a very distorted, muffled audio signal. 

[Stevenrb718]

No schematic?

[PRR]

> No schematic?

Reply #1, link hidden in the middle of the text.

Unless you have a different one?

[Stevenrb718]

> No schematic?

Reply #1, link hidden in the middle of the text.

Unless you have a different one?

That's a Boss OD-1 schematic

[Stevenrb718]

Still hoping for that schematic lol