Muff Pedal Distortion Repair

[amonarch71]

I am fixing a Muff Pedal and I accidently blew the red LED out while testing it with a 9 volt battery directly to the LED.
I have a schematic of the circuit, but I do not have a schematic that shows the LED in the schematic for the
Muff Pedal.

I have tested LEDs quite often with a 9 volt battery, and I have never blown one up... large pop noise, smell but no smoke.
I was wondering if the diode was damaged or maybe there is another part of the circuit that was damaged to cause this
malfunction?

Thanks

 

[Harald Kapp]

I have tested LEDs quite often with a 9 volt battery, and I have never blown one up

You've been just lucky then. LEDs need current limiting.

 

[AnalogKid]

During normal operation, is the LED on continuously, does it flicker with the music content, etc?

ak

 

[amonarch71]

You've been just lucky then. LEDs need current limiting.

What value should I use in series with a 9 volt battery?

 

[amonarch71]

During normal operation, is the LED on continuously, does it flicker with the music content, etc?

ak

I have not seen it in normal operation. It was given to me from a friend who didn't want to fix it.±

 

[Alec_t]

If the LED is intended simply to confirm the pedal is active, then to conserve battery power you could connect between the power rails a high-efficiency LED in series with, say, a 2.2k resistor. That would give a LED current of about 3mA.

 

[kellys_eye]

I accidently blew the red LED out while testing it with a 9 volt battery

Why were you testing the LED? They are usually very reliable. If the circuit wan't working properly it is unlikely to be the LED that caused the fault.....

 

[AnalogKid]

Another thought is to put the LED in series between the battery and the circuit. The opamp static current limits the LED current.

ak

 

[WHONOES]

There isn't an opamp in the circuit diagram!

 

[AnalogKid]

ok, oops (but there is an opamp version of this product). The static current of the circuit is the LED current.

ak

 

[WHONOES]

I think the only opamps available in 67 was the μA702 (from 1962) and the much improved μA709 (from 1965) but I could be wrong.

 

[73's de Edd]

Sir amonarch71 . . . . .


That LED must have been an add on . . .probably for power indicator . . . as it certainly isn't being shown on the shematic.
Can you track down what two points it is / was wired into on the board ?

As far as the basic circuity design proper, it almost looks bulletproof.
No high current / power consumption . . . . .Fairchild silicon planar transistors . . . .no electrolytics . . . .using carbon composition resistors . . . .so . . . .might check the higher value 390K-100K-470K units to see that they haven't gone astray in value.
Also check the pots, in case certain ones had normally got a LOTS of use.

73's de Edd
.....

 

[Cannonball]

Install a 1K resistor in series with the LED and the on/off switch and ground.

 

[AnalogKid]

There is no on/off switch. Plugging in the guitar completes the battery connection to GND through the input jack. This is very common in instrument pedals. When you plug a mono 1/4" plug into a stereo jack, the ring contact is connected to GND through the plug sleeve.

ak

 

[Cannonball]

Connect the resistor the + side of the battery, the other side of the resistor to the anode of the LED and the cathode of the LED to the switched ground the that that is made when the guitar is plugged in.