Bad Diode?

[Drewmeyer]

So I just bought an MXR distortion+ on Ebay to fix up. It arrived and I plugged it in and the LED turns on, but no signal. I look inside and I noticed that one of the diodes doesn't look like the rest. It's subtle, just slightly discolored inside if you know what I mean. I'm wondering if the diode could be bad? Would a bad diode cause the pedal to not pass signal?

[jimilee]

Yes, what diode is it? I guess I'm curious what made the diode go bad, for example is it a protection diode?

[Drewmeyer]

Yes, what diode is it? I guess I'm curious what made the diode go bad, for example is it a protection diode?

It's one of a pair next to each other facing different directions. That would make it a clipping diode, right?

[Quackzed]

yep. if its dead (diode) and shorting to ground, it'll kill the output(no sound). though its odd for that diode to die...   without that diode, the circuit will pass sound so if you can de solder it, or just one side of it, you can check.
if you have a multimeter, you can just measure the resistance on either side of the suspect diode, if its zero or practically zero, then replace it.

[Drewmeyer]

yep. if its dead (diode) and shorting to ground, it'll kill the output(no sound). though its odd for that diode to die...   without that diode, the circuit will pass sound so if you can de solder it, or just one side of it, you can check.
if you have a multimeter, you can just measure the resistance on either side of the suspect diode, if its zero or practically zero, then replace it.

Oh great, thanks! I'll try that out and see what's up. Also, interesting to note: It uses a Carling DPDT footswitch, which I have never seen in these old MXR pedals. Did MXR ever use DPDT footswitches?
EDIT: Ignore the bit about the footswitch, been looking at pictures of the wrong block logos.

[Drewmeyer]

Ah, well I took out the diode and still no signal passed. And to add to that, now when I have the pedal in bypass, I get hum and popping which gets more/less severe as I move the board around, but doesn't ever go away totally. I put the diode back in and the noises continue. Any help here?
EDIT: Also, just noticed there is significant structural damage to the diodes. Very significant. They actually started cracking and bits broke off and the leads on both came right out. It definitely needs new diodes. But that still doesn't seem to be the only problem since it still doesn't pass signal and with the new noise.

[MrStab]

ideally you'll wanna audio probe it and check voltages, as this could be just about anything, but the Dist+ isn't too complicated a circuit, which probably lowers the possibilities. how old is it? a cap(s) might've gone bad. the hum could be because ground might've become disconnected somewhere. the board moving around thing could be caused by it shorting against the case or a loose wire. less-likely are opamp dying or the traces on the back of the board having been damaged along with those diodes. resistors are probably fine. just throwing ideas at you here.

[duck_arse]

can we see pictures? how did those diodes get bust? also, what mrstab said.

[Drewmeyer]

can we see pictures? how did those diodes get bust? also, what mrstab said.

Yeah, I'll post some pictures soon. About the diodes, I literally just was moving the leads to position them back into the wholes and I heard cracking and they broke open. Also, what are the odds the chip is bad?

[Quackzed]

i'd check all the wiring first, but it could be. could it have had anything spilled on it? residue etc? usually first thing to go is any moving parts... pots, jacks, wires etc... if all that checks ok maybee someone reversed power or wrong powered it, then the chip may be dead...

[Drewmeyer]

i'd check all the wiring first, but it could be. could it have had anything spilled on it? residue etc? usually first thing to go is any moving parts... pots, jacks, wires etc... if all that checks ok maybee someone reversed power or wrong powered it, then the chip may be dead...

There is some residue on the chip, but there was also tons of foam with seemed to have dissolved and gotten over everything so it could just be that. All the wiring looks pretty solid. Is it possible some of the caps could be bad. If they are, would they cause the signal to not pass?

[duck_arse]

I just tried seaching a circuit, but the scripts at googoo means I was just sitting and waiting too long.

the only circuit for this I have shows a 1uF tantalum at the opamps output. that would be the first place to look for trubble, just yank it and replace w/ a film cap, if you can fit, otherwise an ordinary electro will do.

and measure the voltage at pin 3 on the ic, it should be about 1/2 V+, otherwise you get probs.

[Drewmeyer]

I just tried seaching a circuit, but the scripts at googoo means I was just sitting and waiting too long.

the only circuit for this I have shows a 1uF tantalum at the opamps output. that would be the first place to look for trubble, just yank it and replace w/ a film cap, if you can fit, otherwise an ordinary electro will do.

and measure the voltage at pin 3 on the ic, it should be about 1/2 V+, otherwise you get probs.

Cool, I'll try that out. If I were to use an electro, how would I position the positive and negative leads?

[Marlowe]

If it's any help, if that Dist+ is a 70's or 80's, the two diodes are germanium and REALLY easy to damage (don't ask me how I know....). usually, when soldering or unsoldering, one or both ends will come off.  It does have a 1uf tantalum on the 741's output. There is also a 1uf tantalum on the junctions of the 2 -  1M resistors used to provide the virtual ground.

You can replace the diodes with 1N4148/914 silicons, but trust me, the sound will change. The tone becomes less mushy and the pedal sounds more aggressive.

There's a good schematic at Tonepad.com

With the meter ground on the chassis, and a good 9V battery, my voltages read:

Pin3- 4.5VDC
Pin4-O V
Pin6- 4.5VDC
Pin7- 9VDC

Pin 1, 5,8 Unused.

The positive of the cap points positive toward chip output, neg towards resistor and diodes.

Hope this helps!

[Drewmeyer]

If it's any help, if that Dist+ is a 70's or 80's, the two diodes are germanium and REALLY easy to damage (don't ask me how I know....). usually, when soldering or unsoldering, one or both ends will come off.  It does have a 1uf tantalum on the 741's output. There is also a 1uf tantalum on the junctions of the 2 -  1M resistors used to provide the virtual ground.

You can replace the diodes with 1N4148/914 silicons, but trust me, the sound will change. The tone becomes less mushy and the pedal sounds more aggressive.

There's a good schematic at Tonepad.com

With the meter ground on the chassis, and a good 9V battery, my voltages read:

Pin3- 4.5VDC
Pin4-O V
Pin6- 4.5VDC
Pin7- 9VDC

Pin 1, 5,8 Unused.

The positive of the cap points positive toward chip output, neg towards resistor and diodes.

Hope this helps!

Oh awesome, thanks! That's very helpful. I'll have to check those voltages as soon as i get a new fuse for my voltmeter. I replaced the cap, but no luck. Still no signal passed. Would there be a way to sort of jumper the chip to bypass it to see if signal still doesn't pass? Then I'd know it was the chip if signal passed, right?

[Marlowe]

Assuming the chip is getting the input signal, and the voltages look proper, you could lift the negative end of the 1uf output cap from off the board. Connect a jumper wire to it and connect that to the tip of a guitar cable. Connect a jumper from the chassis to the ground of the guitar cable. Plug the cable into an amp after turning the MXR's Distortion control down. Without the diodes connected, you're going to get a large boost in volume. By doing this you bypass your diodes and Level pot. If you don't get a signal playing guitar into it then, it ought to be the chip.

Honestly, if you haven't checked the switch first, I'd try that and make sure it's wired correctly. The Dist+ in the 70's and 80's used a Carling SPDT switch. Mine had a bad switch. No output at all. Is this switch board mounted, or flywired? Yours is either a rework trying to make it true-bypass or a later model like Dunlop's.

Hope this helps, Marlowe.

[Drewmeyer]

Assuming the chip is getting the input signal, and the voltages look proper, you could lift the negative end of the 1uf output cap from off the board. Connect a jumper wire to it and connect that to the tip of a guitar cable. Connect a jumper from the chassis to the ground of the guitar cable. Plug the cable into an amp after turning the MXR's Distortion control down. Without the diodes connected, you're going to get a large boost in volume. By doing this you bypass your diodes and Level pot. If you don't get a signal playing guitar into it then, it ought to be the chip.

Honestly, if you haven't checked the switch first, I'd try that and make sure it's wired correctly. The Dist+ in the 70's and 80's used a Carling SPDT switch. Mine had a bad switch. No output at all. Is this switch board mounted, or flywired? Yours is either a rework trying to make it true-bypass or a later model like Dunlop's.

Hope this helps, Marlowe.

Alright, I'll try that. About the switch, this model has an LED so they actually used a Carling DPDT switch, but it looks like it's basically wired like a SPDT but with the second pole being used for the LED. I actually thought about it being bad and wired the pedal straight through the effect and it still didn't work. Also, the LED comes on so it seems like the switch is working fine. But yeah, I'll do what you suggested and let you know the results.

[Marlowe]

Sorry! I was concentrating on the no output problem and missed the bit about yours having the LED! My mistake. Mine is the no LED model. If you try the lifted cap, be careful about your input levels to the pedal from the guitar as you'll have no level pot. With the diodes off, the pedal becomes much like the Microamp- lots of boost, little distortion!

Sorry again about the error! Marlowe.

[Drewmeyer]

Sorry! I was concentrating on the no output problem and missed the bit about yours having the LED! My mistake. Mine is the no LED model. If you try the lifted cap, be careful about your input levels to the pedal from the guitar as you'll have no level pot. With the diodes off, the pedal becomes much like the Microamp- lots of boost, little distortion!

Sorry again about the error! Marlowe.

Oh no problem, I think it was my fault, I never mentioned the LED. Tried what you said, however, and no good. Still no signal. I'm thinking it's the chip now. I can get a replacement chip pretty easily so I'll just do that and see what happens. Thanks for all the help, it's been very useful!

[Marlowe]

Man, I hate that it was the chip. If you don't mind me suggesting the obvious, you might want to consider putting a socket on the board. Not only will it make replacing the chip easier, but you can always try a TL071 in there for it's tonal difference (it makes the Dist+ sound a bit clearer, somewhat like an overdrive).

Sorry I was no help! Marlowe.