Dunlop phase 90 eating zener

[Randall]

Customer says he plugged this Dunlop MXR Phase 90 into the wrong power adapter and now it doesn't work. I found a bad 5.1v zener diode, so I thought I would replace it. I did, and plugged it into a battery and it works, although it wasn't very rich or deep sounding. Then I noticed it was pulling a new battery down pretty fast. I tried it into a wall wart ser to 9v tip neg, and it made a loud hum and pretty quickly killed the new zener. I replaecd it again, and when plugged into a battery, the zener gets quite hot, hell I could even smell it.

Now how can this zener be pulling enough current to make it fail? I should think if any of the surrounding components were failing, it would take the load off the zener. I have not found a drawing that matches the one I have here. This one has D2 and C8 fed from the top side of the 10K and D1 from the bottom as drawn. The trimmer resistor is on the other side of the board, and I have not been able to trace the path, but there is not direct continuity to the supply side of it, and there is a 7K resistor between the ground side and actual ground.

I am not wanting to spend a whole lot of time on this, but I wonder why that zener is burning out?

https://www.electrosmash.com/images/...atic-parts.png

[Helmholtz]

This one has D2 and C8 fed from the top side of the 10K and D1 from the bottom as drawn.

Not clear what this means without a drawing. In any case there must be a current limiting resistor (10k) in series with the zener between the battery poles.

[Randall]

"Not clear what this means without a drawing."

Can you not see the one I posted?

[Helmholtz]

"Not clear what this means without a drawing."

Can you not see the one I posted?

Doesn't this mean that your unit differs from the posted schem?>

I have not found a drawing that matches the one I have here. This one has D2 and C8 fed from the top side of the 10K and D1 from the bottom as drawn.

What is top side and bottom? Is it wired: battery plus to 10K to zener to minus as in the schem? Is the 10k O.K.?

[Randall]

Mine is not wired as the drawing. Mine is battery plus to D2 and C8 to ground, and battery plus to 10K to D1 to ground. I hope that is correct, it's a small double sided board. All of these components measure as good.

[Helmholtz]

Mine is not wired as the drawing. Mine is battery plus to D2 and C8 to ground, and battery plus to 10K to D1 to ground. All of these components measure as good.

Does this mean that the zener (D2) is wired directly between battery plus and battery minus? This would mean trouble. If not, please draw and post the actual circuit to avoid further misunderstanding.

[Enzo]

Sounds like battery is connected to wrong end of 10k R15.

[olddawg]

Are you sure your aren’t confusing the crowbar diode with the zener? I think I would shotgun the 15uf cap, the crowbar diode, the series resistor, and the zener. Make sure there is no charcoaled (resistive) board.

[Mark Hammer]

There will be two diodes in the pedal. One is for protection against power-supply boo-boos, and the other (the 5.1V zener) is to provide a predictable DC voltage to use for setting the DC bias on the FETs. Quick power drain suggests that there may be something amiss with the first diode but NOT the second. Alternatively, consider that perhaps one or more of the electrolytic capacitors are bleeding current and need replacing.

[Randall]

Going by the drawing, it shows D1 and D2, with D2 being the zener. Initially I found D2 to be bad, so I put a 5.1v zener in there. They both look the same. D1 tests good in circuit. I am going to rule out something being connected wrong, because after all, it was an untouched production pcb. The only drawing I find is for the script logo version, mine is a later Dunlop block logo, so it appears there have been some changes.

How do I discern which is the zener and which is the crowbar diode?

[Enzo]

The diodes are in parallel and face the same way. D1 is there to shunt wrong polarity power to ground. D2 is a zener to set 5v. Since they are parallel, you can put one of each in one position or other. Note each must have its anode end to ground. So if you have a plain diode and a 5v zener, pick a spot for each, they are parallel, so you can't get them "reversed".

[Jazz P Bass]

Maybe this will help: MXR_Phase90_1993.pdf

[eschertron]

R15 (10k) and D2 (zener) make a voltage divider, to provide an upper limit to the bias voltage. If the battery does not have D15 between the 9vdc and the zener, it cannot work right. Could R15 have been mounted wrong at the factory?

How long did your customer have the unit before it immolated? Years? Hours?

edit: just saw jpb's post. It IS possible to have D1 and D2 swapped. The zener needs to be the one 'behind' the 10k resistor.

[Enzo]

I was working from the first post drawing, the Jazz post one is clearly different, so the plain diode connects to the battery direct, the zener has the 10uf cap in parallel.

[nevetslab]

Customer says he plugged this Dunlop MXR Phase 90 into the wrong power adapter and now it doesn't work. I found a bad 5.1v zener diode, so I thought I would replace it. I did, and plugged it into a battery and it works, although it wasn't very rich or deep sounding. Then I noticed it was pulling a new battery down pretty fast. I tried it into a wall wart ser to 9v tip neg, and it made a loud hum and pretty quickly killed the new zener. I replaecd it again, and when plugged into a battery, the zener gets quite hot, hell I could even smell it.

Now how can this zener be pulling enough current to make it fail? I should think if any of the surrounding components were failing, it would take the load off the zener. I have not found a drawing that matches the one I have here. This one has D2 and C8 fed from the top side of the 10K and D1 from the bottom as drawn. The trimmer resistor is on the other side of the board, and I have not been able to trace the path, but there is not direct continuity to the supply side of it, and there is a 7K resistor between the ground side and actual ground.

I am not wanting to spend a whole lot of time on this, but I wonder why that zener is burning out?

https://www.electrosmash.com/images/...atic-parts.png

I had to come back to this original post, as it sure seems the 10k resistor that should be between the battery and the zener is NOT 10k, but a very low value, in order to make the zener get THAT hot, and to fail so fast when plugging in the wall wart. Also, following the suggestions from olddawg and Mark, replace that 15uF cap across the 5.1V zener, just to be sure it too isn't draining the battery if it's bad. Are any of the IC's getting hot? What ARE the IC's? your schematic provided shows TL061's, Jazz P-Bass' scheamtic shows TL062's, and I think the original had 741's.