Fried mutronIII

[BillyJ]

A friend toasted his trying to use a power supply on the battery jacks..LOL
Is there any sort of rules of what to look for in a device that has had reverse polarity applied?

[Peter Snowberg]

I'm sorry to hear that.

When reverse polarity is applied, the components that are most likely to die are transistors, chips, and tantalum capacitors. Luckily the Mutron III is a fairly simple circuit. Since there are no transistors in there my guess is that the 4558s may have been toasted. I don't know if Musitronics used tantalum caps.... I would doubt it. If there are any, they should be suspect.

I would power it back up and if it doesn't work right away, feel the chips for any heating. Any chips that warm up should of course be replaced. I would use something like an NE5532 but I don't know if that would change operation in any way other than lowering noise and providing better output drive.

Best of luck with it,
-Peter

[BillyJ]

Thank you Peter.
Yeah he called about in tears  :cry:
Thanks for the info. I am sure it will be very useful.
Should the elctros be suspect or are they less suspect for some reason?
Thanks again.

[Peter Snowberg]

Aluminum electrolytics don't mind reverse bias very much. It takes a long exposure to cause damage. Dry tantalum electrolytics (the ones we see all the time) are a different story unfortunately.

My guess is $3 in new opamps and the pedal will be as good as (or even better than) new.

To protect against polarity reversal damage in the future, consider installing schottky barrier diodes in the supply connections. They don't eat much voltage and they keep the other parts safe.

Some circuits contain a reverse diode across the power input to cause an immediate short if you hook up the power in reverse. If you have a pedal like this, then reverse power can cook the diode. The Mutron does not have these, but it's something to watch for elsewhere.

Take care,
-Peter

[Mike Burgundy]

I've built MIII's with TL072's, without noticing anything different in the pedal's response. There might be a slight difference in filter response due to different impedances though.
My guess is 5532's will work equally fine.

[Eb7+9]

... I'd also check the easy-to-blow photocell diode, and replace the opamps anyway ... the Mutron-III can receive a lowered noise-floor updgrade by replacing signal path 1458 op-amps by low-noise NE5532 or the like.

... jcm~