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Pedal not working after connecting to AC PSU.

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Pedal not working after connecting to AC PSU.

Postby Arpangel » Sun Sep 20, 2020 8:55 am

Yep, I just tested it after I got back, completely kupput.
I really don’t know what I was doing, I just grabbed a PSU from a pile that was sorted into 9v DC jobs, it just got mixed up with the others, but I should have checked anyway.
It’s a T Rex Replay, and I’m just wondering what to look for if I poke around inside, thank good it wasn’t an expensive pedal.

:(
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Re: Pedal not working after connecting to AC PSU.

Postby Folderol » Sun Sep 20, 2020 9:38 am

If you're lucky it'll be a S/C smoothing and/or an O/C series resistor.
If it's an 'old fashioned' one with discrete transistors much of the stuff is likely to be undamaged, if chips, not so likely.
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Re: Pedal not working after connecting to AC PSU.

Postby CS70 » Sun Sep 20, 2020 12:10 pm

Arpangel wrote:Yep, I just tested it after I got back, completely kupput.
I really don’t know what I was doing, I just grabbed a PSU from a pile that was sorted into 9v DC jobs, it just got mixed up with the others, but I should have checked anyway.
It’s a T Rex Replay, and I’m just wondering what to look for if I poke around inside, thank good it wasn’t an expensive pedal.

:(

If it's any consolation, we've all been there. I, at least :D
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Re: Pedal not working after connecting to AC PSU.

Postby ef37a » Sun Sep 20, 2020 3:35 pm

Folderol wrote:If you're lucky it'll be a S/C smoothing and/or an O/C series resistor.
If it's an 'old fashioned' one with discrete transistors much of the stuff is likely to be undamaged, if chips, not so likely.

A lot of pedals used a unique and totally stupid form of polarity protection consisting of a diode (of not massive rating) ACROSS the DC rail. If you are lucky there might be a series 'fusible' resistor before it. The diode will have exploded and the resistor body turned to dust.

Quite possibly, all else will be fine. Fit a 1 Ohm fuzzy R and forget about the diode.

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Re: Pedal not working after connecting to AC PSU.

Postby Sam Spoons » Sun Sep 20, 2020 8:56 pm

I don't understand why they don't build in robust protection to pedals, it can't be that expensive or difficult can it?
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Re: Pedal not working after connecting to AC PSU.

Postby James Perrett » Sun Sep 20, 2020 9:50 pm

Sam Spoons wrote:I don't understand why they don't build in robust protection to pedals, it can't be that expensive or difficult can it?

A standard diode will reduce the voltage available by 0.6-0.7V which could shorten the time you can use a battery if the circuit requires a certain voltage. Even a more expensive Schottky diode will still rob you of 0.3-0.4V. Pedals are often working close to the lower supply voltage limit of a typical audio op amp chip so anything that reduces the supply voltage is bad news.
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Re: Pedal not working after connecting to AC PSU.

Postby Folderol » Sun Sep 20, 2020 10:31 pm

James Perrett wrote:
Sam Spoons wrote:I don't understand why they don't build in robust protection to pedals, it can't be that expensive or difficult can it?

A standard diode will reduce the voltage available by 0.6-0.7V which could shorten the time you can use a battery if the circuit requires a certain voltage. Even a more expensive Schottky diode will still rob you of 0.3-0.4V. Pedals are often working close to the lower supply voltage limit of a typical audio op amp chip so anything that reduces the supply voltage is bad news.
These days I think the best answer is a series polyfuse and a shunt diode. The poly fuse has a very low resistance, but temporarily goes open with reverse voltage due to the diode - the combination protecting everything else. Stick a cap on as well and you get a bit of filtering too.
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Re: Pedal not working after connecting to AC PSU.

Postby ef37a » Sun Sep 20, 2020 11:24 pm

I think it is impossible (must investigate that!) to put a series diode in the switched power line because it would also be in the battery circuit.

However, most guitar pedals are, for some unknown reason, centre negative so there is no problem putting a diode in the 'barrel' circuit?

But it all comes down to cost. I built a PA system for a rifle range that had to be powered from a car battery. There was EVERY chance someone would croc it the wrong polarity so I used a meaty relay and a diode to only pull in for the correct connection. A small 1A bridge rect' always put current the right way through a flashing red LED to warn "Get the leads the right way round yer Pelican!"
But such idiot proofing would cost more than the pedal electronics in most cases!

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Re: Pedal not working after connecting to AC PSU.

Postby Exalted Wombat » Sun Sep 20, 2020 11:38 pm

The Sony Walkman Professional, a high-end and expensive cassette recorder, could be completely trashed by connecting a 6v power supply with incorrect polarity. A protective component would have cost pennies.
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Re: Pedal not working after connecting to AC PSU.

Postby James Perrett » Mon Sep 21, 2020 12:38 am

Folderol wrote:These days I think the best answer is a series polyfuse and a shunt diode.

It is worth checking that the circuit doesn't mind the resistance of the polyfuse - especially after it has done its job a few times. You could easily end up with what started as a few tens of millivolts drop across it turning into something more appreciable after it has gone high resistance once or twice.
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Re: Pedal not working after connecting to AC PSU.

Postby ef37a » Mon Sep 21, 2020 7:22 am

Exalted Wombat wrote:The Sony Walkman Professional, a high-end and expensive cassette recorder, could be completely trashed by connecting a 6v power supply with incorrect polarity. A protective component would have cost pennies.

Yes but then Sony and the dealer would lose out because they could not sell you another one!

But seriously, yes, I agree gear should be made as bombproof as possible but so long as Joe Pub' is always chasing down the last groat stuff will be built down to a price.

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Re: Pedal not working after connecting to AC PSU.

Postby Arpangel » Mon Sep 21, 2020 7:35 am

ef37a wrote:Yes but then Sony and the dealer would lose out because they could not sell you another one!
Dave.

No, I wouldn’t buy something that had that built in to its design philosophy, by a company that could well afford to do the job properly.
Build from the ground up, not down to a price, but that practice isn’t wide spread, if you want that sort of stuff you still have to pay big money.
I’ll have a look at this pedal today, thanks for the tips.
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Re: Pedal not working after connecting to AC PSU.

Postby ef37a » Mon Sep 21, 2020 8:27 am

Arpangel wrote:
ef37a wrote:Yes but then Sony and the dealer would lose out because they could not sell you another one!
Dave.

No, I wouldn’t buy something that had that built in to its design philosophy, by a company that could well afford to do the job properly.
Build from the ground up, not down to a price, but that practice isn’t wide spread, if you want that sort of stuff you still have to pay big money.
I’ll have a look at this pedal today, thanks for the tips.

No, YOU wouldn't but many companies don't see things that way. I know from experience that building in a bit of protection (from the collateral damage a blown power vlave can cause e.g.) puts firms at an economic disadvantage with their competitors.

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Re: Pedal not working after connecting to AC PSU.

Postby Arpangel » Mon Sep 21, 2020 9:12 am

ef37a wrote:
Arpangel wrote:
ef37a wrote:Yes but then Sony and the dealer would lose out because they could not sell you another one!
Dave.

No, I wouldn’t buy something that had that built in to its design philosophy, by a company that could well afford to do the job properly.
Build from the ground up, not down to a price, but that practice isn’t wide spread, if you want that sort of stuff you still have to pay big money.
I’ll have a look at this pedal today, thanks for the tips.

No, YOU wouldn't but many companies don't see things that way. I know from experience that building in a bit of protection (from the collateral damage a blown power vlave can cause e.g.) puts firms at an economic disadvantage with their competitors.

Dave.

Power valves! what! valves??!!!! once you start talking about those things it’s not the firms that are at an economic disadvantage it’s me!
"There is no such thing as a good cheap valve amplifier sir, I really can’t continue with this unless your budget is at least £20,000.......sir"
I had that exact conversation at a well known London hi-fi shop.
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Re: Pedal not working after connecting to AC PSU.

Postby Folderol » Mon Sep 21, 2020 9:47 am

Hmmm, less than 1k in costs - that's quite some profit margin :thumbdown:
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