Fried voltage regulator?

[Les Paul Lover]

I built a box of hall reverb a while back - very nice, and lent it to a friend for him to try out for a gig.

He brought it back not working. The PSU I lent him was dodgy and the wire cut off at the base of the PSU.
After investigation, all mechanical contact are good, so investigated the components and I quickly found the V regulator LM78L05 is getting a healthy 9.2V on leg 1, with nothing coming out of leg 3. And so the Belton brick doesn't get any power.....

Now what could cause the LM78L05 to die?
I thought they were hardy things. Should I expect other components to be affected?

I know the PSU can possibly give out temporarily more than its stated 9v when it is already plugged and you connect it to a pedal, but I thought that shouldn't be enough to fry the regulator.
Would the dodgy PSU i lent him give out lots more than stated when the wires cut off?

I'm just very intrigued.......

Here's a link to the schem:

http://pedalparts.co.uk/docs/Verb.pdf

Any insight greatly appreciated.

[Rixen]

If the wiring of the power supply was dodgy, could it have shorted causing C3 to discharge back through the regulator (something they don't like) ?

Julian
www.rixenpedals.com

[blackieNYC]

Here's the thing about power supply failures: did a power supply component die? Or is the circuit pulling it down?  PS or circuit - one can be fried and the other can be fine. Easier to start with the 5 volt regulator but, after you pull out the first one, measure DC resistance at the point where it's output pin was, to ground. Before putting the new regulator in. I don't know resistance it should be, but if it is under, I don't know -200 ohms? then something I. The circuit has failed.
Or start with the circuit - do you get your 5 v if the brick is removed?
Or the cap on the 5volt line- with the thing off, check the cap.

[Les Paul Lover]

If the wiring of the power supply was dodgy, could it have shorted causing C3 to discharge back through the regulator (something they don't like) ?

Julian
www.rixenpedals.com

Thanks a lot Julian, that's as clear an explanation as I was hoping for.

Yes with both positive and neutral wires frayed, it will most definitely have shorted, I don't see it happening any other way really.

Here's the thing about power supply failures: did a power supply component die? Or is the circuit pulling it down?  PS or circuit - one can be fried and the other can be fine. Easier to start with the 5 volt regulator but, after you pull out the first one, measure DC resistance at the point where it's output pin was, to ground. Before putting the new regulator in. I don't know resistance it should be, but if it is under, I don't know -200 ohms? then something I. The circuit has failed.
Or start with the circuit - do you get your 5 v if the brick is removed?
Or the cap on the 5volt line- with the thing off, check the cap.

I guess that will be the next step! I have more lm78l05 on the way, I just hope the brick is unaffected, that's the expensive part!!!!

[Phoenix]

I just had a look at the schematic/board layout, and there's no reverse polarity protection at all, which seems like a massive oversight for a kit.
This circuit doesn't need the headroom, so I'd highly recommend a series diode for better protection than the parallel reverse biased diode trick. Don't even need to worry about it being a schottky, any old 1N400x will do.

[samhay]

If the voltage regulator was fried by capacitor discharge back through the regulator, then polarity protection won't help (but is still an excellent idea.).  A small signal diode (I use a 1N4148) from regulator output to input (cathode to input) should prevent this from being fatal as it clamps the outut voltage to < 1 diode drop above input.

I have had voltage regulator fail as a short from input to output. This invariably fries whatever is fed from the regulator.
It sounds like this one failed as an open circuit, so with any luck the brick will still be fine.

[Phoenix]

If the voltage regulator was fried by capacitor discharge back through the regulator, then polarity protection won't help (but is still an excellent idea.).  A small signal diode (I use a 1N4148) from regulator output to input (cathode to input) should prevent this from being fatal as it clamps the outut voltage to < 1 diode drop above input.

That's important if the output of the regulator can be connected to something external that might reverse bias it - like in a bench power supply or wall wart, but in this case there's no way for the regulator to be reverse biased unless an exceedingly unlikely series of events takes place: both the op amp and Belton brick would have to short to Vcc for that to happen.
The most plausible cause of the dead regulator is reverse polarity power being applied, which most likely fried the TL074 too. Reverse polarity will often cause a 3 pin regulator to go short circuit, but it appears the OP has been lucky and that hasn't been the case here, so the Belton brick is probably undamaged.

EDIT:
Ah, I just realised that what was discussed was an intermittent short on the power supply, which could cause the regulator to become reverse biased. The recommendation for a reverse biased diode across the regulator is a good one which would prevent damage from occurring in that circumstance.

[antonis]

...Valid only in case you have never verified well working pedal... 

Many batches of 78Lxx regulators have reversed IN-OUT pins..

[samhay]

>...unlikely series of events takes place.
The cap on the output of the regulator charges up to 5V. The regulator input gets grounded. The output is now (temporarily) at 5V while the input is at 0V. Bang.
In fairness, you usually have a large cap on the input side of the regulator that will discharge more slowly than the output cap, so you get away with it.

>Many batches of 78Lxx regulators have reversed IN-OUT pins..
On dear. I almost always use a 7805 in preference to the little brother option, so haven't come across that little surprise.

[Phoenix]

>...unlikely series of events takes place.
The cap on the output of the regulator charges up to 5V. The regulator input gets grounded. The output is now (temporarily) at 5V while the input is at 0V. Bang.

Yep, I realized that's what you were talking about after I posted, and added this edit:

EDIT:
Ah, I just realised that what was discussed was an intermittent short on the power supply, which could cause the regulator to become reverse biased. The recommendation for a reverse biased diode across the regulator is a good one which would prevent damage from occurring in that circumstance.

[duck_arse]

one other method to kill a regulator - your supplied psu goes on the fritz while you are out of sight, friend picks up nearest psu, which happens to fit, also happens to be AC output.

[Les Paul Lover]

Thank you all for the replies.

I received a batch of new 78l05 last night, so I'll try to replace the existing one and then I'll find out if something else is fried......

I hope not.

I noticed now that I'm more experienced that this lay out didn't have any protection diode. Actually, on the next revision of this circuit, pedal part does include a diode 1n4001.

I've got quite a few spare - where exactly should I try to fit it?

With my circuit being on a pcb, I'm not sure I could fit it without having to cut out a trace (is that correct? Is it recommendable?).

Thanks again for all the advice and explanations!!!!

[antonis]

I noticed now that I'm more experienced that this lay out didn't have any protection diode. Actually, on the next revision of this circuit, pedal part does include a diode 1n4001.
I've got quite a few spare - where exactly should I try to fit it?

Anywhere between the IN & OUT of the voltage regulator (i.e. +9V input & +5V filter capacitor(s)..)

[Les Paul Lover]

Thank you all for your answers so far.

I've received my batch of 78l05 and replaced the faulty one - and now get a healthy 4.97 V on leg 3. Woohoo!!!!


Oh, I meant boohoo.... Cos i still can't hear the reverb......
So I've prodded around with my trusty multimeter.......

- Bypass is fine
- with the effect engaged, I clearly get power through (led lighting up, 9v on leg 1 of new 78l05, 5v on leg 3 of 78l05 and pin 1 of belton brick.... But no sound.

I think I get 4.75v dc on pin 3 of tl074, 9c on pin 4, 0v on pin 10, and 9 pretty much 0.55v dc everywhere else on the tl074.

How can I tell if it's fried too? I'm not sure what the input/output voltages should be on that one?

I'm pretty sure the belton brick is fine (though I have been wrong before!!!!!!) as when testing for current, it made a clearly reverberated scratch noise on pin 3. So It seems to be reverberating fine..... But The guitar signal doesn't go through.


My main questions are:

- how can I test the TL074?
- I haven't quite understood how to fit a 1N4001 - antonis, I read you quite clearly, but still not sure exactly where, and where the cathode should be?
- Is there something else I'm missing out? Is the belton brick really OK as I think it is?

[bluebunny]

- I haven't quite understood how to fit a 1N4001 - antonis, I read you quite clearly, but still not sure exactly where, and where the cathode should be?

Something like this:

[Ice-9]

- I haven't quite understood how to fit a 1N4001 - antonis, I read you quite clearly, but still not sure exactly where, and where the cathode should be?

Something like this:

As Samhay said, fit a regulator protection diode. just like  bluebunny has shown in the picture.

[Les Paul Lover]

I like pictures!!!!!!

Thank you so much, that's very clear now!!!!!

Now, how can I tell if a TL074 is fried or not?

I've tried a search for assumed normal reading I should see there, but couldn't find anything of use / i could comprehend!!!!

Sent from my D6603 using Tapatalk

[samhay]

All the pins should be at about half supply (4.5V) except the 2 supply pins, which should be at 9V (pin 4) and 0V (pin 11).
However, if the pins read ok, that is no guarantee that op-amp is ok. What's more, if it's not ok, it suggests that something a little more catastrophic has happened beyond shorting the supply to ground.

[Les Paul Lover]

So if I understand correctly, the way that Diode is placed, it doesn't allow voltage to go from leg 1 to leg 3, but if a capacitor discharge were to happen, like it had on my reverb, it would allow the current to go through as it would show less resistance than the regulator, therefore protecting the regulator?

Is that how it would work?

[Les Paul Lover]

All the pins should be at about half supply (4.5V) except the 2 supply pins, which should be at 9V (pin 4) and 0V (pin 11).
However, if the pins read ok, that is no guarantee that op-amp is ok. What's more, if it's not ok, it suggests that something a little more catastrophic has happened beyond shorting the supply to ground.

So all pins, from 1 to 14 (except pin 4 & 11 as described above) should show 4.5v ish?

I'll recheck those tonight - I'm pretty sure only pin 3 showed 4.73 (which is roughly 1/2 my 1 spot supply), and the rest around 0.5v.
But I'll recheck them now that i know what I should be looking for.