How is my amp still alive after this?

[lightningfingers]

It was working fine for 3 months, then afew weeks ago it started making these incredibly loud popping sounds,, well it turns out that some F :evil: CKHEAD wired the plug wrong, such that the live and neutral wires were reversed. I have corrected the problem but i'm reluctant to use it now...how could it survive this and work perfectly? Its a Marshall Valvestate, I believe it has a solidstate poweramp....

[BD13UK]

Hi,
I think that you should be ok with this now, although I believe you could have been in serious trouble had you been gigging and using a mic and stand through a PA system where you could quite easily have been electrocuted, having been there I KNOW !!!!!!!! I suggest that you get the amp safety checked by a qualified amp tech just to be sure and more importantly safe !!!!!

[SaBer]

I think It doesn't matter which way they go (live and neutral). Some reasons:

1. Here in Finland plugs can be plugged into the wall either way.
2. Both ends of a power-transformer are symmetrical
3. AC moves electrons back and forth. No difference in direction there.

If you have multi-phase AC, it makes a difference, but with only one live wire It makes no difference, as long as earth is wired right.

Am I wrong?

[lightningfingers]

Here in England I think it matters which way they are orientated because the amp works fine now. Even the chorus still works......wierd.
Thanks guys

[Paul Perry (Frostwave)]

If switching the live and neutral makes a difference, then there is something seriously wrong somewhere.
But if your "tech" switched neutral and GROUND, then that would easily give the pops etc.. and yes, you (and the amp!) are lucky to be alive, because witth the ground & neutral swapped, there is no voltage on the case.. until you plug into a supply that has active & neutral reversed, and then you are dead, quite likely. :shock:

There used to be double adaptors in Australia that swapped neutral and active!!! to say nothing of home-made extension cords.

[niftydog]

swapping active and neutral probably won't make it more dangerous, but it may compromise the protection circuitry (ie; fuses etc) causing major damage should a fault occur.

Mesa Boogie amps sold in Australia almost exclusively had active and neutral swapped around for some reason. It seems to have been rectified in the last few years.

Best solution is to fuse BOTH active and neutral, then it doesn't matter how it's hooked up. After all, you can't trust every venue you plug into!

BTW, it's unlikely that simply reversing the active and neutral wires could cause the problems you have described. There must have been something else going on, like bad connections or something.

[Mike Burgundy]

As stated, it really doesn't matter if live and neutral are reversed.
In most of Europe, plugs are symmetrical, and can be plugged in either way. There is no added security for products here, and none needed.
There even are still some places here where they have (and this IS a VERY bad idea) not 230V, neutral and ground, but 110 , 110 opposite phase, and ground. This IS dangerous. Old system.
How you wire live/neutral is only important for wall switches and can-I-touch-this-wire-safety, and where you put your mains fuses. Not for an amps function, not if it's well designed. Not even for the amps fuse, assuming you cannot touch the AC wires between the power switch and the supply. It's AC, after all. The transformer doesn't care wether live goes positve, or "neutral" goes negative, as long as they do something with respect to each other.
Neither should *in any way* be connected to bits of the amp aside from the powersupply, which is isolated by the transformer.
hih

[Eb7+9]

... fixed a few of these, two things to look at :

(i) check that the main filter cap isn't buldged out or is conspicuous looking ... if in doubt replace it with high-Temp equivalent (85degC at least) ... the amp will make loud popping sounds if that cap starts shorting out intermittently ...

I strongly recommend you DO NOT use the compact size surface mount electrolytics like Marshall, Traynor and others are using in some of their amps - IMO they are unreliable in output stages and should be upgraded without question ... mount the new cap on the underside ...

(ii) the main power resistor tends to run hot and can burn the board underneath it ... if so, there is a possibility that the resistor is cracking on the inside ... replace it with a higher power tolerance (same Ohm-age) and install it a little further away from the board ... look on the underside of the board to make sure no traces have been lifted or baked from the heat ...

... running the power primary backwards won't produce the symptoms you describe ... for your safety at least make sure all your gear is grounded, including the PA (!)

... jc

[lightningfingers]

So the general consensus is...there is something else that needs attention?
As stated, the amp worked before, but then It started to have problems, and correcting the wiring seems to have rectified it.
Maybe I should look for a voltage on the chassis? That said, adjusting knobs (which are made of metal and connected to the chassis) while holding the guitar yielded no pain/death/electrocution.
I will swap out the mail filter cap and resistor and see if that rectifies the problem.

[niftydog]

If all that was done was to reverse the polarity of the incoming AC, then I would suspect that you had some dodgy connections to begin with... like the screws were loose or the soldering was dodgy.

[dave h.]

i once had a friend who played bass and sang. he also had braces. one day he got shocked, knocked on his ass, and couldn't talk right for a good 10 minutes, and his hair (although short) stood on end. he said his mouth touched the mic and while his fingers touched the strings and he just got shocked.

what is this, and how does one avoid it?

[R.G.]

...he said his mouth touched the mic and while his fingers touched the strings and he just got shocked.

what is this, and how does one avoid it?

It's AC leakage from one piece of equipment to another, and one avoids it by only using three-wire, safety grounded equipment.

PA equipment is usually safety grounded, so the signal ground (all exposed pieces of metal) is hard-wired to a ground rod or cold water pipes in the ground. Older guitar amps are often not safety grounded, using only a two-wire plug. If these develop a little AC leakage, like through the so-called "line reverse" switch capacitor or a little leakage in the decades-old power transformer or a crack in the insulation of the fractional-century old AC power wiring, the AC leaks to the chassis, and to signal ground, which is connected through the guitar cord to the guitar's strings.

With that setup, you put fingers on strings and mouth - braces or not - on a microphone, and you'll have a high-voltage 60Hz religious experience, if not a sudden-death experience.

I tell you three times: mod your amps to three-wire safety ground power lines.

You can help this some by rewiring your guitar. There is a wire in there that grounds the bridge. Break that and sub in a 330K resistor paralleled with a 0.01uF **AC power line safety rated** capacitor. The capacitor "grounds" the strings at RF, but is effectively open to 60Hz power line. This will help you some if you plug into someone else's dangerous two-wire amp.

[Johnny Guitar]

i once had a friend who played bass and sang. he also had braces. one day he got shocked, knocked on his ass, and couldn't talk right for a good 10 minutes, and his hair (although short) stood on end. he said his mouth touched the mic and while his fingers touched the strings and he just got shocked.

what is this, and how does one avoid it?

A simple test before you sing into a mic while holding a guitar should be made after all the gear is set up, plugged in and powered up (though the amps can be in stand by).  

What you do is to hold the tunning machines of the plugged in guitar up to the mic. As the machines get about 1/4" to 1/8" from the mic you will see arcing if there is a grounding issue (of course you can also use the plug of the guitar cord without the guitar). If you see arcing you need to switch the ground switch, then check it again.

I always put my finger between the plug and mic to make very sure there isn't a voltage between the two. If you've ever felt a line level shock go through your heart, and lived to tell about it (like bare feet and bare hands), you know the difference between that and getting shocked on your finger.

Sweat and saliva both make conduction a lot easier too.

[R.G.]

Poor grounding on stage give you a whole new understanding of "Kentucky Fried"

[gez]

With that setup, you put fingers on strings and mouth - braces or not - on a microphone, and you'll have a high-voltage 60Hz religious experience, if not a sudden-death experience.

I tell you three times: mod your amps to three-wire safety ground power lines.

When I was a lad my brother and I had an American amp with only a two prong plug on it.  No sudden-death experiences, but we treated mike stands with respect!!

I wish I'd followed my friends advice and stuck a 3 pin plug on the end of the lead, like he did!  

All character building stuff (if you survive)... :oops: