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DIY Monitor Repair Help

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DIY Monitor Repair Help

Postby OneTrakAudio » Mon May 24, 2010 6:43 am

Hi

i have a pair of old tapco s8's taking up space. One of the boxes has started blowing fuses. The same thing happenned to a friend with the same speaker and he got it repaired at a rather grandiose price. He said "They replaced the Power Supply"

So, here is a picture of the inside panel. If i know what i'm looking for, i'm 100% i can replace it. But the rub is i need to know what to look for! Heres where you guys come in

As you can see, the power is broken over three bits... IEC socket, Power Switch and 220/110 switch.

The IEC socket looks fine, two wires [no ground], solder, some metal and plastic. What can possibly go wrong there?

What is the consensus on what might be causing the fuses in the IEC socket to short?

Thanks!!

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Re: DIY Monitor Repair Help

Postby Martin Walker » Mon May 24, 2010 9:57 am

Hi OneTrakAudio!

Dodgy area this - you have active monitor loudspeakers that blow their fuses, and suspect replacing the PSU will solve the problem?

PSU fuses generally blow because the current drawn exceeds the rating of the fuse. You might be lucky in that whatever caused the fuse to blow was a one-off surge, or that the PSU itself is getting flaky, but it could just as easily be an issue with the active speaker circuitry.

I’m afraid there can be no ‘concensus on what might be causing the fuses in the IEC socket to short’ - repairing any electronic unit requires step-by-step testing to determine the cause of the problem. Beefing up the PSU might simply do more damage elsewhere


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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Mon May 24, 2010 12:46 pm

Hi Martin

Thanks for the input... i'll go through it here...


Martin Walker wrote:Dodgy area this - you have active monitor loudspeakers that blow their fuses, and suspect replacing the PSU will solve the problem?

I'm going off the theory that i've heard of this on more than one occasion with this particular brand [and age of brand]... My friend i mentioned plus others. Theres a whole bunch of guys down here that bought tapco's when we were young and stoopid.

Now, each time, the response has been that the people they payed to fix it simply 'replaced the power supply', and all is dandy afterwards.

Thats great if i can figure out which part is actually the PSU!!

I'm willing to give it a try for 10 bucks and an hour of fiddling and soldering etc.

Martin Walker wrote:PSU fuses generally blow because the current drawn exceeds the rating of the fuse. You might be lucky in that whatever caused the fuse to blow was a one-off surge, or that the PSU itself is getting flaky, but it could just as easily be an issue with the active speaker circuitry.

Well, first thing i tried was some replacement fuses of the same rating, all with the same net result. it turns on, and a second later the fuse pops and the lights go out.

I then tried the fuse from the other tapco [yeh yeh i know!] and bam. same thing.

Martin Walker wrote:Beefing up the PSU might simply do more damage elsewhere

Well, not really looking to beef up anything ...

Just replace the broken old for working new.

Now my main problem... just a bit of confusion... when we say PSU, on a unit like this in the piccy, WHAT exact part are we talking about?

I never thought a simple plastic IEC socket would qualify as a PSU? its just a connection.

Thanks a bunch!!
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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Mon May 24, 2010 12:52 pm

BTw

I get what you are saying about the active circuitry... that it might be starting in its senility to draw more than its share...

Then thats out of my league...

But for now, i'm gonna stack the odds and hope its the exact same problem others have faced with this brand..

cheers!
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Re: DIY Monitor Repair Help

Postby jc.hunt » Mon May 24, 2010 3:53 pm

the power supply is the big circular thing at the top with all the copper wire & the metal plate on top. you are mistaking the power iec Socket as being the power supply. from your post i really would suggest getting someone qualified to look at them as electricity can kill if you are not sure what you are doing.
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Re: DIY Monitor Repair Help

Postby *INACTIVE USER* » Mon May 24, 2010 4:03 pm

That "big circular thing" is the transfomer. Very rare to get these in a state where they work for a few seconds before blowing the fuse. I would count on working correctly, not at all or blowing a fuse very fast.

But I'm a bit confused, is this all there is in the monitor of electronics? To me it looks as if at the bottom right there is the audio input, the ic's clamped to the metal look like integrated amps (from the labels at the small connectors left & right top).
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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Mon May 24, 2010 4:59 pm

OneTrakAudio wrote:Now, each time, the response has been that the people they payed to fix it simply 'replaced the power supply', and all is dandy afterwards.... Thats great if i can figure out which part is actually the PSU!!

A faulty PSU is quite possible. However, it is clear that you have a very limited understanding of electronics, and I am obliged to point out that anything involving mains power is potentially lethal. If you don't know what you are doing here, you stand a very good chance of destroying the speaker, and possibly of killing yourself.

The power supply unit (PSU) comprises many parts. The mains inlet IEC socket, the fuse holder, the transformer primary voltage selector, the transformer itself, the AC rectification components, the resevoir capacitors, and the voltage regulation circuitry. Most of which is mounted on that circuit board. To find out which bit -- or bits -- is (are) faulty will require appropriate test equipment and a knowledge of PSU design.

With all due respect, it doesn't sound as if you have either, and on that basis, the best and safest advice is to get the unit serviced by a competent professional.

Well, first thing i tried was some replacement fuses of the same rating, all with the same net result. it turns on, and a second later the fuse pops and the lights go out.

Most fuses are designed to blow only with considerable over current, so there is clearly something seriously wrong with this unit. Find a competent technician to repair this unit for you.

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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Mon May 24, 2010 5:02 pm

Havoc wrote:But I'm a bit confused, is this all there is in the monitor of electronics? To me it looks as if at the bottom right there is the audio input, the ic's clamped to the metal look like integrated amps (from the labels at the small connectors left & right top).

Yep, spot on. Incredibly cheap and not very good. That's why I'm not a fan of budget active speakers. You lose far more than you gain. A set of decent passives with a good quality second-hand external power amp would perform way better.

You have to pay a lot of money to get decent amplifiers...

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Re: DIY Monitor Repair Help

Postby *INACTIVE USER* » Mon May 24, 2010 7:19 pm

Yep, spot on. Incredibly cheap and not very good.


If that's the case, then a first test with only the transformer might be a good idea. You can unplug the transformer by disconnecting the white connector just above the cap (while the mains is off). The reconect the mains, switch on and watch if the fuse blows or not. If not, then the transformer is ok.

Next, I would check the caps. Cheap caps in a closed box that can get hot don't tend to live long. Under no circumstances, power the circuit with the caps removed! remove the caps and measure them with a simple Ohm meter. If anything less than a couple hundred kilo-ohm, replace them. It will probably be lower in the beginning but slowly climb. Observe the polarity! If you replace them, make sure to respect the polarity, getting it wrong will mean a bang and probably a very dead monitor.

Check the rectifiers with a multi-meter set to diode function. But if this makes you scratch your head, ask someone who knows this kind of stuff.

For the next step we'd need more on the circuit I'm afraid. But from the look of the pcb it seems as if there's nothing more (?) Just an unregulated power supply and the power amps.
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Re: DIY Monitor Repair Help

Postby Folderol » Mon May 24, 2010 7:35 pm

Is there a fairly loud thump as you switch on? If so, then something is putting DC on the speaker -most likely either a faulty cap, or slightly less likely one of the amps half-dead. This would certainly account for the slight delay before the fuse pops.

In any case, I'd have some doubt whether it was worth the effort unless you know what you're doing and just want to sort it out for fun - as was said before a schematic would make a lot of difference.

Those amps look cheap-and-nasty (tm). The PSU proper couldn't get much simpler and the voltage probably wanders around like a drunken spider.
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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Tue May 25, 2010 7:22 am

great responses guys! Here goes...


Havoc wrote:Very rare to get these in a state where they work for a few seconds before blowing the fuse. I would count on working correctly, not at all or blowing a fuse very fast.

As in, too fast to notice? or up to 1/2 a sec?

Havoc wrote:But I'm a bit confused, is this all there is in the monitor of electronics? To me it looks as if at the bottom right there is the audio input, the ic's clamped to the metal look like integrated amps (from the labels at the small connectors left & right top).

Yep, thats all there is. And you are 100% on the placement on the boards..

Hugh Robjohns wrote:Yep, spot on. Incredibly cheap and not very good. That's why I'm not a fan of budget active speakers. You lose far more than you gain. A set of decent passives with a good quality second-hand external power amp would perform way better.

You have to pay a lot of money to get decent amplifiers...

Hugh

While this is kinda irrelevant [?] i'll let it pass... Hugh, i thank you firstly for your concern and secondly, your input. While i am obviously not competent now, theres no reason i cannot learn from this and possibly do a great job of fixing it. I'd much prefer this outcome to handing it over to someone and emptying my wallet at the same time. I'm an intelligent person and NOTHING if not pragmatic. I'm aware of the dangers of electricity. I actually grew up building guitar amps, radios, synthesizers etc and i built two electric guitars from scratch. However all that knowledge i have let slip away since i was 16... I'm now thirty... I barely remember 23-28 years of age. I became much more focused on parties and girls etc. It happens.

But, i also digress.

The resale value on these is only twice what i might pay to get it fixed. I'd be nuts not to see if i can hash this out myself. Especially as i would like to continue using these boxes as a reference pair after my Opals and Avantones.

Havoc wrote:If that's the case, then a first test with only the transformer might be a good idea. You can unplug the transformer by disconnecting the white connector just above the cap (while the mains is off). The reconect the mains, switch on and watch if the fuse blows or not. If not, then the transformer is ok.

Next, I would check the caps. Cheap caps in a closed box that can get hot don't tend to live long. Under no circumstances, power the circuit with the caps removed! remove the caps and measure them with a simple Ohm meter. If anything less than a couple hundred kilo-ohm, replace them. It will probably be lower in the beginning but slowly climb. Observe the polarity! If you replace them, make sure to respect the polarity, getting it wrong will mean a bang and probably a very dead monitor.

Good stuff, fantastic. Thankyou!

Havoc wrote:Check the rectifiers with a multi-meter set to diode function. But if this makes you scratch your head, ask someone who knows this kind of stuff.

For the next step we'd need more on the circuit I'm afraid. But from the look of the pcb it seems as if there's nothing more (?) Just an unregulated power supply and the power amps.

I will see what i can do Re: schematics. Dont fancy my chances though.



Folderol wrote:Is there a fairly loud thump as you switch on?

I'm pretty sure it was the same as when it was working i.e. a thump, yes sure, but not overly loud.


Folderol wrote:If so, then something is putting DC on the speaker -most likely either a faulty cap, or slightly less likely one of the amps half-dead. This would certainly account for the slight delay before the fuse pops.

I see. Thanks.

Much respect and thanks peoples.

If theres anything i find out, i will put it up here and would be glad to have opinions.

Ta.
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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Tue May 25, 2010 9:32 am

OneTrakAudio wrote:While this is kinda irrelevant [?] i'll let it pass...

Most generous of you While my comments might be irrelevant to you, they were relevant to Havoc since it was his earlier post commenting on the amp construction that I was repsonding to.

While i am obviously not competent now, theres no reason i cannot learn from this and possibly do a great job of fixing it.

There is always that possibility... But there is also the possibility that you could end up killing yourself or cause serious damage through a lack of knowledge combined with mains power.

This is inherently dangerous stuff. You would be well advised to learn about power supply design before attempting to bodge your way through a fix... and helpful forum posters would be best advised to be very careful in the recommendations and advice they give in public lest someone tries to follow their advice and ends up badly injured.

Any advice given here is not authorised, approved or recommended by SOS and anyone attempting to work on equipment do so entirely at their own risk.

I actually grew up building guitar amps, radios, synthesizers etc

I don't understand how this statement fits with the earlier question about what a power supply is! Clearly, some expansion or revision of knowledge is required.

The good news is that the power supply would appear to be very simple... but seriously, messing about with this stuff isn't clever. Do make absolutely sure you know what you're doing. I'm not trying to be patronising, I'm trying to make sure you or anyone else reading this thread don't harm themselves inadvertantly.

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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Tue May 25, 2010 10:25 am

Hugh Robjohns wrote:Clearly

Crystal.

And no i wont sue SOS or any of its members when i blow my cat up. The cat will be the poor soul flicking the switch, you see...
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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Tue May 25, 2010 10:30 am

Hugh Robjohns wrote:
Most generous of you While my comments might be irrelevant to you, they were relevant to Havoc since it was his earlier post commenting on the amp construction that I was repsonding to.


I don't see what expousing the virtues of passive monitors and powered amps over cheap active monitors has to do with anything, is all.

I'm asking for problem solving advice on how to - safely - locate the fault in a broken active.

But taking a deep breath....

Hugh Robjohns wrote:The good news is that the power supply would appear to be very simple... but seriously, messing about with this stuff isn't clever. Do make absolutely sure you know what you're doing. I'm not trying to be patronising, I'm trying to make sure you or anyone else reading this thread don't harm themselves inadvertantly.



I intend to be absolutely sure. If i wasn't, i wouldve 'bodged' something up already. I'm here as part of an information gathering quest, to pick your knowledgable minds. I understand your point here, and appreciate what your saying.
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Re: DIY Monitor Repair Help

Postby OneTrakAudio » Tue May 25, 2010 11:46 am

"and helpful forum posters would be best advised to be very careful in the recommendations and advice they give in public lest someone tries to follow their advice and ends up badly injured."



Well, I don't want that on anyones shoulders so i guess this thread is dead in the water...

thanks for the help and replies anyway, you've all given me some clues for further investigating so all is not lost.

Cheers and have nice day.
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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Tue May 25, 2010 12:28 pm

OneTrakAudio wrote:And no i wont sue SOS or any of its members when i blow my cat up.

I'll sleep easier in my bed now...

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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Tue May 25, 2010 12:35 pm

OneTrakAudio wrote:I don't see what expousing the virtues of passive monitors and powered amps over cheap active monitors has to do with anything, is all.

As I explained, I was responding to Havoc's post where he commented: "is this all there is in the monitor of electronics?" Those familiar with conventional amp designs are often shocked with the chip-amp design often employed in budget active speakers.

Furthermore, 'active speakers' are often purported to be better than passive designs because of the better control afforded by each driver having its own amp, and by using aan active cross-ver instead of a passive one.

However, while these points are generally true, the chip-amps often employed in budget designs undo these potential benefits several times over... and from the point of view of informing the forum users generally, I thought that was a useful point to draw out of the ongoing discussion.

I'm asking for problem solving advice on how to - safely - locate the fault in a broken active.

Yes, you are... and you've had some answers. But you're not the only forum user and threads often go off in tangents to satisfy the interests of all those taking part.

I understand your point here, and appreciate what your saying.

That's good to know -- thanks.

If you have the appropriate tools and test gear, I should think the most likely problem will be the rectifiers or smoothing capacitors, or possibly the regulators if there are any. Not very likely to be the transformer...

... but if there has been a power supply failure it is quite likely that the chip amps will have been toasted too.

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Re: DIY Monitor Repair Help

Postby *INACTIVE USER* » Tue May 25, 2010 7:15 pm

As I explained, I was responding to Havoc's post where he commented: "is this all there is in the monitor of electronics?" Those familiar with conventional amp designs are often shocked with the chip-amp design often employed in budget active speakers


It isn't the chip amps I'm shocked of. There are some decent ones around where you would need some serious discrete amp to improve upon them in this application. In an active amp the power amp does have it a bit easier. But from that photo there isn't a decent psu (a smallish transformer, 2 diodes and 2 caps at most). It looks as if there isn't even a serious input buffer, and from the looks of it the filter (what's in a name?) isn't buffered from the input/output. So how will it work with whatever there is connected?
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Re: DIY Monitor Repair Help

Postby Hugh Robjohns » Wed May 26, 2010 12:17 am

I expect there's surface mount stuff on the underside of those boards....

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Re: DIY Monitor Repair Help

Postby discomb » Sun Nov 11, 2012 12:34 pm

Hello all,

It's nice to be back on SOS, I haven't been here for a long time (working too hard I guess) and it's good to see it's still the place to come for detailed info on all things audio.

OneTrakAudio, did you get anywhere in fixing your S8? I have had pretty much the same problem several few months ago and I'm researching a DIY repair myself. It's either that or sell the pair for less than it would cost to have the faulty one repaired!

In case it is helpful for you or anyone else with a similar problem, here's what I've done. I do need to state that what I've done is potentially very dangerous so as with all things involving mains electricity, it is always best to seek the help of a certified professional because you can be putting your life in your hands, literally.

I managed to get a copy of the circuit diagram from the manufactures, basically by asking very nicely. I can't share it though, on their request. It was helpful to see where to start...

My monitor goes immediately: on powering up the LED blinks briefly and there's a dull pop. I am treading very carefully because as Hugh etc have pointed out this is a potentially lethal task. I've had a chat with a friend who knows his electronics. The first thing he said is "DON'T GO ANYWHERE NEAR THE CAPACITORS, THEY CAN STORE ENOUGH CHARGE TO KILL YOU!!!" He reckoned having left the thing unplugged for a month would be long enough for them to self discharge to a safe level. I still haven't touched them though

After a brief investigation, my mate suspects the transformer. We measured the resistance across the pins in the power socket (where you plug the mains lead in) with a DMM, after fitting a new fuse and flicking the power switch to on to complete the circuit. We found on the faulty monitor a resistance of 18 ohms (and 7.6 ohms on the US 115V mode), whereas the working monitor is 34 ohms (and 8.9 ohms for 115V). A big difference, suggesting the transformer is not quite right. Less resistance = more current = blown fuse.

To confirm this I disconnected the transformer from the rest of the circuit (the white connector block just above the VERY DANGEROUS capacitors), and checked to see if the fuse blew. And it did.

To confirm that it was only the transformer that was the problem, I got my working S8 involved - I used the transformer from the working S8 and plugged that into the dodgy S8 (at the same white connector above the VERY DANGEROUS capacitors). After double checking I hadn't done anything silly, I powered up and... the fuse blew. It turns out my double check was not as comprehensive as it shouldn't have been. This is a very important lesson - check your checks! Turns out I had left the working S8's voltage switch on 115V. I unplugged, replaced the fuse, switched to 230V and tried again, and the LED came on, and stayed on.

Hopefully this means I just need to replace the transformer. I'm going to see if I can source a direct replacement, going by the part number on the original.

Good luck everyone. I hope to report it's all working OK at some point soon...
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Re: DIY Monitor Repair Help

Postby dmills » Sun Nov 11, 2012 2:08 pm

While I am somewhat reluctant to downplay dangers, it is VERY unlikely that the capacitors in that speaker present any hazard.

In a big valve amp, sure, hundreds (Or even, rarely thousands) of volts and you do sometimes get a faulty or missing bleeder, but in a small solid state amp I would be very surprised if the voltage across each of those caps exceeded 35V or so, no real hazard there.

For the most part the hazards in small solid state amplifiers are all in the primary side of the transformer, at least for anything less then 50 watts or so.

Regards, Dan.
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Re: DIY Monitor Repair Help

Postby discomb » Thu Nov 15, 2012 10:47 am

Cool, have managed to source a direct replacement for the transformer for £22. Much better than the ~£120 for the whole power supply part (excluding labour) that I was quoted before I started poking around with the DMM. The DMM was the best £5 I ever spent in Maplin!
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Re: DIY Monitor Repair Help

Postby Madman_Greg » Fri Nov 16, 2012 12:48 pm

Please remember charged capacitors take time to loose their charge, unless you dissipate the charge.
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Re: DIY Monitor Repair Help

Postby James Perrett » Fri Nov 16, 2012 4:31 pm

Madman_Greg wrote: Please remember charged capacitors take time to loose their charge, unless you dissipate the charge.

But typical linear power supplies like the one discussed here will cause the capacitors to discharge quickly when the supply is switched off so, in this case, it won't be a problem provided you don't dive in for a few seconds.

TV's, microwave ovens and other high voltage circuits are a different matter entirely but most traditional audio gear is safe after a few seconds.

James.
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Re: DIY Monitor Repair Help

Postby Madman_Greg » Fri Nov 16, 2012 11:34 pm

James Perrett wrote:
Madman_Greg wrote: Please remember charged capacitors take time to loose their charge, unless you dissipate the charge.

But typical linear power supplies like the one discussed here will cause the capacitors to discharge quickly when the supply is switched off so, in this case, it won't be a problem provided you don't dive in for a few seconds.

TV's, microwave ovens and other high voltage circuits are a different matter entirely but most traditional audio gear is safe after a few seconds.

James.

I realise that James, I just get manic about electrics and electronics and safety, especially as its not my day job, and I dabble from time to time, so always safety first for me.
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Re: DIY Monitor Repair Help

Postby JackCGuitar » Thu Dec 13, 2012 1:52 pm

Hi discomb, do you mind me asking where you sourced the transformer from as i think iv'e got exactly the same problem with one of my Tapco S8's?
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Re: DIY Monitor Repair Help

Postby MarkOne » Thu Dec 13, 2012 3:11 pm

Madman_Greg wrote: Please remember charged capacitors take time to loose their charge, unless you dissipate the charge.

Oh yes, what japes we had as trainee test engineers (back in the dim and distant 70s when H&S at work wasn't something people had heard of ). Charge up a 100uF/35v coaxial electrolytic bend the wires across so they nearly touch and then toss it across the bench to a mate, who would instinctively catch it. BANG!

Of course that was the irresponsibility of callow youths. And a different time... And I would be first in line to condemn such things now
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Re: DIY Monitor Repair Help

Postby Kendo Phil » Thu Dec 13, 2012 8:17 pm

This thread reminded me of my college days, we were doing AC resonance experiments with a huge capacitor, an inductor and a load (resistor). We were measuring the voltage between the capacitor and the inductor under varying AC frequencies.

We were told under no circumstances to touch any of the circuitry when it was live as the experiment results in huge power spikes.

Guess what? Yup one of the guys in my group prodded the capacitor with a screwdriver, there was a white blue flash and the tip of his screwdriver was gone.

That was a good lesson in respecting capacitors!
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Re: DIY Monitor Repair Help

Postby IvanSC » Wed Dec 26, 2012 8:07 pm

Decades ago, I was cleaning a solder sucker with the suction rod exposed when I leant on the power supply caps of a recently-unplugged valve amp.
The rod went clean through the end of my thumb and out the other side!

Have to say I am 100% with Hugh on his attitude towards enthusiastic amateurs and mains electricity in any guise.

And since we havent heard from the OP recently, it does make you wonder....

Not sure how to take someone who claims to have built equipment and doesnt realise what the basic components of just bout ANY power supply are.

Scary!

One for the older techs on here: "I think it's the just output transformer"
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