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Spot the deliberate mistake
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simonp1100 View Drop Down
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Points: 1110 		Post Options Post Options   Thanks (0) Thanks(0)   Quote simonp1100 Quote  Post ReplyReply Direct Link To This Post Posted: 27 April 2018 at 4:18pm
Originally posted by Earplug Earplug wrote:

Originally posted by simonp1100 simonp1100 wrote:

Originally posted by odc04r odc04r wrote:

Overrated tbh. Just stick a crowbar triac on it. When have they ever failed.


Most of the amplifiers i have repaired with a faulty output stage with crowbar protection has always made the triac go short circuit as well and hence needed changing, so i think it's horses for courses really.


It´s usually easier to swap out a triac than a relay...   LOL




Depends how good / quick you are with a de-soldering pump and where the triac is mounted in the amplifier, if its a Chevin then most of the heatsink needs to be taken apart as it's mounted near the bottom of the heatsink and if you have a spare hour then fine and it's a pain in the arse to do as well.

again 6 of one & half a dozen of the other.   ClapClap  LOLLOL




Edited by simonp1100 - 27 April 2018 at 4:29pm
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kedwardsleisure View Drop Down
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Points: 4947 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 28 April 2018 at 12:29am
that relay hadn't been soldered shut, it would have been the result of the relay opening on a DC fault and arcing across the contacts 'til they melted.
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MattStolton View Drop Down
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Points: 4234 		Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 28 April 2018 at 10:02am
Originally posted by kedwardsleisure kedwardsleisure wrote:

that relay hadn't been soldered shut, it would have been the result of the relay opening on a DC fault and arcing across the contacts 'til they melted.

My first guess was self welding, but the sheer volume of material deposited looks like it has been "added to". I know the contact pads are usually some form of 2-3mm thick cylinder of metal, which is the likely source of volume of material, but that is a fecking mess!

My welding often looks like that, but involves me holding the trigger down for ages to amass that much of a blob! I would have run out of gas making that.
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
"Sparkius metiretur vestra" - "Meter Your Mains"
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odc04r View Drop Down
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Points: 5482 		Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 29 April 2018 at 9:30am
Originally posted by MattStolton MattStolton wrote:

Originally posted by kedwardsleisure kedwardsleisure wrote:

that relay hadn't been soldered shut, it would have been the result of the relay opening on a DC fault and arcing across the contacts 'til they melted.

My first guess was self welding, but the sheer volume of material deposited looks like it has been "added to". I know the contact pads are usually some form of 2-3mm thick cylinder of metal, which is the likely source of volume of material, but that is a fecking mess!

My welding often looks like that, but involves me holding the trigger down for ages to amass that much of a blob! I would have run out of gas making that.


I figured the same, shurely too much melting there for it not to have been soldered shut! Was the mains fuse also replaced with a nail? Not the result of your average arc, the sustained current to melt that much metal must have been significant.
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ceharden View Drop Down
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Points: 11776 		Post Options Post Options   Thanks (0) Thanks(0)   Quote ceharden Quote  Post ReplyReply Direct Link To This Post Posted: 29 April 2018 at 10:50am
Believe it or not, I can confirm this is indeed what can happen!  Just proves how much current a Crest PSU & Output stage can deliver.  Probably doesn't take as much current as you might think.  Also I don't believe there is any fusing on the secondary side of the transformer, only the input breaker.

Demonstrates if you go down the crowbar route, how much current the crowbar has to handle before something upstream fuses/trips.  The result would still probably be a huge charred mess.


Edited by ceharden - 29 April 2018 at 10:50am
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MattStolton View Drop Down
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Points: 4234 		Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 30 April 2018 at 10:41am
On my Mig/Tig plant, as little as 20ish A at 20ish Vdc, will happily weld 1.6mm mild sheets together! 6mm to 6mm only needs 50A/60Vdc ish! More for aluminium or metals that really conduct heat away well, but don't need much to sustain an arc over a couple of mm, achieving several thousand degs C!

Off course, if you forget to turn the shield gas on, not that I ever would forget of course, the speed that a tungsten Tig electrode melts is scary. And tungsten really doesn't melt that easily....around 3400degC!
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
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kedwardsleisure View Drop Down
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Points: 4947 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:01pm
MC2 MC1250

A picture of my copious pile of birthday presents to the first correct answer




Edited by kedwardsleisure - 16 July 2018 at 12:03pm
Kevin

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APW View Drop Down
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:08pm

Is that an escaped TO126 transistor I see near the bottom right?

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APW View Drop Down
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Points: 1174 		Post Options Post Options   Thanks (0) Thanks(0)   Quote APW Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:11pm

BTW... Happy 101st!!


Edited by APW - 16 July 2018 at 12:20pm
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Earplug View Drop Down
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Points: 7216 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Earplug Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:32pm
Odd electrolytic cap between the boards in the middle there?  Confused

Are the o/p devices BJT, or FET?
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kedwardsleisure View Drop Down
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Points: 4947 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:34pm
Bipolar. The caps are right
Kevin

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MattStolton View Drop Down
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Points: 4234 		Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 16 July 2018 at 12:42pm
Is the second transistor up from bottom, on the left, centre pin not connected to anything? Could just be my eyesight/photo combo?


The escaped transistor, bottom right, is also of concern!
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
"Sparkius metiretur vestra" - "Meter Your Mains"
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