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Could it be my fault ?

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 15 February, 2010 » Could it be my fault ? « Previous Next »

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davesch
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Username: davesch

Post Number: 63
Registered: 05-2005

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Posted on Sunday, 24 January, 2010 - 10:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've had the two output transistors (matched pair) on one channel of my electronic keyboard fail, nothing to do with a previous post by me about hum on an electronic organ.

One of the transistors has gone short circuit across all 3 pins, the other across 2 pins.

The internal speakers are 8 ohm, but only about 4" diameter, so don't give a great sound especially at bass frequencies. I therefore wired in 2 1/4" switched jack sockets so I could plug in much bigger 8 ohm speakers, the internal speakers being cut when the jacks inserted.

The internal speakers are 8 ohm, the external speakers although much bigger are 8 ohm, so I figured this shouldn't be a problem. It all seemed to work fine for a few weeks, but it failed recently after only a couple of minutes of use.

Could the bigger speakers be the cause in spite of being the same impedance ? or is it likely just a co-incidence ?
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phonoplug
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Username: phonoplug

Post Number: 29
Registered: 08-2009

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Posted on Monday, 25 January, 2010 - 12:06 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just a thought, with some jack sockets, its possible when plugging or unplugging, to briefly short the connections. When you have connected or disconnected this external speaker has the keyboard always been switched off?

The impedance, being the same, shouldn't have caused the damage. Think of it like using a 5 watt resistor to replace a 1 watt type, but they are both the same value. The larger one simply has a larger capacity to dissipate heat.

Its always possible that one of the transistors has failed due to random failure, and if its a push-pull output, this is likely to cause its opposite one to be damaged too.
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terry
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Username: terry

Post Number: 560
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Posted on Monday, 25 January, 2010 - 07:49 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I go for the socket shorting as I have had that happen to me.

Terry
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davesch
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Username: davesch

Post Number: 65
Registered: 05-2005

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Posted on Monday, 25 January, 2010 - 08:56 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It failed whilst already plugged in, in fact I connect up the external speakers prior to powering up.

Yes, I think it is a push-pull output
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twintub
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Username: twintub

Post Number: 57
Registered: 02-2007

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Posted on Monday, 25 January, 2010 - 07:11 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Maybe there's an intermittent short circuit in the jack plug! Make sure there isn't a whisker of wire sticking out from where it shouldn't (I always use heatshrink sleeving to cover the connections).
Also check that the 'crimp' section hasn't been crimped too tight which could cause a s/c between the inner & outer of the screened wire.
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twintub
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Post Number: 58
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Posted on Monday, 25 January, 2010 - 10:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

....that's if you've used screened wire of course! If you've used figure-of-eight cable (whose insulation is quite soft) then a short is even more likely to be caused by a too tight crimp.
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davesch
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Username: davesch

Post Number: 66
Registered: 05-2005

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Posted on Wednesday, 27 January, 2010 - 09:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just had a check of the leads which are made using what looks like 2 core mains flex. They look okay and as the sheath goes under the crimp, I would have thought it was unlikely to be that. I've given them a good waggle with an ohmeter across the terminals, no sign of a short, so don't think it was these.
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hamar
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Username: hamar

Post Number: 26
Registered: 05-2006

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Posted on Sunday, 31 January, 2010 - 12:26 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Could it be a Parasitic HF instability causing thermal runaway, does the output from the matched pair have an inductor/capacitor (zorbel)network in series with the output capacitor? Do they test ok It happens on higher power amps (albeit 2.5KW).

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