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davesch
Frequent Contributor
Username: davesch
Post Number: 63
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, 24 January, 2010 - 10:16 pm: | 
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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 ? |
phonoplug
Regular Contributor
Username: phonoplug
Post Number: 29
Registered: 08-2009
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 12:06 am: |
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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. |
terry
Frequent Contributor
Username: terry
Post Number: 560
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 07:49 am: |
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I go for the socket shorting as I have had that happen to me.
Terry |
davesch
Frequent Contributor
Username: davesch
Post Number: 65
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 08:56 am: |
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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 |
twintub
Frequent Contributor
Username: twintub
Post Number: 57
Registered: 02-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 07:11 pm: |
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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. |
twintub
Frequent Contributor
Username: twintub
Post Number: 58
Registered: 02-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, 25 January, 2010 - 10:06 pm: |
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....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. |
davesch
Frequent Contributor
Username: davesch
Post Number: 66
Registered: 05-2005
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, 27 January, 2010 - 09:32 pm: |
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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. |
hamar
Regular Contributor
Username: hamar
Post Number: 26
Registered: 05-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, 31 January, 2010 - 12:26 am: |
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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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