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Old 14th April 2009, 08:46 PM   #1
Buhl is offline Buhl  Denmark
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Default Pearl: one channel down :(

A pearl of mine suddely quit working - it is always turned on, and one day, the left channel was suddenly very, very low. Got out my voltmeter, and checked voltages. All looks good, except Q4 (ZTX450 that supplies 13.8 volt to the double JFET output) instead of 13.8 volt is reading 0.8 volt - supposedly eliminating the last gainstage of the pearl.

Beeing a engineer of trade (computer however ....LOL) - I keep spares lying around, just in case - soldered in a new 450 in a few minutes - same case

This is where I whished I chose EE insted of Computer Science - anybody of you sharp brains got some clues to what I should start checking??

Kind regards
Hans
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Old 14th April 2009, 09:42 PM   #2
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Perhaps Q4's emitter is being shunted to ground, possibly by a
failed JFET. What is the voltage across R14?

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Old 15th April 2009, 05:10 AM   #3
Buhl is offline Buhl  Denmark
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Hi Nelson - thank you very much for replying
I think you are on to something - voltage across (the hot!) R14 is 27,5 volt on the bad side - the working curcuit measures 9,5volt across R14. - is it the 309 FET that's down then?

Kind regards
Hans
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Old 15th April 2009, 07:44 AM   #4
Fox is offline Fox  Netherlands
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A few years back, I experienced the same problem while changing interconnects with the Pearl still on. It turned out that the double JFET failed and in its failure the ZTX450 got smoked as well. Apparently, the output stage of the Pearl is quite fragile.
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Old 15th April 2009, 08:19 AM   #5
Buhl is offline Buhl  Denmark
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Hmmm, this could very well be the course of it - like I wrote: It's always on - so you may have found the cause of it there Fox

Darn - the 2SK389-BL is a part I do not have liying around

/Hans
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Old 15th April 2009, 12:30 PM   #6
cviller is offline cviller  Denmark
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That seems to be a very common problem with the pearl. You can use two 2sk170BL instead of the 389, but you have to sort out the correct way to place the legs...
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Old 15th April 2009, 05:49 PM   #7
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Happened to mine as well. That's why I now keep two spare sk389s in my drawer.
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Old 15th April 2009, 07:19 PM   #8
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You will note that the Pearl schematic shows about 9 volts across
R14, which means that 9 mA is going through the pair of JFETs.

This is not enough to damage the 2SK389's. However, if the Idss
of the FETs used is high, it will cause too much DC across R14 and
saturate of Q4, at which point the whole think collapses.

Rather than assume that the 2SK389 is bad, I suggest you try a
higher value for R25 and see what happens.

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Old 20th April 2009, 09:57 AM   #9
h_a is offline h_a  Europe
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Quote:
You can use two 2sk170BL instead of the 389
Exactly!

The pearl is an example where you can use 2x SK170 - even unmatched ones - as perfect replacement for the 2SK389.

Have fun, Hannes
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Old 20th April 2009, 11:28 AM   #10
Buhl is offline Buhl  Denmark
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How much would higher values on R25 (default 10 ohms) be - 15, 20 ??

Kind regards
Hans
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