Pearl: one channel down :(

Buhl · 14th April 2009, 08:46 PM

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

Nelson Pass · 14th April 2009, 09:42 PM

Perhaps Q4's emitter is being shunted to ground, possibly by a
failed JFET. What is the voltage across R14?

Buhl · 15th April 2009, 05:10 AM

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

Fox · 15th April 2009, 07:44 AM

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.

Buhl · 15th April 2009, 08:19 AM

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

cviller · 15th April 2009, 12:30 PM

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...

d to the g · 15th April 2009, 05:49 PM

Happened to mine as well. That's why I now keep two spare sk389s in my drawer.

Nelson Pass · 15th April 2009, 07:19 PM

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.

h_a · 20th April 2009, 09:57 AM

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

Buhl · 20th April 2009, 11:28 AM

How much would higher values on R25 (default 10 ohms) be - 15, 20 ??

Kind regards
Hans

14th April 2009, 08:46 PM	#1
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

15th April 2009, 05:10 AM	#3
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

15th April 2009, 08:19 AM	#5
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	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 __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

20th April 2009, 11:28 AM	#10
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	How much would higher values on R25 (default 10 ohms) be - 15, 20 ?? Kind regards Hans __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

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14th April 2009, 09:42 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Perhaps Q4's emitter is being shunted to ground, possibly by a failed JFET. What is the voltage across R14?

15th April 2009, 07:44 AM	#4
Fox diyAudio Member Join Date: Dec 2002 Location: The Netherlands	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.

15th April 2009, 12:30 PM	#6
cviller diyAudio Member Join Date: Dec 2005 Location: Copenhagen Blog Entries: 2	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...

15th April 2009, 05:49 PM	#7
d to the g diyAudio Member Join Date: Aug 2006 Location: minimalopolis	Happened to mine as well. That's why I now keep two spare sk389s in my drawer.

15th April 2009, 07:19 PM	#8
Nelson Pass The one and only Join Date: Mar 2001	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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