Pearl - one channel down

Buhl · 31st January 2012, 09:03 PM

Hi everybody
My wonderfull pearl suddenly stopped playing music in one channel - that is, only a very faint signal was heard.

All voltages check out OK, except Q4 (small ZTX450 which supplies the 2sk389 jfet in the output with power) - which reads 0.8 instead of 13.8 volts.
As mentioned, all other voltages marked on the board reads OK, including the 9 volt reference point.

Any ideas? Is it simply a matter of "swap ZTX450 to a new device" or should I look elsewhere?

Best regards
Hans

Zen Mod · 31st January 2012, 09:50 PM

swap
check

Buhl · 1st February 2012, 05:20 PM

Unfortunately, swapping the ZTX450 did not change anything, same result -about 0.8 volt on the emitter.

What could cause this - a burned jfet output device, which then shorts/overloads the ZTX450

Dont know if it makes any sense to measure the JFET output device, since the device supplying it seems to be down?

Best regards
Hans

Buhl · 3rd February 2012, 05:56 PM

No ideas?
Only got one spare 2sk389, so want to be sure theres notning more i can do before swapping thar one also

Best renards
Hans

blackdod · 3rd February 2012, 06:47 PM

If you change the 2SK389 and recheck the solderings ?

The 2SK389 is a fake?

Regards

Buhl · 4th February 2012, 04:38 PM

Yup - since no one had any objections, i swapped til 389 and te ztx 450(again) - that did the trick, got the pearl up and running again.
What do you mean by the 389 being fake?

Best regards
Hans

d to the g · 11th February 2012, 04:33 AM

The 2sk389 output blowing on these is not all uncommon. It happened to me when I pulled the RCA cable before the power supply discharged. I purchased a handful from a gentlemen in the swap meet section and they were all legit. I suggest stocking up.

h_a · 14th February 2012, 07:16 AM

Yeah, this is the typical problem: the K389 blows and takes the ZTX with it; replacing only the ZTX means it will blow again.

For those that slowly run out of K389, one can replace it of course with K170BL or K369GR. In fact they don't even need to be matched devices as they are used in parallel (no distortion cancellation going on).

If one intends to replace the K389 with K369GR one should keep in mind that the K369 has a smaller pinch off voltage (and therefore also a larger transconductance) so produces slightly more distortion. One can remedy that by using a slightly larger degeneration resistor.

Enjoy, Hannes

31st January 2012, 09:03 PM	#1
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	Pearl - one channel down Hi everybody My wonderfull pearl suddenly stopped playing music in one channel - that is, only a very faint signal was heard. All voltages check out OK, except Q4 (small ZTX450 which supplies the 2sk389 jfet in the output with power) - which reads 0.8 instead of 13.8 volts. As mentioned, all other voltages marked on the board reads OK, including the 9 volt reference point. Any ideas? Is it simply a matter of "swap ZTX450 to a new device" or should I look elsewhere? Best regards Hans __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

31st January 2012, 09:50 PM	#2
Zen Mod diyAudio Member Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain	swap check __________________ my Papa is smarter than your Nelson ! tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; BAF Forum & Gallery;I'm dumb

1st February 2012, 05:20 PM	#3
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	Unfortunately, swapping the ZTX450 did not change anything, same result -about 0.8 volt on the emitter. What could cause this - a burned jfet output device, which then shorts/overloads the ZTX450 Dont know if it makes any sense to measure the JFET output device, since the device supplying it seems to be down? Best regards Hans __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

3rd February 2012, 05:56 PM	#4
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	No ideas? Only got one spare 2sk389, so want to be sure theres notning more i can do before swapping thar one also Best renards Hans __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass

4th February 2012, 04:38 PM	#6
Buhl diyAudio Member Join Date: May 2002 Location: Aalborg, Denmark	Yup - since no one had any objections, i swapped til 389 and te ztx 450(again) - that did the trick, got the pearl up and running again. What do you mean by the 389 being fake? Best regards Hans __________________ "Calculations? We don't need no stinkin calculations! We just make them really big!" Quote: Nelson Pass


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3rd February 2012, 06:47 PM	#5
blackdod diyAudio Member Join Date: Jun 2011 Location: Canarias	If you change the 2SK389 and recheck the solderings ? The 2SK389 is a fake? Regards

11th February 2012, 04:33 AM	#7
d to the g diyAudio Member Join Date: Aug 2006 Location: minimalopolis	The 2sk389 output blowing on these is not all uncommon. It happened to me when I pulled the RCA cable before the power supply discharged. I purchased a handful from a gentlemen in the swap meet section and they were all legit. I suggest stocking up.

14th February 2012, 07:16 AM	#8
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	Yeah, this is the typical problem: the K389 blows and takes the ZTX with it; replacing only the ZTX means it will blow again. For those that slowly run out of K389, one can replace it of course with K170BL or K369GR. In fact they don't even need to be matched devices as they are used in parallel (no distortion cancellation going on). If one intends to replace the K389 with K369GR one should keep in mind that the K369 has a smaller pinch off voltage (and therefore also a larger transconductance) so produces slightly more distortion. One can remedy that by using a slightly larger degeneration resistor. Enjoy, Hannes __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

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