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
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
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
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
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
Only got one spare 2sk389, so want to be sure theres notning more i can do before swapping thar one also 🙂
Best renards
Hans
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
What do you mean by the 389 being fake? 🙂
Best regards
Hans
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.
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
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
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