Hi everyone, I am stumped on fixing a problem with my self-built Pearl Phono v1.
Any comments would be great however cursory, please.
The right channel has stopped working.
Both channels were working but when trying to root out some hum problems the right channel died. It *might* have been related to my plugging in interconnects while the amp was powered up.
All voltages on Fig 5 “The Unsimplified Circuit” of the Pearl Phono Stage article check out (within a bit of variance) except around Q4 (ZXT450) and Q6 (2SK389), where I get the following voltages:
Location: between Q4 and C9
Fig 5: 19.3v
Left ch: 18.5v
Right ch: 1.126v
Location: base of Q4
Fig 5: 14v
Left ch: 14.4v
Right ch: 1.833v
Location: between Q4 and Q6
Fig 5: 13.4v
Left ch: 13.9v
Right ch: 1.112v
Location: base of Q6
Fig 5: 0.05v
Left ch: 0.11v
Right ch: 0.30v
The right channel is the dead one. I first replaced Q4 – it made no difference.
I then figured it must be Q6 and replaced Q6. It made no difference.
At this point I ran out of ideas.
Any suggestions would be much appreciated. (The voltages shown above are the current circuit values – after replacing Q4 and Q6).
Any comments would be great however cursory, please.
The right channel has stopped working.
Both channels were working but when trying to root out some hum problems the right channel died. It *might* have been related to my plugging in interconnects while the amp was powered up.
All voltages on Fig 5 “The Unsimplified Circuit” of the Pearl Phono Stage article check out (within a bit of variance) except around Q4 (ZXT450) and Q6 (2SK389), where I get the following voltages:
Location: between Q4 and C9
Fig 5: 19.3v
Left ch: 18.5v
Right ch: 1.126v
Location: base of Q4
Fig 5: 14v
Left ch: 14.4v
Right ch: 1.833v
Location: between Q4 and Q6
Fig 5: 13.4v
Left ch: 13.9v
Right ch: 1.112v
Location: base of Q6
Fig 5: 0.05v
Left ch: 0.11v
Right ch: 0.30v
The right channel is the dead one. I first replaced Q4 – it made no difference.
I then figured it must be Q6 and replaced Q6. It made no difference.
At this point I ran out of ideas.
Any suggestions would be much appreciated. (The voltages shown above are the current circuit values – after replacing Q4 and Q6).
Hi, just to update those who generously contributed to my thread:
I replaced the Q4 ZRX450 (again), and still the same voltage measurements, including the base of Q4 being at 1-2 V instead of 13-14 V.
So now I have ordered replacement Q6 and will replace Q4 and Q6 simultaneously. I will let you know how it goes.
cheers
I replaced the Q4 ZRX450 (again), and still the same voltage measurements, including the base of Q4 being at 1-2 V instead of 13-14 V.
So now I have ordered replacement Q6 and will replace Q4 and Q6 simultaneously. I will let you know how it goes.
cheers
Typically the jfet blows and takes the ZTX with it; this is the very most common failure for the pearl. In that case replace both as otherwise the remaining damaged jfet will again take down the ZTX.
The K389 is not needed in this position, you can as good use the K170BL or even selected K369GR. These also have twice the power dissipation of the single jfets in the K389, so reliability will no longer be a problem with these replacements.
Hannes
where DO you get a genuine 2SK389?
The K389 is not needed in this position, you can as good use the K170BL or even selected K369GR. These also have twice the power dissipation of the single jfets in the K389, so reliability will no longer be a problem with these replacements.
Hannes
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Typically the jfet blows and takes the ZTX with it; this is the very most common failure for the pearl. ....
Ha! NOW I discover this! I have been alternately blowing each one when I replace the other....
Anyway, I have bought 25 K170BL and will replace Q4 (ZTX450) and Q6 (with a matched pair of 170's) together and report back.
Thanks again all.
Q6 (with a matched pair of 170's)
Just for your information - there is absolutely no matching necessary in this position. Of course you can do it if you simply feel like it.
However, use the highest IDSS BL-jfets that you have in this position and you'll get a bit lower distortion.
Hannes
Fixed!
Great!
A quick listen gave the impression of less output on the repaired channel,
Might happen if your jfets are on the other end of the IDSS scale compared to the ones in the other channel, but still the difference should be very small.
Hannes
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