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Old 31st October 2006, 03:23 PM   #1
Lucas_G is offline Lucas_G  Netherlands
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Question Problem with Pearl. Please Help

Hi guys,

I am afraid I need a little help.

I have finished my Pearl phonoamp a few days ago.

The left channel is working, the right does not (very low sound, hardly any level).

I checked the voltages:

Q2 & Q12 = both channels 8.5 Volt
Q3 = both channels 24 Volt

Q4: Left = 13.3 Volt Right = 1.0 Volt
Q5: Left = 12.5 Volt Right = 10.6 Volt
Q6: Left = 13.3 Volt Right = 1.0 Volt

Q8-Q11: Left = 12,5 Volt Right = 10.6 Volt

I checked the ground connections, but sofar I could not find any faults...

Can anyone offer some advice where to look now?

Much appreciated. The one working channel sounds very good!

Regards,

Lucas
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Old 31st October 2006, 03:35 PM   #2
Lucas_G is offline Lucas_G  Netherlands
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I include this here.
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Old 31st October 2006, 05:51 PM   #3
Lucas_G is offline Lucas_G  Netherlands
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Here are the measured voltages. Problems are situated around Q6.

Any advice is highly appreciated!
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Old 31st October 2006, 06:23 PM   #4
Lucas_G is offline Lucas_G  Netherlands
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I just measured the impedance on various resistors (mounted), in order to find descrepancies between the workin left-channel and the broken right channel.

I found one major difference: the resistance over R24 (100k) is indeed 100k on the working channel, but it is only 40k on the broken channel. Is this another sign that the 2SK389 (Q6) is rotten?
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Old 31st October 2006, 06:30 PM   #5
Lucas_G is offline Lucas_G  Netherlands
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I immediately add another question:

Where can I order a new 2SK389
?
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Old 31st October 2006, 06:41 PM   #6
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Quote:
Originally posted by Lucas_G
I immediately add another question:

Where can I order a new 2SK389
?
Digikey has them. Sorry to hear you're having trouble.
Love my Pearl BTW.

Jeff
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Old 31st October 2006, 06:59 PM   #7
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by vinylkid58


Digikey has them. Sorry to hear you're having trouble.
Love my Pearl BTW.

Jeff
Digikey will be a bit probelmatic for me, I guess. Shipment to Europe and minimal order...

Yes, the one working channel does sound promising....

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Old 1st November 2006, 04:17 AM   #8
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It looks like it is shorted.
I have contacted you per Email about the 2SK389 BL.
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Old 1st November 2006, 04:32 AM   #9
RoboMan is offline RoboMan  Hong Kong
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I don't have a Pearl, but according to the voltages of Q6, it seems that Q6 is not biased properly and caused it to conduct. The gate voltage should be 0 and source should be 0.05V so that the gate is negative to the source.

I think either C11 or C12 may be leaking and supplies a positive voltage to the gate of Q6. Try disconnect both C11 and C12 from Q6. If the voltage of Q6 becomes normal, then find out which cap is bad.

If voltage of Q6 are the same after disconnecting both caps, then may be Q6 is bad. I have bought a few 2SK389 from a shop in Toronto a few months ago. I think they may be able to ship small quantity. Let me know if you find that your Q6 is bad.
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Old 1st November 2006, 07:54 AM   #10
Lucas_G is offline Lucas_G  Netherlands
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Quote:
Originally posted by RoboMan
I don't have a Pearl, but according to the voltages of Q6, it seems that Q6 is not biased properly and caused it to conduct. The gate voltage should be 0 and source should be 0.05V so that the gate is negative to the source.

I think either C11 or C12 may be leaking and supplies a positive voltage to the gate of Q6. Try disconnect both C11 and C12 from Q6. If the voltage of Q6 becomes normal, then find out which cap is bad.

If voltage of Q6 are the same after disconnecting both caps, then may be Q6 is bad. I have bought a few 2SK389 from a shop in Toronto a few months ago. I think they may be able to ship small quantity. Let me know if you find that your Q6 is bad.
Hi Roboman,

Thanks for your help. A clever thought concerning C11 and C12.

I have just desoldered them but the poblem remains, so I guess we can be pretty sure now that Q6 is rotten...

Thanks too for the address to order 2SK389 from.

Fortunately Promitheus has also offered to send me these from Germany, which will be faster and cheaper (customs...).

Thank you both! I will report back here when I have a new 2SK389 fitted, and when it works, I will also post some pictures here.

Regards,

Lucas
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