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Old 25th December 2012, 11:53 PM   #1
6L6 is offline 6L6  United States
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Default Pearl Phono (original) problems

Here's the skinny - I absentmindedly had my PSU turned on while I was plugging the umbilical and had it not lined up properly and something zapped.

The PSU is fine, and one channel is fine. The bad channel measures ok except by the final gain stage.

Here are the voltages, the good channel is 'G' and the bad is 'B'

The ZTX has already been replaced with a fresh one. (I didn't test it, and am just assuming it works...)

The obvious next course of action is to replace the K389, (with a pair of matched K170's, because, lets face it, K389 are complete unobtanium these days...)

(And I''ve no idea how to unsolder a 7-pin widget cleanly...)

Does anything else seem wrong?

Thanks in advance.

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Old 26th December 2012, 12:15 AM   #2
Zen Mod is offline Zen Mod  Serbia
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check that ZTX trice , same as those two 10K resistors

these two 10K-ers are just voltage divider and you must have there Ub/2 , not some wakoo point something level

desolder 389 with vacuum pump or solder wick

bloody decade long DIY-er - doesn't know how to desolder 7-legged fuse
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Old 26th December 2012, 01:03 AM   #3
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If the voltage has been connected wrong then I would replace semiconductors and the electrolytic.
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Old 26th December 2012, 01:34 AM   #4
6L6 is offline 6L6  United States
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Quote:
Originally Posted by Zen Mod View Post
bloody decade long DIY-er - doesn't know how to desolder 7-legged fuse
I'm relatively new to solid state and PCB. Most of my projects in the past use toobz. Replacing those active devices is easy!!!!
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Old 26th December 2012, 02:09 AM   #5
6L6 is offline 6L6  United States
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I had time to check ZTX before running out of time (my workbench is in storage room accessed through toddler's room... it's a small house...)

hFE is good. Measures in the ballpark of other ztx I have on hand.

Resistors measure (in circuit) the same as the other channel.

I'm more confused.
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Old 26th December 2012, 02:53 AM   #6
Zen Mod is offline Zen Mod  Serbia
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desolder both ZTX and 2SK389

check 389 with Idss test
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Old 4th January 2013, 03:20 PM   #7
6L6 is offline 6L6  United States
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Ok, I finally had time to fix it...

The K389 was no longer a live parrot --- it is no more, it ceased to be, bereft of life, it rests in peace.

The circuit wasn't drawing current, therefore the measurements higher up the chain were getting strange.

I replaced it with a matched (Idss) pair of K170, and everything measures fine now. Will listen to it next, once it's buttoned back together.

Gentlemen, thank you for your help , it was invaluable.
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