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Old 8th April 2012, 05:09 AM   #1
tnargs is offline tnargs  Australia
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Default Pearl Phono 1 - diagnostic help please

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).
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Old 8th April 2012, 07:28 AM   #2
mjf is offline mjf  Austria
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hello.
can you have a look at r24 (100k),perhaps contact lost.......bad soldering point or so.
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Old 8th April 2012, 07:50 AM   #3
tnargs is offline tnargs  Australia
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hi mjf, thanks for your comments.

I just looked into your suggestion and can see no problems. All connections around R24 are good. R24 itself measures 100k.
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Old 8th April 2012, 10:11 AM   #4
mjf is offline mjf  Austria
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i woud look at q4........the volt reading at the emitter - and collectorpins are very similar, looks like some kind of short? (built in reverted,soldering bridge or something else....)
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Old 8th April 2012, 11:05 AM   #5
mjf is offline mjf  Austria
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......i think there is something built in wrong/reverted.......if q6 is fully conducting........could be a mistake.
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Old 8th April 2012, 01:59 PM   #6
tnargs is offline tnargs  Australia
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thanks mjf, I will look into it tomorrow and get back
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Old 8th April 2012, 05:59 PM   #7
wayne is offline wayne  United States
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The base of Q4 should be fixed at half supply. Heavy current due to a short can pull it low through the emitter. The gate of Q6 should have almost no voltage unless Q6 is bad or the coupling cap is leak and that is pretty rare for a film cap.
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Old 9th April 2012, 08:07 PM   #8
6L6 is offline 6L6  United States
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2SK389 are pretty rare anymore - are you sure that your replacement was genuine?
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Illustrated Build Guides - Pass F4 - Pass B1 - Pass F5 - Pearl 2 - O2 HPA - Hagerman Bugle 2 - Pass Aleph J - Me at BA '13
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Old 9th April 2012, 10:43 PM   #9
tnargs is offline tnargs  Australia
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Maybe it wasn't.... where DO you get a genuine 2SK389?
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Old 9th April 2012, 10:45 PM   #10
6L6 is offline 6L6  United States
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I haven't the foggiest... The last one I got was most of a decade ago. Sorry.
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