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Repairing Hi-End Preamplifier
Repairing Hi-End Preamplifier
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Old 2nd April 2018, 06:47 PM   #11
jahsavage is offline jahsavage  United Kingdom
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Thanks a lot for all your help I shall have a read with a fresh head tomorrow morning.
Cheers
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Old 2nd April 2018, 08:42 PM   #12
stocktrader200 is offline stocktrader200  Canada
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the 2 diodes connected to the plate may still be ok as there is no high current path.
83uF as the output cap? this is HUGE.
as the diodes are shorted one of them may be ok when removed from the board as the board shorts their pins together. the 3 shunts accoss the 22uF cap are likely dead, 50 - 60 volt diodes here likely.
Guess here SX15 = 1.5ma TJ20 = 2.0 ma QR35 = 3.5ma current sources
5687 has max plate current of 36ma, bias is much lower then this as diodes did not run hot
JFET "diode" current sources usually have a voltage max of 50v across them before popping

Last edited by stocktrader200; 2nd April 2018 at 08:50 PM.
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Old 3rd April 2018, 12:04 PM   #13
jahsavage is offline jahsavage  United Kingdom
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So thanks to all for identifying the diodes as constant current and not plain old diodes or zeners. CRD or CLD diodes are new to me, I must have been hung over during that particular lecture but now at least I understand how the voltage regulation is happening!

While researching I discovered that CRD are quite rare and expensive and made by just a few manufactures so I'll try asking them to determine if the part ids are familiar.

I shall also try and determine the three zener diode values so I can determine the voltage at that mid point in the circuit and therefore deduce the value of the CRDs.

I'll keep you all updated with my progress – thanks again for your help.
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Old 3rd April 2018, 03:50 PM   #14
Aleks1 is offline Aleks1  United Kingdom
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That pre-amp looks like a Zanden 3000, I have repaired one of those previously. I believe they are a Japanese company, English may not be their first language.

Last edited by Aleks1; 3rd April 2018 at 03:54 PM. Reason: more details
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Old 3rd April 2018, 03:55 PM   #15
jahsavage is offline jahsavage  United Kingdom
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I confess its a Zanden 3000 Mk I which has been as good as gold until my accident while changing tubes. What issue did you fix?
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Old 3rd April 2018, 04:06 PM   #16
Aleks1 is offline Aleks1  United Kingdom
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It had battery issues and a few other things, it was a while ago so my mind is a bit blank. Recognised the build though.
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Old 3rd April 2018, 06:09 PM   #17
analog_sa is online now analog_sa  Europe
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Repairing Hi-End Preamplifier
Don't want to sound like an old auntie, but this thread upsets me a bit. The Zanden is a much better preamp than 99% of the so called high-end and destroying it while trying to learn basic electronics just ain't cool. The "circuit" as drawn already speaks volumes.

Hopefully someone experienced will take pity and prevent a cruel death.
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Old 3rd April 2018, 06:23 PM   #18
rayma is offline rayma  United States
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Agreed, and this is costly preamp. At least the mfr should supply the correct parts, though probably at a high cost. There may be other damage as well, if the power supply was connected after being powered on.
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Old 3rd April 2018, 09:21 PM   #19
stocktrader200 is offline stocktrader200  Canada
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The Zanden 3000 appears to be a Line preamp not phono preamp
you could request the parts from them first. then draw a correct schematic from the board
to ensure no mistakes are made. replacing with similar parts or resistors will work but the sound may not have the same flavour. Complicating matters, they may have matched the current diodes to provide identical currents to both plates of the tube. A transistor can replace a current diode but the exact current value must be known.

Last edited by stocktrader200; 3rd April 2018 at 09:27 PM.
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Old 3rd April 2018, 11:52 PM   #20
analog_sa is online now analog_sa  Europe
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Repairing Hi-End Preamplifier
I still maintain that an experienced technician should mess with the repair, but could not stop myself looking at a couple of pics

The minimalism of the power supply just boggles the mind. Hard to tell what are the exact values of the wrapped caps but perhaps a safe upper limit judging by size and rectifier would be 47uf. As the grid bias is fixed @3.6v, a very likely operating point would be 90v, 10mA. There are no candidates for plate resistors, so load has to be another ccs. I see a (very) small chance of the tube being a follower and the ccs being at the cathode, while the voltage gain is handled by the transformers.

Used LTSpice only because i had it handy. Actual transformers are not what the circuit indicates. D4/5 is the tube rectifier after the power transformer secondary. Whatever the cc diodes are, two are connected in parallel in all locations.

Of course i may be wrong.
Attached Files
File Type: pdf 5687 Zanden.pdf (24.2 KB, 192 views)

Last edited by analog_sa; 3rd April 2018 at 11:54 PM.
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