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Repairing Hi-End Preamplifier
Repairing Hi-End Preamplifier
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Old 4th April 2018, 12:07 AM   #21
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
There appear to be separate CCS/zeners/decoupling caps for each channel. Chances are some of the current diodes are still alive. As they are paralleled, a short measured across does not guarantee both are bad. It is a very good idea to have them tested for current. Unless someone can recognise the type.

The batteries definitely need a change after 12 years.
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Old 4th April 2018, 02:04 AM   #22
anatech is offline anatech  Canada
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Repairing Hi-End Preamplifier
Hi,
Those lithium batteries are commonly good for 20 years + in test equipment as battery back-up. However, replacing them now that you have it apart is a good idea. These batteries are still manufactured by various companies. I just bought one for an HP 5372A, so I'm certain you can get them new.

Otherwise I agree with you completely as to the probable supply circuit.

-Chris
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Old 4th April 2018, 08:06 AM   #23
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
Oops. The pot and biasing cannot work as shown. Interposing them works but is a compromise. Perhaps this part of the circuit is different.
Attached Files
File Type: pdf 5687 Zanden.pdf (24.2 KB, 87 views)
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Old 4th April 2018, 08:31 AM   #24
jahsavage is offline jahsavage  United Kingdom
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So here's some more info, the power supply box has a semiconductor regulated supply for the 5687's heater. There are relays in both boxes to switch the final power on and provide a delayed power up. I believed the visible transformer and the one in the bottom right is just for balanced input and output (I think I read that in a review). As regards the capacitor values in my original diagram I read them with a fluke meter, generally but not in all cases with one end disconnected from the circuit.

I also posted a couple more images.

Last edited by jahsavage; 4th April 2018 at 08:32 AM. Reason: More inof
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Old 4th April 2018, 08:36 AM   #25
jahsavage is offline jahsavage  United Kingdom
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Tried again with the images.
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File Type: jpg undersideFull.jpg (902.8 KB, 438 views)
File Type: jpg aboveFull.jpg (1,006.4 KB, 428 views)
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Old 4th April 2018, 09:21 AM   #26
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
This is probably sufficient for an accurate circuit. The output Tamura is clearly identifiable, but what is the input?
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Old 4th April 2018, 10:24 AM   #27
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
Indeed, the input transformer is not used as a means to provide bias. Unless the zeners and current diodes can somehow be identified, these should all be removed and tested with the hope that some still work in order to establish the original bias points.
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File Type: jpg Zanden.JPG (57.0 KB, 416 views)
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Old 4th April 2018, 10:55 AM   #28
scottjoplin is offline scottjoplin  Wales
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What makes this preamp cost $17,000?
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Old 4th April 2018, 10:57 AM   #29
merlin el mago is offline merlin el mago
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Japanese craftmanship.
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Old 4th April 2018, 11:32 AM   #30
analog_sa is offline analog_sa  Europe
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Repairing Hi-End Preamplifier
That is why such objects should not be deconstructed but admired under a closed lid. Otherwise the temptation to squeeze a nice choke inside the PS may become overbearing.
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