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Old 5th June 2007, 10:18 AM   #1
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Default Mark Levinson 38 Preamp Schematic

I have in the workshop a Mark Levinson 38 preamp with odd fault. There is NO volume control even though the 'numbers in the display' change with rotation of the volume control or with the remote control. The output remains at a fixed level (high).
It does work when 1st turned on so I guess it is out with the freezer spray on the processor/surrounding caps etc.
Anyone experienced this problem?
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Old 5th June 2007, 12:46 PM   #2
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How the volume control works:

After passing through a couple of opamps the signal is applied to the Vref inputs of a current output 12-bit 8222 DAC. If you're constantly getting full output this probably means that all data inputs to the dac are constantly high or the dac is blown - easy to check. If true check the shift registers feeding the dac and whether they are getting volume data/strobe from the micro.
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Old 5th June 2007, 05:01 PM   #3
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Hi
I sort of worked out how the volume control worked in the short time after posting the enquiry. What is strange is that problem is intermittent. I'll delve deeper this evening and keep you updated. Thanks for info.
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Old 7th June 2007, 01:43 PM   #4
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As suspected the DAC inputs are all constantly high so I'm back to the shift registers. I reckon the DAC is culprit but will know when the inputs are disconnected.
Anyone with schematic?
Ta
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Old 12th June 2007, 03:55 AM   #5
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Intermittent...
could be dry solder

Gajanan Phadte
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Old 12th June 2007, 08:27 AM   #6
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Hi
Dry joints on a Mark Levinson? I don't think so. Anyway the repair is progressing. The unit works when 1st turned on but then the volume control fails to operate on one channel after short time. All the inputs to the DAC on that channel remain high (no strobe).
The inputs to the 4049's are strobing when the control is turned. I have replaced the DAC8222 but no better. I am investigating the 4049's a bit deeper. Thanks for the suggestion.
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Old 13th June 2007, 04:08 PM   #7
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A pair of SCL4094 shift register chips did the trick
Thanks to everyone for their suggestions.
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Old 9th October 2011, 01:13 PM   #8
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Problem with a 380 now. All inputs fine except 4 designated SSP for a surround processor input. When this is selected it gives full volume and the volume and balance do not work.
Help!
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