DIY GFP-750 Power Supply Repair - diyAudio
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Old 6th May 2009, 08:44 PM   #1
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Join Date: Apr 2008
Default DIY GFP-750 Power Supply Repair

Just got a call from an Adcom technician who told me that part of the circuit board for the power supply on my recently purchased GFP-750 was melted/fried (major power surge?), resulting in voltages high enough that he was surprised that it was working at all (~50V!).

A'gon ad had stated that passive mode worked fine, one channel was out in active mode, and that repair had been quoted at $100. Ouch, hurts to get burned.

So, instead of a stop-gap before I jump into a DIY pre, now this looks like a new power supply might become a DIY project itself.

Haven't located a service manual yet, so don't know the details on the GFP-750's power supply or how close it is to the BOSOZ or BOZ power supply.

Anyone with experience with these units or a service manual have any advice for a good power supply PCB?

Any thoughts on squeezing a new power supply in there vs. making the power supply external?

Without changing the toroid, any mods that folks would recommend
to improve from stock sound?

Pic from technician is attached
Attached Images
File Type: jpg adcom gfp-750 pre amp damage.jpg (76.0 KB, 233 views)
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Old 7th May 2009, 11:32 PM   #2
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Join Date: Mar 2005
Location: Bakersfield Ca
Since this forum is dedicated Pass Audio maybe you would have better results to ask in the solid state or power supply design forum. I know there are some service teks that frequent them, I have come across a few of their posts but I can't remember who they are. You may try some of the search functions.

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Old 7th May 2009, 11:36 PM   #3
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Join Date: Apr 2008
Thanks, I will try over there, but figured that since this preamp employs Nelson Pass circuitry this would be the best place to start, especially as Pass Labs DIY power supply designs have good odds of being a good fit for the GFP-750
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Old 7th May 2009, 11:56 PM   #4
Join Date: Oct 2005
Location: Central Florida
More pics are needed but it looks like a zener string like in the Aleph P.
Should be easy to replace. I have a couple boards for an Aleph P ps that I'll never use.

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Old 8th May 2009, 01:10 AM   #5
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I expect it will be similar enough to the simple BOSOZ.

I also believe Adcom can supply a proper schematic, perhaps at a
small charge.

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