I have this console with 28 mono+4 stereo channels with 1 850 PSU (no Virtual Dynamics),when powered on after 2/3 hours +17,5 volts drops down to 12 volts causing a lot of noise thru the tape inputs (wind/bubbles noise) due to IC malfunction.A competent tech had look and replaced some components (trimmers,small electrolitics) and checked the voltage regulator the other semiconductors and big electrolics and told me that everything seem ok.After the intervention the voltage drop is only 2 volts over 17,5 but enough to still worsen the s/n ratio of the console,everything happens whit the PSU transistors are very hot after at least 2 hours of use,if I switch off then for some hours is fine.i even tried to replace the regulator U1 and Q2 but the problem remains.Any suggestion from the experts?? Regards.
Big by Langley PSU voltage drop
- Thread starter Pomello
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Help Support GroupDIY Audio Forum:
mitsos
Well-known member
- Joined
- May 4, 2007
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- 2,886
Is your PSU like the one in the pic?
These PSUs are known to go into thermal shutdown, google and you'll find a bunch of info.
A big problem is that the regulators and transistor are on sockets, and heat doesn't transfer efficiently enough to the heatsink. Colin at audio maintenance made a PDF with pictures of the modification process, send him and email if you want to DIY. Basically you remove the sockets, placing the regs and transistor directly on the heatsink (keep the stock mica insulators!) and solder everything directly to the PCB.
Here is a link to GRS's ebay post (where I got the pic from). There is a list of what they do to the PSU. It's not too expensive, as it will probably take you quite some time to get everything done. But this is group DIY... so, I would at least do the mod, recap and thorough cleaning (dust bunnies = heat buildup). Make sure to replace ALL the electrolytics, especially the large main filter caps, since you're already taking the thing apart. Up those main caps to 15000 or 20000uF.
http://www.ebay.com/itm/AMEK-TAC-MATCHLESS-POWER-SUPPLY-REBUILD-UPGRADE-750-850-/290600023454?pt=US_Live_Studio_Mixers&hash=item43a91c759e
These PSUs are known to go into thermal shutdown, google and you'll find a bunch of info.
A big problem is that the regulators and transistor are on sockets, and heat doesn't transfer efficiently enough to the heatsink. Colin at audio maintenance made a PDF with pictures of the modification process, send him and email if you want to DIY. Basically you remove the sockets, placing the regs and transistor directly on the heatsink (keep the stock mica insulators!) and solder everything directly to the PCB.
Here is a link to GRS's ebay post (where I got the pic from). There is a list of what they do to the PSU. It's not too expensive, as it will probably take you quite some time to get everything done. But this is group DIY... so, I would at least do the mod, recap and thorough cleaning (dust bunnies = heat buildup). Make sure to replace ALL the electrolytics, especially the large main filter caps, since you're already taking the thing apart. Up those main caps to 15000 or 20000uF.
http://www.ebay.com/itm/AMEK-TAC-MATCHLESS-POWER-SUPPLY-REBUILD-UPGRADE-750-850-/290600023454?pt=US_Live_Studio_Mixers&hash=item43a91c759e
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Slenderchap
Well-known member
Usually this fixes it:
www.audiomaintenance.com/downloads/750_series_to3_socket_modification.pdf
Colin
www.audiomaintenance.com
www.audiomaintenance.com/downloads/750_series_to3_socket_modification.pdf
Colin
www.audiomaintenance.com
