Dolby CP 750

[Brian Davidson]

Hi I'm a long time theatre tech but new to Cinema systems. I'm learning on the fly. I was recently called in to the theatre I'm associated with for a "sound stopped working problem". It was intermittent at first and then stopped completely. The Dolby CP 750 unit was showing an odd random pattern on the front panel display. Then it went blank. My first thought was power supply issue as I've read several posts about it on here. When I checked it the next day after leaving it unplugged, it came on for about one minute and then powered off. Tried it again a few minutes later after pulling it from the rack, with the same results. My question since I haven't found the answer yet in the manual is, does this unit stay powered on or does it have a signal sensing circuit that powers on when it senses audio. A definitive answer will help me with my next step. Upgrading is planned but they want to get up and running asap. Cheers

[Leo Enticknap]

No; the display stays lit constantly if the unit is working as it should. The front panel LED flickering out is very likely a bad power supply board. The good news is that, the last I knew at any rate, Dolby does still have them available for purchase. Furthermore, they are not pricey (low three figures), and the swapout is very easy.

The other fault that frequently affects CP750s is the so called "level drop disease," whereby you can barely hear anything, even with the fader cranked up to 10. The cure for that is a new motherboard, and the bad news is that as a result of a recent a fire at a factory in Japan that supplies Dolby with D to A converter chips, they have had to bring forward their planned date for discontinuing the sale of replacement motherboards, and these are no longer available.

If you contact your Dolby dealer, you should still be able to order a power supply board.

[Steve Guttag]

The last we checked, Dolby is out of the CP750 power supply. It isn't discontinued, just out of stock with no set date of restock. We are OEM suppliers of that power supply and we were given a 16-week lead time on our last order. We have found that the capacitors on that power supply, in particular on the output side have swollen, after many years and can often be the source of its issue. That said, it is a relatively service unfriendly supply for recapping.

I wonder how many of the CP750 MB and PS failures would have been avoided had they put a fan on the unit. Right by the power connector on the motherboard is a pair of terminals labeled "Fan."

[Brian Davidson]

Thanks for the input. My initial thought was the power supply and now finding out the potential difficulty of finding a replacement, any recommendations for a replacement for the CP 750 would be welcomed.
Thanks

[Kyle Mikolajczyk]

I would suggest placing a post on the "Equipment and supplies WANTED" part of this forum if you are looking for another CP 750 and/or possible replacement parts. When we were looking for parts for our CP 650 people were very helpful

[Carsten Kurz]

You may have or dig up a used Dolby DMA8 - it uses the same power supply. With some soldering on the connector, you may also find a suitable external (backup) power supply, the voltages are not too unusual. The Dolby external backup supply is insanely expensive.
The internal Dolby CP750 power supply often suffers from an easy to fix cheap component breakdown. However, not sure if your device has the same issue. If it is the standard issue, it would be easy to fix (Dolby has a technote about it).

[Jim Cassedy]

CP-750's also show up on e-bay quite frequently.

[Mark Gulbrandsen]

Just recap the boards! I did that with a lot of 650's.

[Dave Macaulay]

"Just recap the boards!"
Have you looked at a CP750 board? There are a LOT of capacitors.

[Mark Gulbrandsen]

"Just recap the boards!"
Have you looked at a CP750 board? There are a LOT of capacitors.

There were a lot in the 650 too. I did it right in the booth in about 90 minutes, but also carried a new supply just in case that was not the problem. But it was always the capacitors because those were made back in the capacitor plague days. Worst part is they run a bead of glue along the tops of them. Never cared much for the 750. I mainly installed JSD-100's and JSD-60's once those came out.. So far not a one dead JSD-100 and going on ten years.

[Dave Macaulay]

That's the PSU... not the CP750 board. A Dolby tech told me the level drop could be fixed by replacing those many little caps. On a multilayer board... no, thanks.

[Marco Giustini]

I think the level drop is caused by resistors going out of specs - not capacitors. Those would be smd resistors. Never replaced them myself though

[Vern Dias]

Resistors don't go out of spec unless they have seriously overheated. Capacitors do deteriorate over time. Some were improperly manufactured some years ago which led to a much shorter life span for those products.

[Jon Dent]

There were several CP750s available for sale on the Equipment for Sale section last fall. Not sure if that guy still has any around.

In any case the CP750 is pretty old at this point and not getting any younger. I think a CP950 without Atmos is a few grand.

[Greg Routenburg]

Has anyone ever tried connecting an external power supply to a 750 using the backup power supply connector on the back of the chassis? Does it functionally bypass the internal supply? If the internal supply was completely removed, would it work exclusively with the external supply?