Film-Tech Forum ARCHIVE: CP-750 cooling

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Posted by Ken Lackner (Member # 1002) on 04-05-2019, 07:50 AM:

What rackmount cooling solutions have you found to be effective at preventing CP-750 failures due to overheating?

Posted by Scott Norwood (Member # 30) on 04-05-2019, 08:48 AM:

Is this a known problem?

I can't speak to the CP750 specifically, but various manufacturers sell rack panels with cooling fans mounted in them. An example would be here:

https://www.rackmountsolutions.net/accessories/fans-filters-controllers/

Note that these things tend to be noisy (which might not be a problem).

Posted by Leo Enticknap (Member # 534) on 04-05-2019, 02:18 PM:

There are at least two known issues with the CP750, both of which have been the subject of Dolby service bulletins. The first is that a resistor in the power supply boards of early production units can go bad. Dolby used to offer a resistor swapout kit: not sure if they still do, but the cost of a new power supply board is probably about the same as that of the kit plus a tech's time in doing the fix, and so it really doesn't make sense to do anything other than replace the PSU.

The second is that prolonged operation of the unit in a very warm ambient temperature can cause a motherboard fault whereby the output signal level falls to an unusably low level. The fix is a new motherboard: again, the economics of doing that are such that a new CP750 makes more sense (or a 950 now!).

I have personally had to deal with three PSU resistor crapouts, and one mobo crapout. All four CP750s were in a warmer than comfortable environment: one was in a rack, the inside of which was in the 90s (of Fahrenheit) when I left my little portable thermometer in there for a few minutes.

Therefore, I'm of the opinion that keeping them at least down to room temperature is a good idea, even if that does mean putting up with some fan noise.

Posted by Ken Lackner (Member # 1002) on 04-05-2019, 04:08 PM:

Thanks, Leo. I had no idea about that field bulletin. I'll take a look at it. Perhaps it's worth some time and money to proactively replace the PSU's in these things. I had a failure a couple weeks ago; the unit would not power up at all. Replacing the PSU did not change the condition at all; I suppose it will need a new motherboard. From the prices I've seen, this is FAR cheaper than buying a new 750 or 950.

Scott, as Leo mentions, this is definitely a known issue among users of this product. I've had failures like what Leo describes (reduced output), and also total failures where the unit would either not power up at all (as mentioned above), or it does power up and all appears well but there is no output. That last one happened just two days ago. That unit was extremely hot to the touch when I went to remove it, so I am fairly confident overheating caused the issue.

In an unmanned booth with already loud projectors, I'm not concerned about a little fan noise.

Posted by Steve Guttag (Member # 268) on 04-05-2019, 04:10 PM:

While I've had a couple power supplies crap out (and we always put an emergency supply at every complex (there is a 4-pin XLR connector on the rear of the unit so anyone can hook it up). We've yet to have a MB fade out. I hear it affected early units.

Note, we always allowed 1* breathing room on it above/below so that may have contributed to our "luck."

On new builds, I always try to put in a cooling system by pressurizing the rack with fans that pump into the rack. This actually reduces dust as much as anything by keeping the rack pressurized, it won't settle in there. We use QSC amps so they draw air from inside the rack to outside so by filtering the air I pump in, they stay cleaner too.

And before you drop that CP950 in place of a failed CP750...be sure the CP750 wasn't using its multi-channel analog input...as the vast majority of ours do.

Posted by Ken Lackner (Member # 1002) on 04-05-2019, 04:20 PM:

I just read Bulletin Number 262 on the power supply varistor issue.

quote:
Dolby identified a trend of failing Dolby CP750 power supply modules in late
2014, and found through an extensive investigation, that the power supply
vendor had substituted a part in the power supply.

Does this mean that units produced in 2014 are subject to this failure, or that the issue was discovered in 2014?

Posted by Leo Enticknap (Member # 534) on 04-05-2019, 09:26 PM:

I'm guessing the latter. The most recent CP750 I fixed after its PSU went out was made in late 2012, according to the sticker on the back.

quote: Ken Lackner
I suppose it will need a new motherboard. From the prices I've seen, this is FAR cheaper than buying a new 750 or 950.

I wasn't trying to suggest that there is no significant difference in the up-front cost of these two options (replacing the mobo vs. a new CP750), but rather that if one of these two faults has already materialized, replacing the entire unit will massively reduce the risk of you being hit with the other - at least for a few years. Given that the cost in refunds alone of a single lost evening show would more than absorb the difference, my advice to a customer whose unit has the mobo level fading fault would be to buy a new CP, especially if they don't have a spare that can be swapped in while the bad one is awaiting the replacement mobo).

Posted by Mark Gulbrandsen (Member # 72) on 04-06-2019, 08:36 AM:

Oh my hell! All this goes without saying and is to be expected! [Roll Eyes]

Posted by Marcel Birgelen (Member # 6801) on 04-06-2019, 08:46 AM:

We're all eagerly expecting the Gulbrandsen I. The audio processor that will outlast the universe as we know it. [Wink]

Posted by Mark Gulbrandsen (Member # 72) on 04-06-2019, 09:55 AM:

I already installed close to 300 of them beginning back about 2010. Going on nine years and not one failure. But then again Dolby has that flakey power supply trait to uphold that actually began way back with the CP-50. [Roll Eyes]

Theoretically, thats the power supply is the easiest part of any powered item.