CP750 problems thread

[Steve Guttag]

The CP750, as most now know, has a limited lifespan. It is also, for the most part, unrepairable and definitely no longer supported by Dolby except, perhaps the CAT 994-1 external power supply.

Its most popular failure point is the internal power supply. There was a field bulletin about an improper part on the supply but by now, the predominate issue is the supply itself where its capacitors are likely bulging, if it has been in use for a decade or so. Preemptively changing the supply is recommended and, better yet, move the supply to outside of the cabinet to avoid it contributing to the heat within the cabinet.

If you ever open up a CP750 after it has been powered on for some time, you'll find that the internal parts are quite toasty and this has likely accelerated its failure. It might be a good idea, from a heat standpoint, to remove the cover though this will defeat any sort of RFI/EMI shielding...so this may not be allowable in your area. Perhaps putting fans on the unit would help too and allow the cover to stay on.

Other forms of failure, aside from the supply are diminishing output levels. Most common, it starts with the surrounds. You will find yourself constantly having to turn the levels up (including on the amplifiers). However, this can spread to the other channels. You can also find yourself with little to no output on all channels however I think this primarily happens after a power supply failure (like the chips were damaged by bad power).

I have also seen front panel issues with the unit failure that include frozen buttons as well as a missing or frozen display.

Thus far, Dolby has not indicated which parts affect these issues and I don't think that they will as most people are moving over to something current like the CP950 (which uses different I/O so you will need adapters...even the side of the chassis is opposite, if your cables are dressed just so).

[Stefan Scholz]

Power supplies is one issue, high temperature another. CP 750 uses a certain type of electrolytic capacitors which are known to fail after a certain time. Not only on these products, also in other electronics using the same type of SMD.
Failing supplies can damage chips, as voltages could run high.
At least 10 years, often 15+ years in continuous operation, repairs are not recommended, if this is used in your source of income. A CP 950 is not that expensive.
You may find someone to repair the faulty caps, but other do fail afterwards.
Yes, there' s finally a standardized output pin configuration, which is widely used in the audio industry, unlike the previous cinema industry standards. Ready made cables are available at low price, so this is of no concern to me.
Just that CP 950 is lacking analog multichannel sucks, when there's still a film projection setup.

[Steve Guttag]

The electrolytics that I've seen failed were on the power supply itself. I wonder, as the power supply is losing its ability to provide clean power if the SMD versions near the various chips are not accelerated in their failure rate.

As for repairs, if the parts that degrade are known and the cost of replacement is significantly lower than that of a new processor, why not repair? For instance, and I'm just using very rough numbers...if a CP750 could be repaired for ⅓ to ½ of a new CP950, and the client needs no more functionality, why not? What is to say you won't get another 10-15 years...or certainly the life of the theatre. I come from an era where we repair things, if possible rather than merely replace. In fact, I still repair things, when possible. In the A/V industry, I was repairing a certain wall plate that uses Dante audio and the A/V people looked at me like I was a Martian ("oh-goody"). But really, it was just a $20 PoE board that failed that took an $800 wall plate down. I don't get people that don't think about repairing such things. You can make more money on the repair and the client pays less. Everyone is happier on the result.

As to the CP950 using "standard" cables...that doesn't help the person that has a CP75 that uses an industry standard pinout (THX...which was used on almost all balanced DB25 connectors for the past 40-years within our industry). The Last I checked, the CP950 isn't really used much, if at all outside of cinema.

As for premade cables to the "Tascam" pinout (so more to a specific company rather than an industry), yeah, I can see some appeal. The cables are not all that cheap, unless you use HOSA or like cables that use off brand connectors with pins that aren't exactly aligned to the shell (on the XLR style). Good quality cables are not all that cheap and nothing stops you from using an adapter cable/board to convert the THX to TASCAM, if you choose the pre-made method...or just pre-make your own using bulk cable.

And yes, Dolby should have had a multi-channel analog input option. No argument there.

[James Gardiner]

So, in other words, stay away from a CP750 if you can.
I see piles of them on China cinema second hand hardware sites....

I installed some early units, and based on the thermals experienced. I knew they had issues from the start. Quite a few failed in 6 months due to heat. Had to go back and move everything to give them more breathing space.

Did not touch them for my sites.

Considering this, what second hand sound processors are best? (If you can find them. As I expect the better ones, are going to stay in use.)

[Steve Guttag]

Yeah, I wouldn't start down the CP750 path. It a "when" not "if" it is going to fail. Perhaps, with clean power and if one wanted to modify it for better cooling...then...maybe it could have survived better. I'm not sure I'd bank on any second hand stuff. While I haven't had any issues with the JSD100, there is no support for it or any of the JSD line anymore. The DCP line from QSC really isn't supported anymore.

I'd say, really, just plan on a CP950, OV2 or jump to Q-SYS, as one's needs so dictates.

[Harold Hallikainen]

On lack of support for JSD series, it would be interesting if companies would open source the designs of products they are discontinuing and not selling to someone else (like the USL/QSC IRC or the QSC cinemat speaker line being sold to MIT). If a compnay is not going to make any money on the product anymore, give the design away! I guess there may be a concern that repairs would cut into new product sales. But it would be great to save stuff from the landfill.

I spet the weekend back here in San Luis Obispo ( https://contracarnivale.org/ ). SLO is the home of https://www.ifixit.com/ that does a great job on helping people repair consumer products. They also do product teardowns so you can see how a product was built. They also rate products on repairability. Great company!

[Marco Giustini]

it would be interesting if companies would open source the designs of products they are discontinuing and not selling to someone else

ha! Not going to happen in this universe. If someone can fix the old product, it means the new product won't be sold.
Don't get me wrong, it would be nice indeed. But unless there is some legislation to force discontinued products to be fixable...

[Frank Cox]

I've always thought that copyright reform could go at least some way toward solving this kind of problem.

If your book/movie/software/whatever is commercially unavailable, the copyright immediately lapses and anyone can reverse engineer, modify, reuse, fix and re-release it.

That would solve a lot of problems with things like out-of-print books and, in this case, unsupported hardware. If the original company is no longer interested in selling it, then someone else could.

[Steve Guttag]

I'd go a step further...if you want your design copyrighted, you have to submit the schematic. That is what is being copyrighted. Once upon a time, all products came with schematics and that didn't stop a vibrant market of competitive ideas (witness the stere/hifi market of the 1970s or even Dolby in the 1970s/1980s.

As for worrying about competing...QSC is actively getting out of the cinema market. I don't think anyone that is looking to keep JSD (or even DCP) products alive will cut in on Q-SYS one bit. They still have the DPM line but (and I don't have any inside information on this) there is no way that they keep that going once they've depleted their parts supply to make them. I predict that you'll see the DPM and DCA amplifiers go EOL, at that time. The new MPA-Q amps are going to satisfy their need for an "analog" amplifier since they have a mechanical switch on the back to put it in standalone mode (so you don't need Q-SYS to use them). I would presume that they will expand that line to replace the CX-Q line. At that point, they will be pretty much on new technology (post-C19) for the entire Q-SYS line up. That's my belief, at least.

[Marcel Birgelen]

Around when COVID started, I've "recapped" a dead CP750. I've reported about it here. I've measured all caps with an ESR meter and I found none to be "out-of-spec" but a few to be close to be out of spec. That's when I decided to replace them all, which ended up fixing the issue.

After COVID, I've donated it to a place that was struggling to make ends meet, it's been running there ever since, without any major issues. Keeping the thing well-ventilated is probably part of the deal. I've not replaced any of those micro-SMD caps b.t.w. While it's not impossible for those to fail, most of them are made from thin ceramic sheets and not from "paper drenched in electrolytes", so they essentially cannot vent-out.

I'd go a step further...if you want your design copyrighted, you have to submit the schematic.

That was exactly my proposal too. It's similar how patents work. If you want protection, file your schematics with a copyright office. Once you abandon the product, the schematics will be published. While this doesn't guarantee repairability, it makes it a whole lot more achievable.

[Steve Guttag]

Did you change ALL of the SMDs or were you more targeted? How long did it take you to do the recapping you did?

[Marcel Birgelen]

I replaced all the "electrolytic capacitors", first on the motherboard and later also on the PSU. I started with the motherboard, because I did have a replacement PSU back then and I knew that it wasn't the issue.
The issue here wasn't the volume drop-off, the motherboard simply didn't want to boot anymore.

The reason for replacing all the electrolytic caps was because I couldn't isolate problematic ones and without any schematics and the motherboard being a multi-layer circuit board, it was very hard to point to a specific region being the problem.

It was a pet project back then, more like occupational therapy than anything else. I lost my brother not long before, then we lost our then major contract due to COVID shutting them down, so I decided to take a few months on the back burner. The entire process probably took about a month and a half, from measuring all the caps, figuring out replacements, ordering them and eventually replacing them. I didn't log the time, as I would've done, would this been a billable project, but it was many, many hours. My soldering skills also were pretty rusty, so it might have taken me quite a while longer than anyone doing those kinds of repairs for a living. Still, at any normal commercial rate, this wouldn't have been viable operation, even back then, when the CP750 was just declared EoS by Dolby and still went for quite a bit more than it does today.

[Stefan Scholz]

Steve, I also had the reputation since my high school days that I do and can repair everything that's electronic. (Well, not everything) And I also lived up to that to keep things in operation as long as it makes sense, and no go and buy one new. Having done networked sound systems since digital cinema arrived, first on Yamaha, later diving into QSys, processors like these were for the cinemas without a real budget.
We mostly did USL 80 or 100s, and they do a very reliable job, without any issues so far. Very rare to have failures. And later we did AP20s, where crappy plastic fan and switching supplies are encountering a failure rate over time. Both could be replaced, if the voltage in a failing supply didn't run up and damaged the chipset. And I also had short circuits on electrolytics on the backplane board, that shorted out the PS. They are similar to the ones in CP 750s.
Power switchers are a normal item to swap, on any equipment. Size and power categories are standardized, and buying a reputed brand as a replacement an easy and quick fix.
I also saw the piles James has seen on the web, piles Chinese folks want to sell. Ok, they are a modern society, they are rich these days, and they charge, in comparison to wages, high admission fees.
So I can assume they swap equipment likely more often, when something has a tendency to fail.
Today's boards are mainly multilayer, they use SMD components, and my eyes are not getting better with age. Today's designs offer a high level of reliability on reputed brands, and a new complexity to repair them, so I may swap for a new unit with new warranty and enhanced features. I know, from the time needed, you eventually end up at half the cost of a new 950. The old one may work, and may fail again. The type of discussion with a customer I do not really need. To make proof it is another failure point.

and last, James asked, which second hand units are best? I found a USL 80 in an independent cinema. THat's been "wrapped" in a clear polycarbonate shell, had a 2 digit serial number, and definitively no USB or Ethernet port. Looked like a very early Show West demonstration unit, and has been in operation ever since, without any repairs so far, at least to the owners of the place.
From the type of housing, certainly not FCC or CE compatible.

[Steve Guttag]

I'm having JSD80s fail, at this point. Not in a massive way but the cards are starting to go and without a repair source (nor do I know what on them are failing). We sacrificed a couple of removed units for parts to keep existing units going but have exhausted those cards now. So, I would not favor a JSD80, at this point. JSD60s can definitely get flaky on their outputs but the JSD100, thus far, has had a near perfect record for me.

On the CP750 repair...whereas the problems seem to stem from power supply and capacitor, I'd think, all-in, a repair could be in the ⅓ of new price range. If so, then the reliability is bound to be high and the cost savings significant. Exhibitors (multiple) have expressed a desire to hold their own (not improve) if it can save significant money. These past two years have been notably lean years. Naturally, we do offer current-model replacement systems. I would argue that there is no reason to believe that current offerings will be more reliable than the CP750, however. Typically, older technology has a higher degree of stability given that it is less complicated.

[Carsten Kurz]

A colleague has volume adjustment encoder issues on both his JSD-80 and JSD-100. Not as critical as the typical CP750 issues, as volume can still be set through remote or network. And they can probably still be replaced rather easily, if similar parts are still available.