CP750 - level drop disease presented shortly after power supply swapout

[Marcel Birgelen]

It was a pandemic project... I bought a new ESR meter and I had a new, shiny soldering station standing around I never really used. The particular CP750 was collecting dust on the shelf and I had to decide to get rid of it or to give it a shot, so I did.

I limited myself to all non-SMD capacitors. SMD capacitors are of the ceramic type, not that they never fail, but their failure mode usually is a hard fail.

I did in-circuit testing, which isn't ideal and often fails, but in that case, I simply replaced the capacitor all together, to reduce overhead. In the end, the thing came back to life...

Yeah, I even did give it a proper ultrasonic rinse to get the flux off.

It was a lot of work, a lucky shot and no, it's not commercial viable, especially not for a guy that solders as slow as I do...

[Marco Giustini]

My understanding is that ESR in-circuit will give an accurate result OR a worse one - so if the ESR meter says "good" it should be good. What puzzles me is that you didn't find anything out of the ordinary but replacing all the caps fixed the issue.

Do you have a picture of a 750 board? I've seen it before but I cannot remember how many caps are there.

I suppose that hot tweezers and also some pre-heating might speed up the process considerably on such a board.

[Steve Guttag]

There are a gazillion SMT capacitors on that board. If it is just the few through-hole...then changing those wouldn't be an issue. Depending on how the day goes, I'll have access to the MB today to snap a picture.

[Bruce Cloutier]

I agree with Steve. It seems daunting. I would likely have reversed the design somewhat and tested what I thought were the critical capacitors first. Test the unit for improvement and then move on to the next caps if it still seemed compromised. I might also replace these with higher voltage caps maybe with better temperature characteristics.

It always pains me to discard a device that served me well and that would continue to serve had it not failed. I think a lot of things get tossed that might need only a firmware update. Seems broke so... time for a new one. So much for saving the planet.

Once this issue raised its head too, if I were the manufacturer I would have likely have changed something. I might have even had inventory reworked. It's an expense that shareholders don't appreciate.

[Marcel Birgelen]

There are a gazillion SMT capacitors on that board. If it is just the few through-hole...then changing those wouldn't be an issue. Depending on how the day goes, I'll have access to the MB today to snap a picture.

If I remember correctly, most of them are to the "left" on the output part of things. I replaced a total of 23 electrolytic SMT capacitors.

The plan was as follows:

- Look at any capacitors obviously gone bad. There were none of them.
- Skip all the tiny surface-mount capacitors, as they're usually ceramic.
- Inventarize the hole-through-style caps for replacement. (According to my notes, there were none)
- Measure all the "big" caps with the ESR meter, in circuit. From what I've read, in-circuit measurements are usually pretty accurate, only big inductive loads like motors seem to be able to throw it off considerably. There are none such loads on a disconnected motherboard.
- The ESR meter usually indicates a fault if in-line measurement doesn't give you a reliable result. All those caps got flagged for replacement. This happened 6 times.

In the end, none of the measurements exceeded the "good value range", but about 20% of them were close to the edge, I decided to flag them for replacement.

Then I ordered a bunch of replacements, as close to the originals as I could find them from a reputable source. After my shipment got in, I had a a few lonely nights training my soldering skills.

As for reversing the design: I gave up on finding a smoking gun after none of the most suspicious caps gave any real bad ESR reading.

Edit:After scrolling through my notebook, found some scribbles from back then, adjusted stuff accordingly.
Edit 2: This was a personal thing and a hobby, if you need to get your hours paid, it's certainly not worth the effort, unless you're running some Chinese sweat shop. I later donated the CP750 to a theater in financial troubles, that had a CP650 with a dead PSU. The thing is still in there doing fine.

[Bruce Cloutier]

A lightning strike took out my Klipsch subwoofer. I rebuilt that even though the insurance would cover a replacement. That was a bit crazy too. I had to replace some ICs. That was from the days when custom ICs weren't the rage. But, I could never convince myself that I had restored its performance to near original. Klipsch was helpful then with schematics. Even the insurance company paid out for my labor and, I didn't ask for it. After a few years there was a hum. So I upgraded. That was probably just a cap too.

To Leo's question though, you have to wonder if disturbing the eco system might not have brought on the failure mode. Sometimes PM isn't as preventative as you hope. Falls under the 'If it ain't broke' clause.

[Mark Gulbrandsen]

I've recapped many computer motherboards over the years. Recapping is actually pretty easy, but you do need the correct tools to safely pull it off. A vacuum type solder sucker, solder wick, correct solder and lots of patience. I did probably close to 40 XD-10 motherboards , many right in the booth they lived in. Send me your CP-750 main boards and I'll do it for you!!

[Steve Guttag]

Here is what it looks like. All of those silver caps on the output side would be a nightmare to go through unless on is really board. The handful of larger ones wouldn't be so bad. Hot tweezers would be the tool, I think for doing a mass change out (and some flux). Using a normal hot-air would likely blow the wrong components off while trying to get at the capacitors. To use a normal pistol type solder sucker...one is probably better off, first using hemostats to just twist the capacitors off, first...then clean up the pads.

Here is what it looks like. All of those silver caps on the output side would be a nightmare to go through unless on is really board.

20221012_171507.jpg

[Leo Enticknap]

Are you sure it is the XC-1? I've normally found that the input attenuators are the evil ones.

Yes. When I moved an XC-1 between amps, the fault moved with it.

[Marcel Birgelen]

Here is what it looks like. All of those silver caps on the output side would be a nightmare to go through unless on is really board. The handful of larger ones wouldn't be so bad. Hot tweezers would be the tool, I think for doing a mass change out (and some flux). Using a normal hot-air would likely blow the wrong components off while trying to get at the capacitors. To use a normal pistol type solder sucker...one is probably better off, first using hemostats to just twist the capacitors off, first...then clean up the pads.

Here is what it looks like. All of those silver caps on the output side would be a nightmare to go through unless on is really board.

I'm not your go-to soldering expert and stuff may also depend on your personal prefferences.

You need some decent equipment for this kind of work. Not your granddad's 120 year old soldering iron, but a proper station that can control the temperature should be on your list. Wether you prefer an "old-fashioned" soldering iron or a pistol style tip, is really up to your preferences. You also need some decent magnification, unless your eyes come with spiderman vision.

I also used two irons, one for desoldering and one for soldering, set at a different temperature. I'm not saying you need to, but it worked best for me.

A hot-air tool isn't the solution, unless you want to strip the board clean. I've used a hot-air tool for some SMB IC assembly, which is really efficient, though I'm always worried to cook the IC that way.

Hot tweezers may work for some, but for me, they didn't really work for me. Maybe, because you have to focus on two ends instead of one. If you can handle it, it will probably speed up stuff significantly. I'm not sure if they work in many of those tight spaces on the output side of things though.

You need to get the iron to the right temperature for the particular solder, get both sides hot, try to get it off with a bunch of tweezers or your favorite component-wiggle-tool. Since you're replacing the thing anyway, no need to care if you're cooking the cap or not. Once it's off, clean the pads with a bunch of solder wick. I also bought some newfangled solder-suckering device, but I probably don't understand how to use it or it's just outright garbage. The soldering wick method worked well enough though, but it leaves quite a lot of flux on there.

Then for resoldering, I used a different iron with a different temperature. Resoldering was far easier than I expected. Put some soldering grease/flux over the pads, put the component down and touch both sides wtih the iron and you're done.

With all the flux on there, you really need to give the board an ultrasonic cleaning afterwards. Flux attracts moisture and moisture and electronics aren't the best combination.

[Marco Giustini]

I'm not familiar with the 750's board but if it's multi-layer and has extensive ground planes one issue is that the board will suck away the heat from whatever you're using. Ideally you want to pre-heat the board using a pre-heater or at least use some hot air around the area you're working on.

Some tips I can think are
1. Re-flow the solder pads using lead solder first. That will lower the melting temperature (lead-free melts at 220C, leaded solder at 180) making things easier.
2. Don't apply solder on both pads when replacing the component. You won't be able to melt both pads simultaneously (unless you use hot-tweezers or two irons) and the component won't sink into the solder. Instead, either clean the pads and then apply solder when the component is on - or apply solder on ONE pad, so that the leg of the component can sink into it when melting. Then you do the other leg.

Everybody has different ways they like though. What's important is to understand that if things don't work, cranking up the temp to "make it work" is not a solution! (Well it is but components are not designed to work that way).

[Leo Enticknap]

While attempting board level repairs of this intricacy is great for personal education purposes, it's not economically viable to do on a significant scale in the field, except in unusual, bordering on unique circumstances - e.g. if an EOL device has no, even close to direct replacement that would not incur a bunch of other costs to install as a replacement. What makes the CP750 situation especially frustrating is that the 950 is not a drop-in replacement: jacks on the back are situated such that cabling in the rack might not be long enough, the DB25 pinouts are different (Dolby will sell you an adapter to work around that, but IMHO they should be giving you one in the box), there is no 5.1/7/1 analog input, and you have to retune. But even jumping over those hurdles makes more sense, IMHO, than spending hours replacing dozens of capacitors, with no guarantee that the thing won't break again a couple of weeks later, and the support needed to fix it quickly and easily (i.e. the sale of a new motherboard) not being available if it does.

[Steve Guttag]

Leo, if the failure point is really the capacitors...then there is zero reason to believe that such a repair won't kick the issue down the road 10+ years. The reason I'm considering doing board repairs on them (for a hefty charge because just look at the time on it) is that it allows one to make it a field repair or even a repair exchange if you have a CP750 seed stock. No retuning or rewiring needed...which comes off of both the cost of the board repair (which, honestly, is probably on the order of the cost of the CP750MB, when it was available) and it will be even more cheaper than a new sound processor that won't sell any more tickets.

The reason we have been repairing the power supplies is because we couldn't get new ones for 16 weeks. A repaired supply costs twice what a new one does, when available but if you are down, it is a bargain. It isn't a get-rich quick thing...it is what it costs to pay a person to actually do the work (plus the handful of parts). But again, if you are down...bargain.

I do long for the days of being able to just order things and they show up in a reasonable amount of time (or overnight, if so ordered). Let's all hope that 2023 gets us much closer to that day again, albeit with some wounded on the battlefield (products, brands that are no longer with us).

Here is a handy video on how to remove SMT capacitors without desoldering! I highly recommend Mr. Carlson's Lab videos. They guy is incredibly good/knowledgeable.

https://youtu.be/X8N9O3a9jiM

[Bruce Cloutier]

It is hard to believe that there isn't just one component or solder joint causing the level drop. While the shotgun approach to capacitor replacement leaves the board refreshed, it would really help to find out where in the circuit supply, bias or reference voltages are compromised. Carsen (I think) mentioned some in-rush current circuit failure. Whatever it is, the repair might not consume evenings of meticulous soldering if you know exactly what is at fault.

I agree with Leo that a replacement is probably a more longer term solution. Certainly it is less risky. It is also concerning that in the CP950 design someone didn't think that changing connector position and pinouts would impact the upgrade/replacement market. And, are you really sure that the CP950 won't have cap issues down the road?

Of course, designing in some obsolescence has some business merit.

[Steve Guttag]

Dolby has received flak about the pinout change...a pinout that dates back to the mid 1980s (the THX standard). The argument for the Tascam design is that one could walk into "Guitar Center" and buy a premade DB 25 to XLR cable set in most major cities rather than depending on custom wiring or industry specific cables.

To make matters worse, they transposed all of the connectors from the power input to the audio outputs (left for right) so no existing cables will reach their new destinations. The CP950 omits the multichannel analog input so it isn't a drop-in replacement for any system that is using the CP750's multichannel input. A perfect upgrade, it is not. I think the adapter cable is sufficient support, however, for those that are swapping the units as it takes it off the installer having to redo existing cables. If it were me, I would have stuck with the THX standard. Either make your cables for a net job or go to Odyssey/DargCo to adapt.

Remember too, the CP950 is AES67 out capable so one outputs can be a mere single CAT cable to a Dolby DMA or into an AES67 network.