Dolby CP750 - level drop disease recommendations?

[Marco Giustini]

Is the level drop consistent across all channels or is one channel more affected than others? That might help pinpointing the area.

[Marcel Birgelen]

From my experience, it's pretty consistent all across the board, so that might exclude the channel-related caps.

1. Caps can be tested at various frequencies, maybe your tool was using some test signal which was more forgiving compared to the 750s?

My ESR meter uses 100kHz according to the manual. Also, according to the manual, the frequency should not affect the ESR reading much, especially for relatively low capacitance capacitors, like all of them are. The reference readings have been performed at 100kHz, according to the manual.

2. Some caps would misbehave when under voltage. I've seen a few myself. That said, I'd imagine it's very low voltage there so...

Not all failure modes can be found using an ESR meter and yes, some caps could behave differently under load. Also, there weren't any visually obviously defective caps to be found. But like you indicated, the processor is a "pre-amp", so the voltages and current should both be pretty low across the entire board, unless something is wrong...

[Marco Giustini]

From my experience, it's pretty consistent all across the board, so that might exclude the channel-related caps.

My thought exactly.

Why do I feel this is a specific cap which fails - or, better, they all "fail" but one is in a more critical part of the circuit where being 21% off is making a difference.
I feel that if someone could spend some time on a test board, it could reveal some important data which could greatly speed up any subsequent repairs.

One trick that could be used to ascertain where the issue is could be heat/cold. Heating/cooling an affected capacitor might temporarily restore its full capacity. I'm wondering whether someone could monitor a sinewave output while warming/cooling sections of the board to find out where the issue is. Once it's identified, start heating/cooling single capacitors in that area. Or start replacing them in that area.

[Tony Bandiera Jr]

I'm familiar with Rossmann as well, cool guy.

Electrolytics have 20% tolerance, I'm not so sure that a borderline one could cause issues - but can't say that for sure of course.
Guts feelings is
1. Caps can be tested at various frequencies, maybe your tool was using some test signal which was more forgiving compared to the 750s?
2. Some caps would misbehave when under voltage. I've seen a few myself. That said, I'd imagine it's very low voltage there so...

I was searching for a picture of the 750's MB and found this: https://www.chinahao.com/product/40964349364/

Yes, that's a lot of capacitors indeed. Knowing which one actually caused the issue would help. I suppose someone could replace them one by one and test afterwards. Once the bad ones are identified, then it could be easier?

The store in the link lists "200 in stock" with a price of $33.39 USD. Does that mean they have 200 of those MBs at $33.39 each? Or is the $33.39 what they charge to rework a bad board? In Ronda's case it might be worth finding out, because that low of a price would be a relative bargain in either case (shipping to the US is between $20-34, but the fine print says shipping weights listed for items are approximate and often under the actual weight) But lead time might be considerable, though for this item they claim DHL to be about 3-4
days shipping time.

I might just try to order one of these "Dolby Board Repair" in the near future and see what ends up arriving.

Edited to add: This is the price for the repair service, as I found out with some help for a translator online (Don't use Google's, it just spits out gibberish.) (Bolds, etc. added by me.)

The machine is a special-purpose decoder for theaters. Usually, the failure of the machine is sent to the United States for processing, which has long maintenance cycle and high freight. I am experienced in maintenance of this machine and have cooperated with the major cinema suppliers for a long time. Motherboard, power supply board are also provided. Accessories such as display panels. A common failure is to shut down and not control the volume, with noise or large distortion in one channel.

Maintenance coverage: It includes most European and American brands and most Japanese brands. FIFI Audio, Home Theater Audio, Conference Audio, Stage Audio

It has been used to repair sound equipment, registered companies, factory or upgrade accessories, professional maintenance equipment, more professional and demanding maintenance process.

1. Our shop is a maintenance store, 10 years of professional audio maintenance, Taobao store soon opened, so the credit rating does not represent strength.

2. This link is used to pay for the repair after we provide the repair service, not to sell the product described.

3. Most of the prices listed for POKÉMON are not final repair prices, and the actual prices are either evaluated after we have checked them or after Want Want has diagnosed them online.

We have advanced maintenance equipment and rich experience in sound maintenance in the country, as evidenced by the provision of a large number of maintenance examples. If your power amplifier, stereo, etc. product goes wrong, you will receive complete repair service through us.

The specific process is: Want Want contact — explain the fault — Package and send after consent — Offer after testing - customer service consent — Repair — Taobao guaranteed payment — Seller package and send back — Buyer confirm payment after normal test. Ship-to Address: Cheniandian Lane, Dongcheng District, Beijing Tel: 13641191493 53315819

[Devin McNamara]

Heating has been the one for me. It was this way for every one of those Doremi power supplies. I’ve pulled the cover folded the plastic cover out of the way. At this point I plugged it in. Of course the orange LED never came on like it should (indicating 5 volt standby power). After which I started heating it with my trusty radio shack heat gun. About five seconds of head near the TOPSwitch 5volt regulator and the power supply fired up. If I am quick I could shove it back into the Doremi and power it up. The catch is as long as it stays on it’s good but if you let it cool down it would fail again. Replacement of the caps has solved it. Point is even your wife’s hair dryer can be a pretty good diagnostic tool for hunting down failing caps. I suppose if i could get a hold of one I might be able to as I do have an oscilloscope.

[Marco Giustini]

In Ronda's case it might be worth finding out,

Mark all the caps, send a board, inspect the received board and see which caps were swapped!

[Paul Finn]

From Leo Enticknap earlier: "You will need to re-tune the room, because you can't import EQ settings from a 750". On a replacement basis it would be a manual process, but why would not the same equalizer settings on any equivalent equalizer result in the same or very close sound response in the room? Same amplifiers and speakers and same processor input and output levels. At the most, I think you may(?) have to tweek the room channels' SPL levels but even those should be very close to the original processor settings.

Paul Finn

[Stefan Scholz]

Electrolytics is a problem, if power fails, and remains out after PSU swap. That, as Marcel said, is one of the faults. Others involve chips, that are no longer available.
Even replacing a chip does not warrant the rest of the batch on the board is any better.
I can understand that repair is an option wanted, but neither, nor knowledge, nor tools are actually free in this world.
CP 750, at best an input source switch with limited EQ capabilities and a master volume unit, hasn't been anything great or something to be enthusiastic about. I've never liked them.
They have now served for 12+ years in day to day operation. They were cheap. So what problem is there to install something new and recent.
I've never attempted to repair. A CP 950 initially, before the price rises was a very good option. A flexible unit, modern, and incorporating everything needed today, still is a good option. With an upgrade path to immersive audio.
Yes, output is on the other side of the box, and it is IEC standard (Tascam, not THX anymore), a nice move actually, so you can use readily available cables for analog. AES 67 out, great to do networked audio integration.
My other choices were Datasat AP 25 and Trinnov. As Trinnov does not offer HDMI in and audio deembedding, I didn't really see the point to use it. Analog multichannelö? 20 years ago, probably yes.

So taking efforts and cost into consideration, go modern trash the CP 750 affected and fimiliarize with something new.

[Leo Enticknap]

From Leo Enticknap earlier: "You will need to re-tune the room, because you can't import EQ settings from a 750". On a replacement basis it would be a manual process, but why would not the same equalizer settings on any equivalent equalizer result in the same or very close sound response in the room?

Agreed: taking screenshots of the EQ settings in the 750 app, and manually entering them into the 950's web UI would likely sound better than leaving the 950 flat. But, IIRC, the 950's manual actually tells you not to do that, claiming that the way the DSP works in the 950 relative to the 750 means that you won't be getting an exact carbon copy of the EQ. Furthermore, if the room was last tuned when the failed 750 was installed, say, 8-10 years ago, then the characteristics of the power amps and speakers will likely have changed significantly over that time, meaning that a retune will be needed to take account of that.

[Sam Chavez]

Leo says, "Furthermore, if the room was last tuned when the failed 750 was installed, say, 8-10 years ago, then the characteristics of the power amps and speakers will likely have changed significantly over that time, meaning that a retune will be needed to take account of that."

The power amps and speakers don't really change over time but damage, wear and tear, and as you Brits say, "finger trouble" has more to do with it.

A retune is needed just to see if everything is order.

[Steve Guttag]

Back in the THX days, I did get to observer the sound changes in a screening room environment over a long period time (over 10-years). The only sound degradation that I measured was a gradual HF roll off that was almost immeasurable year-to-year but was measurable decade to decade. You'll get greater variations with humidity than the speaker itself.

[Sam Chavez]

Maybe clogged up holes in screen?

[Steve Guttag]

Nah...diaphragm fatigue. You can get sag in a woofer, particularly 15" and 18" and, over time, they can start to develop a rub. I believe Altec had a recommendation for driver rotation to combat sag. Furthermore, the older doping compounds for the woofer surround (compliance) was a bit amorphous so it would, over the years and decades, pool at the bottom of the woofer.

[Jon Dent]

I think this thread is cursed, as one of our CP750's just failed .

[Tony Bandiera Jr]

I think this thread is cursed, as one of our CP750's just failed .

It knew you were reading this!