Searching Service Bulletin 262 CP750

[Dennis_Dirksen]

Hello, I am looking for Service Bulletin 262 (CP750 Varistor/PSU issues)
which Dolby issued for the CP750.
I have permanent tinnitus due to a bang trauma that was triggered in the cinema, and today I found out which systems use it and that there is a known problem with the power supply of the audio processor in the CP750. Does anyone else have similar experiences with the CP750 or reports that this device often bangs?

[Steve Guttag]

I have not heard of a complaint where the CP750 causes a "bang" in the theatre. However, I would consider any/all CP750 power supplies that were original to the unit to be in need of changing. Those supplies suffer from, among other things, bad capacitors that will be bulging by now with questionable power quality. For the most part, we no longer even put the internal supply back in as it is a heat-source for the unit. We've opted to just use external supplies.

Even with that, CP750 motherboard failure rates continue to escalate with the most common symptom being diminished output levels, starting with the surrounds. A smaller percentage will not boot up or will lockup shortly after booting.

[Caleb Williams]

Steve is right. All of our units have had to have power supplies replaced in the last 4 years.

I also have not heard a bang, but occasionally the units that show signs of failure will send a very loud buzzing to the speakers, causing a manager to panic and ask us to make sure they didn't blow a speaker. I have not found any blown speakers because of this, but usually it's so loud you can hear it clearly in the hallway outside the auditorium with the door shut. Sometimes, just power cycling the unit can get that to go away for a time, but if the power supply is failing it will always happen again.

[Dennis_Dirksen]

What I'm looking for specifically are examples.
There was a short loud bang and then the siren went off. First a very high-pitched sound and then a deep bass rumble.
I have now read several times that when there is a short circuit in the power supply, the internal test tone generators or amplifier stages in the CP750 deliver uncontrolled full power to all channels.

I also have a video that I could post. Maybe someone knows exactly what these sounds are.

[Frank Cox]

I don't really understand what you're asking here.

You have "permanent tinnitus due to a bang trauma that was triggered in the cinema", and now you're asking about what caused the bang?

Is the siren sound part of this tinnitus or is it a "real sound" that was generated in the theatre? Do you still hear a siren after having left the theatre?

Is this a recent event? I'm wondering about the "permanent" part of your question. If it happened yesterday maybe the sound will go away tomorrow?

Were you in the theatre as a customer or a manager/employee at the time of the bang? Did you make any inquiries with whoever was running the show at the time?

Edit: Have you checked with an audiologist regarding your tinnitus?

[Miguel Moreno]

What I'm looking for specifically are examples.
There was a short loud bang and then the siren went off. First a very high-pitched sound and then a deep bass rumble.
I have now read several times that when there is a short circuit in the power supply, the internal test tone generators or amplifier stages in the CP750 deliver uncontrolled full power to all channels.

I also have a video that I could post. Maybe someone knows exactly what these sounds are.

I guess the bang sound came from a bad amplifier

[Ryan Gallagher]

I would share the video before we get too far into the weeds. I assume you are staff at the venue responsible for fixing or contracting the fix?