Film-Tech Forum ARCHIVE: CP750 - main LCD display flickers when mute button is pressed

This is topic CP750 - main LCD display flickers when mute button is pressed in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=003477

Posted by Leo Enticknap (Member # 534) on 10-23-2018, 07:13 PM:

When the mute is engaged, the main (circular) display flickers continuously. When the mute is disengaged, the flickering stops and the illumination is steady. The mute button red LED doesn't seem to be flickering at all - the slow flash is reported to be steady.

Could this be an early warning sign of a PSU failure? This is quite an old unit (manufactured in 2012), and the PSUs on early 750s are known to be unreliable. The theater involved is a very remote one, and none of the staff there would be confident to swap out a PSU, so I'm veering towards recommending replacement at the next planned maintenance visit.

I have the settings file from that unit safely saved and archived, so if it does kick the bucket, we could ship a preconfigured replacement for them to plug and play. The cost of one of us getting there is about the cost of a new CP750, so if emergency action is needed, this would be the more cost effective option. But obviously I'm hoping that it'll last until the next planned maintenance trip (probably next summer - we go once a year, and I last went in August), and if that is the problem, the PSU can be replaced then.

Posted by Harold Hallikainen (Member # 5405) on 10-23-2018, 07:49 PM:

Does it flicker only when muted? Did it just start doing this? I would send a preconfigured unit just in case it is not the PS. That would fix it for sure.

Harold

Posted by Juan Jose Vazquez (Member # 10496) on 10-23-2018, 08:34 PM:

Yesterday I had a similar failure, the front panel does not respond and when I touched the mute button the buttons were turned on but the display showed nothing. The problem is solved by resetting the communication cable from the front panel to the main card.

Posted by Leo Enticknap (Member # 534) on 10-23-2018, 09:41 PM:

The main front panel display flickers only when the CP750 is muted. Immediately the mute is released, the illumination becomes steady again.

I must admit that a mis-seated cable didn't occur to me. That booth can get pretty hot, and so it can't be ruled out.

Not sure where to go from here. Moving towards letting them know that any time they'd like to buy a replacement, preconfigured CP750 they can, but otherwise, we'll measure the output of the existing PSU and check all the internal connections on the next planned maintenance visit, and go from there.

Posted by Harold Hallikainen (Member # 5405) on 10-23-2018, 10:20 PM:

As mentioned before, I wonder if this is a new problem and if anything in the system changed when the problem appeared. I don't know the internal circuitry of the CP750, but on the JSD-100, the front panel bar graphs are driven by voltage measured at the outputs of the processor (this is different than the JSD-60 where the bar graphs are driven by the DSP before D/A conversion). On the JSD-100, we originally had 100k pull-down resistors after the mute analog switches. This high impedance caused some amplifiers to oscillate when the mute switches were open. IF the CP750 similarly measures the output at the rear panel (after mute switches), this COULD be a similar issue. It could be checked by turning off the amplifiers and see if the problem goes away. Further, with the amplifiers off, unplug the amplifiers from the CP750 and see if the issue is still present when muted. On the JSD-100, we resolved the issue by dropping the pull-down resistors to 10k.

On trying to fix something remotely where you really don't know what the problem is, I like just sending a replacement unit. That way you KNOW you've solved the problem. It may or may not be the power supply. If it isn't, you've put time and money into something that did not solve the problem.

Good luck!!!

Harold

Posted by Carsten Kurz (Member # 5396) on 10-24-2018, 03:17 AM:

What about using an external spare PSU? The official Dolby one, however, is not cheap, and if they are okay with the cost of a replacement unit...

- Carsten

Posted by Marcel Birgelen (Member # 6801) on 10-24-2018, 07:36 AM:

I agree with Harold, it would be interesting to see if this problem persists if the amplifiers are off.

An instable PSU somehow feels counter-intuitive to me. The unit should consume less power in "mute"...

Posted by Carsten Kurz (Member # 5396) on 10-24-2018, 07:42 AM:

Well, but switching power supplies sometimes do not like being 'unloaded'. Also, depening on the circuit, a mute may not save any current consumption at all.

- Carsten

Posted by Marcel Birgelen (Member # 6801) on 10-24-2018, 09:00 AM:

Yes, but you wouldn't expect any significant difference on the load on the PSU compared to "No audio being played" and the fader on 0.0, yet the result seems to be different.

I don't know if the CP750 monitors the output after the mute relais and if there are any multi-K-Ohm pull-down resistors there, but Harold story sound pretty plausible.

Posted by Leo Enticknap (Member # 534) on 10-26-2018, 11:37 PM:

This is now entirely academic - the 750 in question kicked the bucket yesterday. Our customer reported that the display went out altogether, accompanied by a burning smell coming from the rack, which dissipated after the power cord was pulled. So it's being replaced.

That answers my question (though not the way I'd hoped, obviously) - a flickering display is indeed an indication of imminent failure.

On the current consumption issue, my original thinking is that the flashing red LED in the mute button will use a little current on one of the rails. If it's the same one as the illumination for the main display, then those few extra miliamps could just be enough to stress out the power supply, if, say, a bridge rectifier (or other critical component in the DC output stage) was starting to go bad.