Film-Tech Forum ARCHIVE: Strange buzzing noise with CP50

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Posted by Michael Schaffer (Member # 1204) on 06-17-2005, 10:23 PM:

We have been haunted for several weeks now by a strange buzzing noise which occurs intermittently, but at unpredictable intervals. Once it even happened while I was there. Problem is, it randomly comes and goes, so there is no way to test it reliably. It is a short but quite loud series of quick buzzes "eek-eek...eek". When it occurs, both meters on the level card jump up. So it must be somewhere in the A-chain.
All the wire connections were checked, all card edges cleaned and firmly reseated. This was one of our remaining auditoriums with an exciter system, so I took that out and replaced it with a new BACP reverse scan. It appeared that the sound was gone for a while then, but news just reached me that it is back in action after about 10 days. Of course I have no way of knowing if it was gone that long or if they just didn't pay attention...
It makes no difference whether the film is playing or not. Apparently it also buzzes in non-sync.
It also occurs when the CP50 is in optical fault and when the preamp for projector #2 is used. I doubt it could be caused by the power supply. It is just a transformer and a few caps, not much in there to fail intermittently. Dolby say the same.
Mysterious [Confused]

Posted by Sam D. Chavez (Member # 1841) on 06-17-2005, 11:46 PM:

I would change the power supply bridges first. Old style ones would overheat and cause just the symptom you described.

This happens more often when SR cards are used as they draw more current.

Posted by Michael Schaffer (Member # 1204) on 06-18-2005, 12:32 AM:

Sam - what exactly is meant by bridges? This one indeed has SR cards. So you are on a hot trail. I guess it doesn't make any difference that it also occurs when in optical fault and - allegedly, according to the projectionist - in non-sync as well, even though the SR cards aren't used and shouldn't draw much current?

Posted by Jack Ondracek (Member # 1466) on 06-18-2005, 12:46 AM:

That sounds a lot like the sound I'm getting in some computer speakers here, due to the proximity of the coaster-pager transmitter I just installed.

I once had troubles with an elevator that had an scr-based power supply, though it didn't pulse like you mention.

I'd vote for looking for nearby utility transmitters... maybe even those new utility cell units that read your gas and power meters. Failing that, I'd capacitor-bypass the power lines & put a scope on them to see if anything's coming in that way.

Posted by Sam D. Chavez (Member # 1841) on 06-19-2005, 01:21 AM:

Bridge rectifier(s)is what I meant.

Posted by Michael Schaffer (Member # 1204) on 06-19-2005, 04:47 AM:

Thanks for clarifying that! I was confused because I had actually thought the Cat.No.114 power supply was just a pair of transformers and a few filter caps, but a fresh glance at the schematic shows that I was mistaken. There are actually two diode assemblies in there. Of course, without rectifier circuits there would be no DC output.
Today I had another look at the patient. I had thought that it should be possible to test-run it successfully without the 2 Cat.No.280T SR cards in there, in optical fault mode. But the buzzing still came back after a while, they told me. During the next test run, when I was present myself, everything seemed fine until the buzzing suddenly occurred insistently after about 30 minutes and disappeared instantly when those 2 cards were removed, only to reappear when they had been plugged back in. Drawing my IE-33 like Wyatt Earp (actually I don't have it in a holster, it's in my backpack, but you get the picture), I found that it was on both channels at the preamp as well. So it does look exactly like Sam said, caused by overheating of the power supply, with the only difference that it occurs earlier with the 2 SR cards in the system. The power supply was actually my very first suspect, but I was told by Dolby there was no way it could cause that problem. I should have asked Sam first instead...

Posted by Mark Gulbrandsen (Member # 72) on 06-19-2005, 08:07 AM:

If this is an old supply it should also be re-capped. I've seen ALOT of leaky caps(literally leaking white stuff)in CP-50's lately. Many of these supplies are approaching 30 years old... electrolytics just don't last that long. New caps are smaller but if you wrap them with tape they will fit into the opriginal clips provided. Parts cost to completely overhaul a CP-50 supply is less then 20 bucks total if you get the parts from a place such as Digi-Key or Allied.

Dolby tech support just aint what it used to be.... if the problem you're having isn't listed in their files then they can't help you anymore, plus the people there now have little experience with the older products.

Mark @ CLACO

Posted by Tim Reed (Member # 137) on 06-19-2005, 11:09 AM:

The answers may be coming from a phone center in India. [Roll Eyes]

From your description, Michael, it sounds like the cell phone handshakes I occasionally hear when it's placed next to my XM receiver. Only it's more like, "eek...eek..dit-dit-dit-dit..eek..chirp-chirp"

Posted by Richard Fowler (Member # 893) on 06-19-2005, 11:35 AM:

My DSL provider technical trouble shooter line has moved to Mumbai, India....and the results have been excellent [Razz]

Service in many businesses have gone to the toilet in line with the low cycle of sales revenue...that is why forums like this are helpful [Cool]

My office cell phone charger is on our computer / server rack and we are very used to the "chirp, chirp" handshake [Big Grin]

Posted by Mark Gulbrandsen (Member # 72) on 06-19-2005, 01:41 PM:

quote: Richard Fowler
My DSL provider technical trouble shooter line has moved to Mumbai, India....and the results have been excellent

Yea, but the guy he replaced is sitting in a nice air conditioned room at home collecting unemployment whilst sipping a cold Budwiser and watching the PGA match in HDTV while the guy you talk to in India is sweating his ass off, makes a few dollars an hour and is lucky to get a drink of decent water and some Government controled B&W TV!

Mark

Posted by Michael Schaffer (Member # 1204) on 06-19-2005, 06:52 PM:

It doesn't sound like the cell phone thing which is more nervous. This thing here is more like a steam boat signalling, each of the eeks is about 1/2-1 second long and they usually come in series of several regularly spaced bursts. Right now I have absolutely no time to sit down and take it apart, so I will see tomorrow if they do a repair/exchange still.

Posted by Sam D. Chavez (Member # 1841) on 06-19-2005, 07:24 PM:

I believe they still do an exchange but the price is beyond extreme. The value of the CP50 is less than a $200 so you probably don't want to spend big money.

You should rig an external supply. You just need +/- 12 and 24 Volts.

It's quite easy to replace the rectifiers, an hour in and out.

Posted by Michael Schaffer (Member # 1204) on 06-19-2005, 08:09 PM:

Do you have a part# and source for the rectifiers? Are the +- 12V referenced to each other or ground? Do they share a common neutral? Same about the 24V, is that referenced to ground? It is a little confusing that Dolby call the connection E. On the other hand, on the backplane, it is labelled 0V. That makes more sense.
How many amps minimum should the external supplies be laid out for?

Posted by Sam D. Chavez (Member # 1841) on 06-20-2005, 12:33 AM:

Any bridge that is close to the right physical size will work. There is no penalty to shoehorning in a bigger one.
There is no exact replacement, and if there were, it would probably fail in the same way.

Something like 200 or more PIV, and 500 mA and up will work.

I would buy some surplus power supplies rather than trying to build them. They are all over the place.

I've forgotten the current draw on a CP50 but something like 1 Amp should be more than enough.

Posted by Ken Jacquart (Member # 369) on 06-21-2005, 10:33 PM:

Michael,

We spec'd out an alternative replacement power supply for the CP50 some years ago. I'll check back at the office and email the information to you. FYI, I'm not (and haven't been for some time) in tech support anymore as I have been doing other things. I'm disheartened to here some of the comments in this thread. For all your reference, the tech support phone number for North/South America is 415-645-5205. There is also a tech support email which is CinemaSupport@dolby.com. I have been in this biz for near 20 years now, 8 at Dolby. If tech support can't figure out your questions regarding old time Dolby products, I would hope that they would ask me.

ken

Posted by Michael Schaffer (Member # 1204) on 06-22-2005, 02:46 AM:

Ken - Thanks for the email. I didn't get it though. Apparently you sent it to Helmut Kaczmarek (now with NEC), the other German haunting the tech scene in SoCal, instead of me. I worked with him earlier today at our open air digital cinema site at the San Diego County Fair, and he was puzzled why he got an email from you about CP50 power supplies...
[Smile]

In any case, the information I got may have been a misunderstanding. It is sometimes hard to figure out stuff over the phone, especially since I was describing something I hadn't at that time witnessed myself, only heard about from the people in the location. Kind of around five corners.
I hope that you will still be reachable in some form or manifestation, I have always valued your support extremely highly.

Posted by Ken Jacquart (Member # 369) on 06-22-2005, 01:18 PM:

Michael, I don't know what I was thinking... I suppose that your accent is a bit similar to Helmut's and there you have it... In any event, one particular CP50 power supply replacement that we've tested here is the PUP110-41. It can be had for around $140 from Foresight Electronics at 800-253-0490. They don't have any on the shelf, and would have to order it (so don't wait until you're off the screen!). There may be other suppliers if you just google the model number.