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I have not had the problem you describe in any of the CP-50's I have upgraded.

Here is what is troubling...you say you achieve Dolby-Level but that the pink noise level is too low.

This, off hand, doesn't make any sense. I say this since all the cards are plugged in when you set Dolby level. What ever your voltage rails are at should still be there when you run Dolby level or pink noise. Sure, pink noise is wide-band (well it is the widest band one can record on film) but it is recorded a full 20dB lower than Dolby level. Therefore sure pink noise should require you to turn up the RTA 20 dB as compared to Dolby-Level.

I find it surprising that the Cat. 108C draws so much more current that it sends the CP-50's power supply over the cliff...as Mark has pointed out, it is really just an optimized circuit and I am not aware of any power hungry devices...the original Cat. 108 used the projector LEDs as well which normally are some of the more power hungry devices.

Now, lets presume what you say is true (since we have Mark confirming your symptom I must accept that it is happing or is appearing to happen).

First off, raising the gain would NOT fix the problem it would only cause others. Dolby Level is used to set the pre-amp so that the level reproduced exactly matches that which is recorded. This is essentail to proper Dolby decoding. Any signal that is recorded above Dolby level is not processed by the Dolby NR circuit...the theory being that levels that loud (over 85dB in the theatre) are so far above the noise in the recording medium that any processing can only add coloration with no noise reducing benefits. So, if you raised the level up, you would really just be pushing up the noise and cause mistracking on the noise reduction which will result in sever coloration and other distortions such as pumping. Dolby-Level is Dolby-level and in the special case of optical movie film it is at 50% modulation or 6-dB down from maximum.

I have received a defective Cat 108C where the two channels didn't match each other at all. Any chance you have one of those? Have you tried another Cat. 108C? Or take your Cat. 108C to your nearest CP-50 or CP-200 to give it a whirl.

As to Williams' comments on the Cat. 114. Since you say your CP-50 is a "side swinging door" it is an early one indeed. It will have the original Cat. 114, for sure unless it has been RXed sometime in it's life. The newer Cat. 114C does have a much beefier supply with more filtration. Incidentially, the Cat. 114C is not a regulated supply...just a beefier brute force supply like the Cat. 114. I haven't had a problem using the Cat. 114 with the Cat. 108 though. The more you upgrade your CP-50 with the Cat. 114 the more likely it will become a problem...also if your incoming power is low you could be closer to the cliff. Generally, the upgrade to the Cat. 560 shows up the Cat 114...it taxes the +/-12volt rails so you MAY notice the shortcomings like a hum that comes from out of nowhere. Also, make sure your voltage selector switch is set for "115".

You can measure your voltage rails on the backplane at the terminals appropriately labled however, if you think this may be a power problem then the simplest thing to do is to (while looking at your RTA) is to start removing cards...I'd start with the Cat. 150 and then move to the EQs (those three are the big power killers. If they bring it back (if it is a power problem then probably any one of those cards will bring the level back)...then you need more juice. My guess is that if you just pull the NR cards you would bring it back as well since that will block all the later cards from receiving the signal.

If it is a power problem you have three solutions...first and easiest is to purchase a Cat. 114C from your favorite Dolby dealer. You might even be able to pick up a used CP-50 for next to nothing with one in it (starting about late 1982 the Cat. 114C was standard. It is easy to identify since it's power button is BEHIND the front panel rather than protruding through it. It will have four active fuses (input line voltage, +12, -12, +24).

The second option is to do the power supply yourself...this isn't too hard since many electronic supply houses have power supplies for purchase...the only trick is that you may have to have one for the 24 volt rail and another to handle the +/- 12 volt rail (this can be as high as 15 volts). You then need to fabricate a power cable using an AMP 4-position "Mate-N-Lock" connector or use the external power input connector on the rear of the CP-50.

Lastly, build your own CAT 114C equivalent using appropriate transformers to get the three power rails (you will either need a multitap transformer or more likely just use two transformers) then add bridge rectifiers and capacitors. Remember that when you rectify the AC you will a larger DC voltage...that is 24VAC rectified will not be 24VDC...closer to 30VDC. The final voltage will be load dependant just like with the Cat. 114. You are looking for at least 1 amp for each of the +/- 12 volt rails and 2 amps from the 24 volt rail. You can also regulate the power rails but then you better properly heat sink your regulators

One big advantage of the off the shelf power supply is that they can be regulated. Linear supplies are bound to be pricier but less noisy. Sola makes some nifty units as do others.

Steve

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The reason you shouldn't look to raise the levels just for the RTA is that this is indicative of a problem...you don't want your signal level to vary with bandwidth...this would cause mistracking in the NR and 2:4 decoding...If there is indeed a problem, then best to fix it or you will forever be chasing your tail.

Steve

Greetings from Finland! This is THE first post I´m sending to this forum while I´ve been registered for a period. First I have to thank YOU ALL (especially Brad) for this magnificent forum for finding help with film-technical problems.

I have also problem with CN108 pre-amp card. We have CP50 processor in our theatre. I have got significant improvement for optical sound when I installed Ernemann Laser Audio readers to our projectors (Cinemeccanica Victoria 12). The reproduction of this reader is similar with reverse scan readers: Improved high frequency response and channel separation and so on… The response with these readers should be flat to 16 kHz (-1dB) without HF slit-loss EQ.

The Pre-Amp card reversion is cat 108-4. The problem is filter designs on the pre-amp. There is some kind of low-pass filter which rejects the high freq response above 12 kHz. Upgrading to cat 108 reversion C might probably solve the problem. But I´m not intend to invest any money for the old CP50 any more so I´d like to try cheaper way first. I have couple of those older CN108 pre-amp spare cards. Does anyone have the schematic diagrams for the cat 108C optical pre amplifier card?
They might have came with the latest reversion of CP50 or CP200 manuals. Or they might came some instruction field bulletin with the newer 108 C card.

I´d like to make comparison between the old cat 108 and the newer 108C-version. There might be possibility to remove the low-pass filter rejecting high freqs. Newer style HF is not needed because frequency response is flat enough without it. Now with the old card the response is perfectly flat to 10-12 kHz with HF controls turned to minimum. No slit-loss EQ is needed.

I have already tried to make some modifications. I have found the high-cut filter. There is an old style parallel resonance LC-circuit based filter which can be simply bypassed. I have done this modification and the frequency response looks nice on RTA. The response is flat to 16kHz (with cat 69P) and 20 kHz comes well at – 3dB level when using older cat 69 printed on B/W –stock. So far so good. The problem is increased distortion which appears whith some films. I would like to know how this part of schematic have been made on C reversion and which newer semi-conductors there have been used. The older cards schematics are quite simple but the op-amp IC´s single-supply biasing is quite strange. I have also find the older connection little unstable. While the card warms up Dolby level increase. The components are so cheap that there is no idea buying new card while old ones can be quite easily serviced.

So this is the reason I´d like to find the cat 108C drawings. Also if somebody have 108C card could take a picture from the component side of this card, I would be grateful. That would be also very helpful if Dolby has not published these diagrams. Hopefully you understand what I mean.

Cheers
Teemu

Steve