Film-Tech Forum ARCHIVE: dolby level in cp50

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Posted by Christos Mitsakis (Member # 143) on 04-25-2007, 11:00 AM:

A situation of a cp50 where I suspect the ballistics of the meters are completely off. Should the level on the preamp test points with dolby tone be 300 or 500mv for correct noise reduction. There is something written in the schematics that is unreadable.

C.

Posted by Raphael Friedland (Member # 3453) on 04-25-2007, 02:50 PM:

Connect a voltmeter to Rt and ground and adjust level to 775mV
and the same for Lt.

Posted by Louis Bornwasser (Member # 3063) on 04-25-2007, 03:19 PM:

Most of those little meters (Cat 109 card) are worthless by now. Louis

Posted by Marin Zorica (Member # 1716) on 04-25-2007, 03:32 PM:

300mV RMS with Cat.No. 69T Dolby Tone!

Posted by Steve Guttag (Member # 268) on 04-25-2007, 06:20 PM:

Yes, 300mV on the Cat 108 test points should correspond to Dolby Level on the Cat 109 meter.

Mind you the Cat 109 was created to take advantage of existing technology, like many things in the CP50. So, the signal that feeds the meter in the Cat 109 comes from the Cat 22 (or Cat 280) from a different signal path than the actual signal itself.

The voltage level coming from the Cat 22 (Cat 280T) is actually 1.85V that feeds the meter circuit. The signal that feeds the rest of the processor is at 500mV after it leaves the Cat 22 but it is at 300mV when it enters the Cat 22.

Electrically, there isn't much to go wrong with the meter circuit...there are no caps to dry out but that does not speak for the line amplifier that feeds it from the NR card. There is always a chance of any component failure though I would find it odd if both meters failed at the same time.

Mechanically, that is another story. I've seen them come unstuck from the PCB, the meters stick...etc. The meter movement itself was never precision...it has exactly one point of accuracy and it was calibrated when the PCB was assembled via resistors R121 and R123 in parallel with each other limit the current to the circuit prior to rectification. Using appropriate values of resistors for R121, R123, the technican would calibrate the meter at Dolby.

Steve

Posted by Cameron Glendinning (Member # 3516) on 04-25-2007, 09:43 PM:

Silly point or perhaps more of a pet hate, Is the dolby tone loop in new condition? It amazes me how many loops from 1992 (exaggeration) are still being used for setting the levels. Obviously if its worn, you will not get the correct levels in the first place.

Posted by Gordon McLeod (Member # 33) on 04-25-2007, 11:03 PM:

Wear will be more of an issue on pinknoise rather than tone

Posted by Christos Mitsakis (Member # 143) on 04-26-2007, 01:55 AM:

300mv I'll do. Thank you all. [beer]

Thank you Steve for the detailed explanation, when I'll have some time I will try tinker with the resistors on cat109. Asside the meter inconvenience the processor works perfectly, with upgraded cards and a SRA5 unit that I attached two days ago.

C.

Posted by Steve Guttag (Member # 268) on 04-26-2007, 08:23 AM:

Again, make sure it isn't the Cat 22s throwing the meters off...try swapping the cards out or even transposing them to see if the levels change.

Steve

Posted by Christos Mitsakis (Member # 143) on 04-26-2007, 03:45 PM:

I already did that Steve, using three cat 22 cards. All of them at L & R position gave me with 300mv reference at cat108 test points: 1.84v at cat22 pin 2 that drives the meters and 504mv at Lt & Rt input of cat109. The left ch. meter always just below dot, while right ch. meter at middle of dot and upper end.

Two more things.
a. Where, at cat109, are the leads that power the meters' light bulbs? I know this consumes power, but its only just out of curiosity.
b. I have a spare older cat117 that someone messed with it damaging two components. I want to replace them so the card can be functional, just in case. Again on the schematic its difficult to read clearly the values. So if you remember or have a large schematic on hand what are the values of C906 capacitor and R909 resistor.

C.

Posted by Steve Guttag (Member # 268) on 04-26-2007, 04:36 PM:

C906 = 22uF/16V
R909 = 8.2K

The meter lights are powered by those two solder on tabs on the top side of the meter. They draw about 20mA at about 12VDC. Both +/- 12V are available on the card (and you probably only have about 10V or so on those rails in a typical CP50 unless you have the Cat 114C (it has a bit more juice to it).

I suppose you could tag one meter to each power rail (to load them idetically so one isn't higher than the other).

I looked at a Cat 109 On of the resistors that sets the meter calibration seems to be uniformly 3.3K and only a 5% resistor that that. The other resistor seems to be the one they varied. On the card I'm looking at...one has a 10K 5% resistor and the other has a 15.4K 1% resistor so there is a bit of a range there. If you really want to dial it in...mount a pair of 25K pots in place of the varied resistor and tweek as needed.

Steve

Posted by Christos Mitsakis (Member # 143) on 04-27-2007, 01:35 AM:

Thanks Steve.

Its an upgraded processor with cat114C

quote: Steve Guttag
mount a pair of 25K pots in place of the varied resistor and tweek as needed.

The best solution. I'll do.

I meant should I connect the bulbs direct to the power rails or there is an outlet on the pcb for that purpose.
C.

Posted by Steve Guttag (Member # 268) on 04-27-2007, 08:49 AM:

I'm unaware of any Cat 109s that lit up the lamps. I think they were only lit in the various 360 series frames. As such, I don't think there are specific tie points to light them up so you would need to tie them to the rails.

Steve

Posted by Christos Mitsakis (Member # 143) on 04-27-2007, 12:56 PM:

Thanks Steve. I don't intend to leave the lights on during normal operation. just because I saw some pics of the cp50 with the meter lights on I presumed there would be a power outlet on the cat109.

C.