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Topic: Dolby level with CP500 & Dolby MPU

Ari Nordström
Master Film Handler

Posts: 283
From: Göteborg, Sweden
Registered: Jan 2000

posted 08-30-2000 03:18 PM

Spent last thursday and friday installing a Dolby MPU to a CP500-70 setup. Everything went fine except that I had to go through a number of soundheads to find a pair that would give me Dolby level. Yet, all of these heads worked just fine with an older, non-Dolby pre-amp and gave Dolby level without any problems. It bugs me that one of the soundheads that finally worked doesn't give a nice, flat response all the way up to 16 kHz with all channels...

So my question is, how come? Why is it so hard to achieve Dolby level with most of the soundheads I tried out? (The projectors are a pair of Zeiss Favorit70.)

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 03:52 PM

The problem is the impedance of the heads
Many european heads had much higher impedance than the standard North American ones
There are straps (solder links)on the backplane of the MPU to change the head sensitivity. Be carefull though as it is possible to make the MPU dramatically unstable (burnt trace mode)
Also R101 and R102 provide loading on the output of the transformer and can be changed to effect increase of gain
The newer and better 92C's are a different story and I don't think I have there schematics
What is the actual make of the head cluster

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 08-30-2000 05:01 PM

I have experienced this problem with both old Norton and Norelco heads. Both were high impedance. Be VERY careful if youmove the links on the backplain of the MPU as there is a good chance that the card may go into oscillation if you 1.) turn the gain up too high, and or 2.) turn the hf equalization up too high. Shut down your power amps and check the output with a scope to be sure it is clean!! The newer 92C cards are vastly improved and work very well with just about any type of mag head. Getting 92C's was the only way to cure my oscillation problem plus it alowed the theater to use a bunch of older heads that were NOS.
Mark

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 05:16 PM

Actually it is rare for me not to be able to reach level with most heads unless they are really badly worn.
Again as I said if removing the links watch out for unstabality if it goes and it is on a large powersupply the rails will burn off
THe only heads I had alot of trouble with are the Racal and the Boggen ones
The 92C is a fine preamp but for most films considering the limited resources in the CP500 (no SR for 70) the older cards are adequate

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 08-30-2000 07:07 PM

Nope, the problem you are experiencing, in this case, is not the head impedance...

The problem is that the CP-500, foolishly wen't with the bastard 388mV sensitivity that only existed on the CP-65 (due to the Cat.300 buss level...it was a bad move then, and it still is a stupid idea).

The Dolby MPU-1 was designed around a 100 mV buss level....a whopping 11.78dB increase in level!!!! They only way you can barely achieve this is with heads with the wider track width (.050", I think, instead of the .035") AND you use films that comform to this standard (ie made after 1983), and you MUST have Cat. 92C set to high output...then you will just barely make it.

What Dolby should have done, even with the CP-65 is keep the Mag buss level at 100mV. By the time the CP-65 came out mag was so old and entrinched in it's ways it could apply for Social Security!

My suggestion is to put some low-noise line amps between your MPU and the CP-500 to make up for the nearly 12dB level missmatch.

Steve

------------------
"Old projectionists never die, they just changeover!"

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-30-2000 10:36 PM

Steve we are going to have alittle disagreement here. Yes the bus level is higher but you will note that some of his heads do produce the correct output.
In the US most projectors were equiped with Teccon or Electromag heads (Ampex or Brush or Norton earlier) if they were of european origan and I can attest to the fact that head impedance is a problem as almost all our cinemecanicas, prevost westrex's and bauers that originallly came to canada had original european heads in them and they rarely met level into a cp200 or 100 until the gain strapping was changed or the head cluster changed out

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Ari Nordström
Master Film Handler

Posts: 283
From: Göteborg, Sweden
Registered: Jan 2000

posted 08-31-2000 07:48 AM

Thanks for all the tips and information. They're much appreciated.

We have 92C's in the MPU. The present soundheads (that managed to produce Dolby level) are not particularly worn, but more so than some of the ones we tried, and have 500 Ohm impedance. They are made by Zeiss, which means that they're actually Prevost if I'm not mistaken.

I know of the MPU's high and low impedance feature in the MPU backplane (50 and 500 Ohms, I'm told) but that didn't help. BTW, somebody had put in switches instead of the solder links so it was easy enough to try.

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 08-31-2000 09:09 AM

Gordon, I said, "in this case," and I ment it. The end user claimed to have Cat 92C preamps which eliminates the high/low links on the MPU anyway. Sure there were impendence mismatches but you can't just wave away a 12dB gain problem. Sure some can "barely" make it but everything has to be going the right way for you. Try it with any 4-track mag at 50% modulation and see if you make 388mV with any Cat 92C or otherwise.

I would ask the Ari to measure the output voltage from his MPU runing D-Tone with one of the heads that doesn't make Dolby level and I would be willing to wager that you are exceeding 100mV

Steve

------------------
"Old projectionists never die, they just changeover!"

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-31-2000 09:14 AM

Funny as I have used Dolby MPU's with super starscope way back when and they had 300mv sensitivity as did the CP100 if memory serves

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 08-31-2000 07:18 PM

Actually i just looked it up and in Dolby's specifications for the MPU it specifies a 100mv level for reference or 300mv to accomodate older cp100
Ari check each preamp card on the 92c there is a gain jumper on the cards themselves J101 and J 201
Also I looked up my notes on the favorite 70
If the machine is labled Zeis it usually had german heads made by Bogen or a head made in belgum (applied)
If it is marked Prevost they typically used a company call foto- (i can't make out th second word on the sketchs)

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Ari Nordström
Master Film Handler

Posts: 283
From: Göteborg, Sweden
Registered: Jan 2000

posted 09-01-2000 12:21 PM

Gordon,

Thanks a lot for the information. I'll check ASAP.

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Ari Nordström
Master Film Handler

Posts: 283
From: Göteborg, Sweden
Registered: Jan 2000

posted 09-01-2000 12:27 PM

Oh, BTW:
My Favorit70 projectors are marked Zeiss so I presume they have Bogen heads, then?!

The corresponding Prevost model is P70, I believe. The only "Favorit70" models I know of are made by Zeiss.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 09-01-2000 09:01 PM

Apparently the Favorite's were made by Prevost and had the zeiss name put on them and used a slightly different motor
The head cluster itself could be anyones guess

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Ari Nordström
Master Film Handler

Posts: 283
From: Göteborg, Sweden
Registered: Jan 2000

posted 09-02-2000 02:40 AM

Yeah, I heard something similar except that according to the version I heard, the Favorites were license-built by Zeiss. I don't know for sure, though.

It's a good projector, in any case.

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