Dolby A 361 - Testing, Calibrating?

[Dr_J]

Hi Everybody,

I just bought 4 Dolby A 361 Units which i want to use as outboard exciter.

Now the first unit is on the bench.
Ok, all bulbs are dead, audio is passing with no notable problems, noise; sounds good.
I ran a sine sweep and with decreasing gain i can see how the high frequencies got compressed.
What's the best, easy way to check the functionality?

I read the manual for calibration and got stuck...
On Bypass I aligned the DAW return level (1kHz sine) by trimming the output.

But how do I set up, check the input?
(The meter doen't move at any time..!?)

Any hints would be perfect! THANXS!

 

[gridcurrent]

What's the best, easy way to check the functionality?

inject +4 dBu @ 1000 Hz signal,
playback mode,  "from rec"  and "to mon"
ensure "check tape" is not engaged.

if the meter is defective (most are),
adjust input level so that there is approx. 1.95 VAC @ the red-white wire leading to the meter.
adjust output level for +4 dBu at  your console/DAW.

will be helpful is you replace the meter (250 uA) or provide connectivity an external device.

 

[JohnRoberts]

I never worked with dolby A but back last century was responsible for manufacturing a cassette deck with dolby C NR.

IIRC the calibration for that was performed at 400 Hz.....  FWIW

JR

 

[Dr_J]

Hi,

THANKS a lot!

Just to make sure ... function testing:

inject +4 dBu @ 1000 Hz signal,
playback mode,  "from rec"  and "to mon"
ensure "check tape" is not engaged.

- Line in: +4 dBu@1000Hz

- Switching the unit to "Playback" (S4 Button)
  so the unit is decoding; right?

- DAW Linie In : "From Rec" (JI4) (alternativ "To Mon")

And now? What to look on?


Regarding the meter, i already had the feeling that it's dead...
I measured "Red-white wire" quickly, i think it was 1.6V - but it was DC!?
I'll check that!

Are mostly the meter them self are defect or something on the meter board?
Is the meter normally working all the time?

ROCK-ON!

 

[gridcurrent]

Just to make sure ... function testing:
- Line in: +4 dBu@1000Hz

- Switching the unit to "Playback" (S4 Button)
  so the unit is decoding; right?

yes.  1000 @ +4 makes little difference in meter level, whether the NR is engaged or not.

Regarding the meter, i already had the feeling that it's dead...
I measured "Red-white wire" quickly, i think it was 1.6V - but it was DC!?
I'll check that!

AC.

Are mostly the meter them self are defect or something on the meter board?
Is the meter normally working all the time?

the meters are notoriously unreliable. 
meter amplifier is on the cat.22 card but there are a few diodes and resistors on the meter assembly.
meter indicates whether printing or playing back.

 

[Gold]

If you just want to use them as an effect then the level calibration is probably close enough.  The actual calibrator which is described in the manual is as rare as hens teeth. Dolby doesn’t have one. There are a few hiding. I traded six units for two restored and calibrated units. None of the eight I had were very close to each other.

 

[radardoug]

To functionally test whether the transfer function is correct, use pink noise as a source. Plug it through one unit in record, and then through another unit in playback. Put the output on a 1/3rd octave display or similar. The pink noise should show flat frequency response.
Then lower the level, the response should stay flat. If not, one/both  of the units is faulty.
But if you are only using it as an effect, you dont need to worry about calibrating it at all.

 

[Dr_J]

Thanks radardoug,

i'll never used pink noise for testing before.
For NR it makes sense...
Maybe I'll give it a try.

I was much more focusing on the compression behavior (maybe build to many compressors...  )


ROCK-ON!

 

[gridcurrent]

regarding "calibration" of a Dolby 361 or 360.
there is nothing internal to calibrate other than the user's  input and output controls.
the model 35 NRM as described in the manual tests:
"all major functions of the Noise Reduction Module,  to check the ripple level of the rough DC supply which powers the module,
and to verify the accuracy of level setting meters used in  the equipment in which the module is installed'

I have a 35 NRM, including the 35a and 35b.

 

[Dr_J]

Perfect!
I put a 1kHz sine @+4dBu (1.228VAC) in and adjusted the red-white meter wire to 1.95VAC (I had to come down from 5.something!)
After that I adjusted the output trim to meet 1.228 again.
I guess now it‘s fine.

Next checking with freq-sweep...

THANKS!

 

[fazer]

Dr-j.  I assume you know by reading your post but just to mention to others,  you mod it to turn off the low and low mid compression  also for the dolby hi freq compression effect. 

 

[pucho812]

testing this.... nothing to really add at the moment

 

[Dr_J]

I didn't applied the mod jet.
Currently the unit(s) are original.

Applying the mods comes next. I like to make them switchable and looking for a
good place for the switch. I don't what to drill a new hole on the front panel...

Additionally I did some quick tests with a blues-rock-kind guitar file. It sounds pretty good with the dolby engaged,
it "smooth out" the hole sound very nice and adds some "air"... 


ROCK-ON.



P.S.:
The meter is dead. When I opened it, it was all messed up inside... and rest was done by me... :

 

[Dr_J]

Here are some frequency response plots.
(Blue Cat's and Screen-Shots overlay.
I hate not to have a data export option... )

Btw. does anybody have a LINK for a meter replacement?
I found a used one on ebay, hope it will fit and i can replace the scale.
But maybe I'll need some more.


ROCK-ON!

 

[gridcurrent]

Applying the mods comes next. I like to make them switchable and looking for a
good place for the switch. I don't what to drill a new hole on the front panel...

Additionally I did some quick tests with a blues-rock-kind guitar file. It sounds pretty good with the dolby engaged,
it "smooth out" the hole sound very nice and adds some "air"... 

the seller of the replacement meters is reputable.  the meter face is proper and original.

suggest you try out the 361/cat.22 with music,  as is,  before attempting the "mod". 
many users like it stock.
some of the modifications shown online either don't perform like the cards pulled on a A301,
or  they produce noise.  one of the illustrated mods produces only 2 dB of boost.
perhaps  our resident poster austex64 can chime in with his experience.

if you do go ahead with the "vocal stresser" fix,  tiny toggle switches can be  mounted on the face of the cat.22.
there is barely enough room to mount 4 (if one wants to individually select any band), 
no problem mounting 1.

 

[Dr_J]

the seller of the replacement meters is reputable.  the meter face is proper and original.

Do i miss something?! Which seller? Any?

I didn't open the cat22 card jet.
Maybe a little switch on the card itself is a way to go. Thanks for that tip!

ROCK-ON!

 

[Tubetec]

Its been 25 years since I set up a dolby unit .
I did pick up two of them but they have just sat there .
As far as I can remember in record mode with the dolby tone pushed in your output from the 361 will output reference level to tape ,you'd record a segment of tape with this level ,then on playback that recorded tone is fed back into the 361 and you can check your reference levels off tape . I dont see any reason why a daw or hard disk recorder cant replace the tape machine for the purposes of line up . Hope that helps

Cat 43a if you can find one turns your 361a/cat22 into a single ended(no decode) four band compressor expander noise reduction .

 

[gridcurrent]

Which seller?

https://www.ebay.com/itm/Used-Dolby-Level-Meter-For-363-And-Other-Models-Complete-Tested-Works-ME/324098413314?hash=item4b75c55302:g:6zwAAOSwOM5eZszU

 

[Dr_J]

Wow! Thanks!
I have to improve my Ebay search skills...

But the price is killing... plus shipping to europe...
the meter is about twice of the unit...
... there must be another way!  :-X

But, THANKS A LOT!

 

[Dr_J]

Hi everybody,

i'm moving (slowly) forward with my project(s).
I replaced all the dead meters now and applied a 3 way switch for the cat 22 mods. Works fine and sounds great!

But on 1 unit the meter pengs hard right with no signal 
There must be something wrong with the meter driver on the cat 22, i guess.
I read 10VDC(!) at red-white meter wire 
This may explains why the old meter was smashed.

Unfortunately my copy of the manual with the component layout of the cat 22 pcb is bad quality, i can't hardly read anything.

Has anybody a readable component layout for me?
(Or a picture of the pcb issue with the component info printed on?)

AND of course any hints where to look at would be absolutely GREAT and welcome!


THANKS & ROCK-ON!