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General Discussions => The Chamber => Topic started by: Gold on February 21, 2018, 03:54:07 PM

Title: Light Beam Meters - Neumann PTMV
Post by: Gold on February 21, 2018, 03:54:07 PM
I have a set of light beam PPM meters in a Neumann SP66 mastering console. I had another set in previous MT66 which is almost the same as an SP66. I could never get them calibrated where I trusted them. 

I recently discovered a Neumann factory calibration procedure for the PTMV amplifiers. I also found a calibration procedure for the meter unit. I’m having someone restore them. The PTMV has nuvistors and that is out of my wheelhouse.

I can’t wait to see a properly calibrated light beam meter. I’ve never seen one before. I don’t imagine anyone under say 60 years old has ever seen a properly calibrated one.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: [silent:arts] on February 22, 2018, 12:07:19 AM
We called them "Lichtfinger"  8)
Title: Re: Light Beam Meters - Neumann PTMV
Post by: ruffrecords on February 22, 2018, 03:04:21 AM
The first time I saw anything like that was in the physics lab at secondary school in the 60s. We had a couple of light beam galvanometers for measuring tiny current. They were about the size of a dinner plate. Saw one or two Neumanns  in studios around London in the 70s but they disappeared soon after.

Cheers

Ian
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Timjag on February 22, 2018, 11:07:32 AM
I'm naive - what's a lightbeam meter?
Title: Re: Light Beam Meters - Neumann PTMV
Post by: ruffrecords on February 22, 2018, 11:12:04 AM
I'm naive - what's a lightbeam meter?

It is like a regular edgewise meter except the needle is replaced by a small mirror. You shine a beam of light at the mirror and shine the reflection on the meter dial. As the meter operates, the mirror is deflected and with it the beam of light. And you get a 2 times optical amplification for free.

Cheers

Ian
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on February 22, 2018, 03:50:44 PM
Here are some photos.
(http://www.saltmastering.com/neumann/Neumann-LightBeamMeter-1.JPG)

You can see the front panel calibration. Set 0% then set 100%. Then move into operation mode.

(http://www.saltmastering.com/neumann/Neumann-LightBeamMeter-2.JPG)

These are the PTMV amplifiers. You can see four trimmers in there. Mysterious.

(http://www.saltmastering.com/neumann/Neumann-LightBeamMeter-3.JPG)

This is the rear. You can see the nuvistors on the bottom PTMV.  You can see the festoon lamps on the rear of the meter unit. Apparently there are tweak point in the unit. I've never opened it up.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on April 05, 2018, 09:20:04 AM
I am restoring a Neumann mixing desk that uses a similar meter and one of the two PTMV units is defective.

I haven't been able to find schematics for it, does anybody have a copy ?

Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on April 05, 2018, 12:36:34 PM
I sent my documentation to the  guy restoring mine. When he’s done with it I can scan it.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on April 05, 2018, 01:05:49 PM
Thank you. I have looked everywhere and couldn't find schematics for this module.

If you could also share the calibration procedure that would be great.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on April 05, 2018, 01:12:00 PM
Thank you. I have looked everywhere and couldn't find schematics for this module.

If you could also share the calibration procedure that would be great.

Sure, I'll scan what I have when I get it back. I'd like to see more working light beam meters.

I just got an email saying he located a NOS Nuvistor to replace the missing one. The sticky thing is at some point the thermistor was replaced with a cap. Apparently the thermistor looks like a tantalum cap. He has to figure out a suitable thermistor to instal. 

He's also just finished a Waveforms 520-A VTVM. I'm excited to get that back. It's very compact and has a large meter with a mirrored scale. I like it better than an HP400.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: leadbreath on April 05, 2018, 07:24:40 PM
Yeah man love these old meters!! Buddy of mines got one on this old Ge Neumann desk that doesnt work so would love to get it back to life...
Title: Re: Light Beam Meters - Neumann PTMV
Post by: guy_4 on April 30, 2018, 06:58:18 PM
We had this type of meters on the EMI TG12345 ( 16  )  and on the  EMI-NEVE ( 24  ) at  the EMI studios, in Paris.

When a bulb was blowing during a recording session, you had to wear sun glasses before opening the back panel of the Vumeters enclosure, because the light of all the bulbs was so intense   !     >:(
Under the instructions of the recording eng. the new bulb had to be slowly rotated until the filament was not making a " shadow "  over the vumeter scale....
Add two fans to blow the hot air out...
What a crazy design it was  ! ::)
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on August 29, 2018, 04:31:46 AM
HI Paul,

any chance I can get the documentation regarding the PTMV amps ?

I am coming to a good stage with my mixer refurbishment and would love to get the meters properly calibrated.

Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on August 29, 2018, 01:28:23 PM
I sent the documentation with the unit and I don't have it back yet. I know the work is complete. I'm not sure when I'm going to get it back because I have him working on a pair of UE100's for me. They are about 200 miles away so I'm going to wait until the UE100's are done to pick up the PTMV.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on August 29, 2018, 02:25:56 PM
Thanks, I will keep in touch
Title: Re: Light Beam Meters - Neumann PTMV
Post by: gyraf on August 30, 2018, 03:51:01 AM
Can I come in on CC when this turns up?

Jakob E.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on August 30, 2018, 01:02:07 PM
Can I come in on CC when this turns up?

Jakob E.


Sure, no problem.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: DerEber on August 30, 2018, 05:04:39 PM
Light Beam Meters are so cool!

10 Years ago I still worked in rooms at the government radio station equipped with similar NTP meters.
I got myself one, but failed very hard in replacing the murky mirror.
After spending a long time finding someone recoating the tiny mirrors I completely failed in reglueing the replacement.
This was one of my most unsuccessful repairs ever. 
;D

(http://www.twin-x.com/groupdiy/albums/userpics/IMG_4072_-_Front.jpg)

(http://www.twin-x.com/groupdiy/albums/userpics/IMG_4086_-_Spiegel_klein1.jpg)

(http://www.twin-x.com/groupdiy/albums/userpics/IMG_4089_-_Spiegel_klein2.jpg)

I still have everything ready to install one like this in front of my desk.
If anyone knows a source, please let me know.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on November 07, 2018, 02:21:44 PM
I have all the documentation back. I will scan everything. I don't have the ability to stitch together the schematic scans. Does someone want to stitch together the schematics and then post the files in the Technical Documents section?
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on November 07, 2018, 02:32:41 PM
Excellent. I could do that
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on November 07, 2018, 03:00:34 PM
Here is a link to the raw scans.

https://we.tl/t-cZDL23E8Qu

I'm not sure how many versions of the PTMV there were. There are scans for 930-02 and 930-03. I don't have scans for 930-01.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: beatnik on November 07, 2018, 03:36:37 PM
Awesome. Thanks a lot Paul
Title: Re: Light Beam Meters - Neumann PTMV
Post by: abbey road d enfer on November 11, 2018, 07:23:54 AM
I have a set of light beam PPM meters in a Neumann SP66 mastering console. I had another set in previous MT66 which is almost the same as an SP66. I could never get them calibrated where I trusted them. 
What's wrong with the calibration? You should be able to do a simple comparison with a trusted meter on steady tones. Maybe they are calibrated at +6dBu (the German broadcast standard of the time). Or is it the response to transients? Do you have other peak-meters you van compare with? Specs for PPM's have evolved over the years, particularly the attack time, which was originally 10ms, gradually shifted to 1ms, and now to zero (True-PPM). That makes a lot of difference in terms of what the eye perceives.
At Barclay studios we made comparisons, since we had all sorts of meters available (we had lightbeam meters - probably Neumann - in the transcription desks, Moving-coil PPM's in the Neve desk, VU-meters in the API desk, bargarphs in the cutting room, and a 30 LED prototype I had built. We ended up concluding they all gave different indications as soon as there were transients in the program. We didn't pursue with the prototype because it was felt that, since it did not agree with any of the other established systems, we could not impose our own standard.
Anyway, the Neve PPM's were recognized as the most usable, followed by the VU meters.
Only one guy favored the Neumann lightbeams, having worked all his professional life with them. OTOH, he did not have to make  big decisions, since his work was copying tapes to tapes.
Me, having been a 2-week chicken at the time, I was comfortable enough with my LED meters and used them regularly  in my recording studio and in the consoles I built at the time.
Now I'm happy with the true PPM's in my DAW.

I recently discovered a Neumann factory calibration procedure for the PTMV amplifiers. I also found a calibration procedure for the meter unit. I’m having someone restore them. The PTMV has nuvistors and that is out of my wheelhouse.

I can’t wait to see a properly calibrated light beam meter. I’ve never seen one before. I don’t imagine anyone under say 60 years old has ever seen a properly calibrated one.
[/quote]
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on November 12, 2018, 01:09:40 AM
I think the calibration problem was that if I adjusted 0% to minimum deflection and 100%  to the red line then 0VU didn’t match up  at +4 dBu. Each channel was different too. That was about 15 years ago so I don’t exactly remember.

There are a lot of calibration points. I’d like to see what one looks like when calibrated to factory specs before I decide if I like it or not.

I like VU meters. I’ve been using mechanical PPM ‘s driven with a board that is more German than British.  They are okay but I have an RTW plasma PPM that I like better. I’m going to use that in my main console instead of the mechanical PPM’s.

The light beam meters are built into the Neumann SP66. I want that restored to original condition. It ‘s a great sounding and looking console IMO. 
Title: Re: Light Beam Meters - Neumann PTMV
Post by: abbey road d enfer on November 12, 2018, 06:53:04 AM
I think the calibration problem was that if I adjusted 0% to minimum deflection and 100%  to the red line then 0VU didn’t match up  at +4 dBu.
Calibrating at nominal level is the first step in the calibration process. However, adjustment of other parameters interact with it; it is often necessary to redo the initial level calibration, sometimes several times. It's a lengthy and tedious procedure.

Quote
I like VU meters.
I respect your opinion, but would you agree they are almost useless in a digital environment? I tend to look at them as decorative signal presence indicators.  :)

Quote
   I have an RTW plasma PPM that I like better. I’m going to use that in my main console instead of the mechanical PPM’s.
That makes sense IMO.

Quote
The light beam meters are built into the Neumann SP66. I want that restored to original condition.
The meters are probably the most difficult in that endeavour.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on November 12, 2018, 11:17:25 AM
Calibrating at nominal level is the first step in the calibration process. However, adjustment of other parameters interact with it; it is often necessary to redo the initial level calibration, sometimes several times. It's a lengthy and tedious procedure.

As I remember no matter what I did something didn’t seem right.  It wasn’t subtle like being picky about integration time.

Quote
I respect your opinion, but would you agree they are almost useless in a digital environment? I tend to look at them as decorative signal presence indicators.  :)

They are obviously useless for seeing overs but still just as useful for judging how the audio will hit a mechanical transducer like a speaker.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: abbey road d enfer on November 12, 2018, 12:35:15 PM
As I remember no matter what I did something didn’t seem right.  It wasn’t subtle like being picky about integration time.
There are a few components in them that are prone to drifting; when they do, the log amp goes all over the place. Finding which is defective is sometimes uneasy. IIRC, they had to be sent back to the factory; even the french distributor was not properly equipped (competent?) to service them.

Quote
They are obviously useless for seeing overs but still just as useful for judging how the audio will hit a mechanical transducer like a speaker.
If you mean they are a rough indicator of how loud it will sound, I agree; that's their raison d'etre, but it's been demonstrated often enough that the perception of loudness is not a simple matter and that it takes a quite complex analysis to achieve proper measurement of perceived loudness.
If you mean that a VU-meter can give a useful indication of how a loudspeaker is stressed, that is far drom the reality; the mechanisms are very complex. A VU-meter may indicate roughly the electrical power delivered to the loudspeaker, that's about it.
Title: Re: Light Beam Meters - Neumann PTMV
Post by: Gold on November 12, 2018, 01:21:26 PM
There are a few components in them that are prone to drifting; when they do, the log amp goes all over the place. Finding which is defective is sometimes uneasy. IIRC, they had to be sent back to the factory; even the french distributor was not properly equipped (competent?) to service them.

That’s why I gave it to an ace tech who is familiar with this old stuff. He’s calibrated them before many times.

Quote
If you mean they are a rough indicator of how loud it will sound, I agree; that's their raison d'etre, but it's been demonstrated often enough that the perception of loudness is not a simple matter and that it takes a quite complex analysis to achieve proper measurement of perceived loudness.

No, I use my ears for that.

Quote
If you mean that a VU-meter can give a useful indication of how a loudspeaker is stressed, that is far drom the reality; the mechanisms are very complex. A VU-meter may indicate roughly the electrical power delivered to the loudspeaker, that's about it.

That’s what I mean. The integration time of a VU meter more closely approximates the breakup behavior of cheap speakers. I wouldn’t use them if I was designing drivers but I get very useful information out of them. I say this from experience. YMMV.