6 Track Mag Head Maintenance

[Ryan Gallagher]

I had a moment today to check the jumper settings on the Cat-92C cards. All were set to High gain and fringing compensation jumper set to "out".

According to the docs, that is actually incorrect and the fringing compensation jumpers should be "IN' for my narrower AMPEX head and "OUT" for the Teccon head.
http://www.film-tech.com/warehouse/m...DOLBYCAT92.pdf

High gain may or may not be correct, it's referenced as a starting point for doing the calibration.

Moved fringing jumpers to "IN" for AMPEX Projector No. 1 cards according to docs, have a band onstage so can't listen test today. If this turns out to resolve my noise issues would changing fringing settings require re-doing the A-chain/Dolby Level? (Assuming YES, and checking the A chain is probably worth doing either way eventually before another mag print comes our way).

Teccon replacement head is still going to be proposed if I can sell em on it.

[Steve Guttag]

You'll see the bass response change with the fringing compensation. It won't be a noise thing. It has to due with the head being narrower than the recorded track.

[Ryan Gallagher]

Separately, this is essentially the vintage interconnecting cable installed from heads to MPU in our booth (although minus the P, non-plenum version)
https://marvacsale.shop/product/25-2...c-electronics/

Another possible source of the noise AMPEX head noise issues.

If we redo anything i'm reading in the forum that Belden 8451 or 9451 (or equivalent) are recommended.
If we are gilding the lily, spring for Starquad bonded pairs?

[Ryan Gallagher]

You'll see the bass response change with the fringing compensation. It won't be a noise thing. It has to due with the head being narrower than the recorded track.

Thanks Steve, that was my instinct (it not relating to noise). But if I leave it "corrected" like this, does that trigger needing to redo A-Chain cal?

[Steve Guttag]

In the world of magnetic playback...everything triggers A-chain calibration. Including use.

[Ryan Gallagher]

In the world of magnetic playback...everything triggers A-chain calibration. Including use.

LOL. heard. Need to get comfy with that process it seems. ;-)

I've also confirmed we have the 22K resistors installed per FB144 (in the 100mv position on our Cat 201 cards) for the modern oxides/record levels.
http://www.film-tech.com/warehouse/m...DolbyFB144.pdf

Will still flip projectors on the MPU to trace confirm noise is in the Ampex or head connections. But read the mag setup section a few times now, we thankfully have a reasonable RTA and Scope in the booth I can use when needed (and a Smart License + mics if really needed elsewhere). What I don't have is a CAT 85B pink noise card (that I have seen), but that is more about B chain and the room from my understanding, and the AP20 does all that, so I just need flat responses on the CP200 channels and confirm Dolby Levels I believe. (And maybe re-confirm it's doing the magnetic aux sub-woofer handling/routing while at it).

We have pink and tone 70mm mag loops and stock thankfully.

All this will be to confirm we really DO need a replacement 6-Track Teccon mag Head of course. ;-) Which we should just order anyway, cause the Ampex WILL die, if it's not already dead.

[Brad Miller]

We have pink and tone 70mm mag loops and stock thankfully.

Waiting for Steve to chime back in to point out all of the pink and tone loops require different calibrations. lol

Actually for that matter, EVERY print you will come across needs a different calibration to play it back properly. The problem is even if you have the original loops that were sounded with the print, the actual mag track wear on the print, or on a certain reel(s) of a print, may very well likely make the loops incorrect to use as a calibration reference. That's where having a well-trained ear and a knack for making adjustments comes into play for 70mm mag prints.

[Ryan Gallagher]

Waiting for Steve to chime back in to point out all of the pink and tone loops require different calibrations. lol

Actually for that matter, EVERY print you will come across needs a different calibration to play it back properly. The problem is even if you have the original loops that were sounded with the print, the actual mag track wear on the print, or on a certain reel(s) of a print, may very well likely make the loops incorrect to use as a calibration reference. That's where having a well-trained ear and a knack for making adjustments comes into play for 70mm mag prints.

Yeah, starting to realize what a beast mag was to do “right”. A step up would be to have the calibration loops from both oxide/width eras, but yeah so as loops wear and prints wear and every mag head that struck a print had non identical alignment.

Perfection is on a per print basis it seems. Perfection is not our goal just yet, I’d be happy with incremental improvements via learning the process, which will be all I can hope for considering we might show one of em a year.

[Tony Bandiera Jr]

Separately, this is essentially the vintage interconnecting cable installed from heads to MPU in our booth (although minus the P, non-plenum version)
https://marvacsale.shop/product/25-2...c-electronics/

Another possible source of the noise AMPEX head noise issues.

If we redo anything i'm reading in the forum that Belden 8451 or 9451 (or equivalent) are recommended.
If we are gilding the lily, spring for Starquad bonded pairs?

I have used that cable from Marvac in the past and had no issues with it. It is stiffer and not as easy to strip as Belden but it worked well.

Any equivalent to Belden, like West Penn, is fine. We are not talking ultra high frequencies or super long distances here, where cable capacitance becomes an issue even at audio frequencies.

Forget the Starquad bonded pairs, waste of time and money unless you are in a severe RFI/EMI environment. Use that money elsewhere, like the Teccon heads (Good idea to change the other one at the same time and keep the old one as a spare) or your rewind table parts.

[Steve Guttag]

I disagree with Tony on the wire. Star-Quad is the the way to go for mag head wires. Failing that Belden's 9452 is proven to be vastly superior to 8451/9451. I've had cables from mag heads pick up all sorts of things on their way to the preamp. Heck, I'd recommend putting the preamp out by the projectors and run line level back to the CP200 (or whatever cinema processor you are using).

I have certainly used standard cable, including snake (e.g. Belden 1411R and its family) and have "gotten away" with it. But, if the installation is an unknown and I don't want to have to do it twice and troubleshoot noise issues...star-quad.

[Jim Cassedy]

Speaking of magnetic sound heads. . . .
Last week I needed to get serial numbers off of a pair of IREM rectifiers that were
located in 4ft high crawlspace under the projection booth of a private screening room,
and I came across these these two 35mm AMPEX penthouses. Unfortunately, I didn't
have the time to inspect the heads to see what kind of shape they're in, but I'll be going
back there again soon since I made a deal to get a rewind bench they're not using.
(They also offered to throw in a Goldberg re-winder, but I've already got 3 of those! )

AmpexMag_1.jpg
Although these haven't been used in years, the penthouses themselves appear to be very clean
I just wish I had thought of inspecting the head clusters, but I was very tight on time that afternoon.
AmpexMag_2.jpg

Another anecdote: Back in the late 90's I was engineer in charge of a 1" videotape editing facility.
All of our videotape decks, timebase correctors, switchers, etc were all made by AMPEX, which at that time
still had a huge facility a few miles south of San Francisco. We had a service contract with them, and about
once every other month or so, the service guy would come up and do scheduled maintenance on the machines.
I once mentioned to him that I had a 35mm projector at home which had a working mag-sound penthouse on it,
and that the head cluster 'wasn't in great shape' but it still worked. Well, on his next visit about 3months later, he
brought me a brand new set of heads for my penthouse. The story he told me was that when Ampex got out of
the film-sound business, somebody forgot to tell 'the head factory' and they kept manufacturing these head
clusters for at least a month or two before they shut production down - and that there was "a warehouse full
of them" - - so he grabbed a set for me. I don't know how accurate that story was, but that's what he told me.

[Tony Bandiera Jr]

I disagree with Tony on the wire. Star-Quad is the the way to go for mag head wires. Failing that Belden's 9452 is proven to be vastly superior to 8451/9451. I've had cables from mag heads pick up all sorts of things on their way to the preamp. Heck, I'd recommend putting the preamp out by the projectors and run line level back to the CP200 (or whatever cinema processor you are using).

I have certainly used standard cable, including snake (e.g. Belden 1411R and its family) and have "gotten away" with it. But, if the installation is an unknown and I don't want to have to do it twice and troubleshoot noise issues...star-quad.

I kinda knew you would.

I have used the Belden in many rooms, including a few studio rooms with those new-fangled VFD drives (notorious for generating all kinds of noise) and never once had any issues, mag or optical. It is all about proper routing, separation and shield termination. I would always have completely separated metal couduits, sealtite and gutters for all audio and low voltage controls. Data and high voltage power or controls never got installed close to audio.

Star Quad would be, as I brought up, great for extreme RFI/EMI hostile environments (like broadcast studios and recording studios where every minute is super expensive), but in all the theatres and screening rooms I'd done over 3 decades, I'd never encountered the need for it. And since Ryan is in a situation with bean counters, I see absolutely no need to spend the money on upgraded wire where it is most likely not necessary. He might as well change all the sound rack receptacles to the $300 gold audiophile ones while he's at it.

[Steve Guttag]

And in my over 4-decades, I've been burned a couple of times and yes, picked up VFD noise even (before it left the projector). My mag wire path is always in grounded metal conduit/flex. The Snake cable, actually has the benefit of being double shielded each pair gets its own drain and the snake has an overall shield). Belden 9452 is a good step between traditional cable and star-quad. It isn't all that much harder to work with. It has about twice the twists per inch and a "tape shield" to contend with but, otherwise, terminates like 8451/9451.

See, I just don't think of Star-Quad as all that esoteric. I've used a couple of different brands but mostly Belden. Again, it isn't all that hard to work with...just match the colors.

Screen Shot 2024-09-23 at 3.23.36 PM.png

For optical, I've had my best luck with Belden 8723 (2-pair with a common drain).

But hey...use whatever floats your boat. I'm amazed that much of any magnetic film is being run anymore. There is about a 10-year span where the film shouldn't have faded and magnetic was the 70mm sound. So, the surviving prints would be dwindling. But for any clients of mine that would plan on putting in magnetic sound, I'd budget in the star-quad as the cable. Fortunately, the clients I already have, that are likely to run magnetic already have their systems.

[Ryan Gallagher]

I’ve definitely listened to the head noise with both the rectifier/switchers off and the VFDs unpowered. Still present, so if it follows the ampex head and a new teccon doesn’t solve anything, re-termination or wiring upgrades may be warranted. Thanks for the input on cable types.

[Jim Cassedy]

The booth I worked at the Telluride Film Fest this year would pick up static/buzz from our
digital walkie-talkies if we stood within several feet of the sound rack, but this was only a
problem when running 35mm. (Optical sound through a CP-65 which fed into a CP-750)
I'm sure it was a shielding issue, as the rack wasn't the neatest job I've encountered. The
festival folks have promised to look into it before next years shows. There were VFD drivers
on those projectors for running silent stuff, although this year we were only running 24fps, but
I was running an archival print of a 1940's movie, so I had programmed a 3sec ramp-up
time into the VFD's to protect the print, but we didn't have any noise from the VFD's.
(TB-Woods "E-Trac X2C" VFD's - almost every venue at Telluride has them. Never heard of
any RFI or noise problems in the 15 years I've worked there)