6 Track Mag Head Maintenance

[Ryan Gallagher]

The Annis is powerful enough for most stuff you'll encounter. BTW, all teeth pointing north means no magnetism. JJ sprockets are also made of 316 stainless, which is the most non-magnetic steel made, but it can become paramagnetic from other magnetic particles.

Yeah maybe just pulling the drive sprockets to get behind them in all the nooks and crannies for a cleaning would eliminate the reads I'm seeing. Or at least I could tell if that source was still present and not associated with the sprockets themselves.

[Mark Gulbrandsen]

There are other steel parts too. Pad roller shaft, and the pad roller arm open, close detent mechanism , the pad roller arm itself is usually cast iron. Not everything in the projector is 316 stainless...

[Ryan Gallagher]

The Annis is powerful enough for most stuff you'll encounter. BTW, all teeth pointing north means no magnetism. JJ sprockets are also made of 316 stainless, which is the most non-magnetic steel made, but it can become paramagnetic from other magnetic particles.

Annis Hand-D-Mag did the trick once it arrived. Had both upper drive sprockets and shafts out... both had some detectable magnetism in places, (.1-.4 mT)... but not after hitting em with Hand-D-Mag, which definitely can handle bigger parts than the VCC (which I'm saving just for degaussing readers).

I think part of my battle was that my meter picks up some flux when the chassis work-bulb is illuminated, making checking the parts in situ hard until I caught that. And now i'm wondering if that weak field is what is magnetizing these parts over time? Time will tell I guess.

Edit to add photo of today's fun:
hand-d-mag.jpg

[Mark Gulbrandsen]

if you have a Teccon head, they generally don't become very magnetized like an iron core head does. The housing is non ferrous brass, the poles are Sendust, but those may give a slight reading on your magnetometer. Also check the mounting hardware as it may or may not be ferrous.

[Ryan Gallagher]

if you have a Teccon head, they generally don't become very magnetized like an iron core head does. The housing is non ferrous brass, the poles are Sendust, but those may give a slight reading on your magnetometer. Also check the mounting hardware as it may or may not be ferrous.

I know our 70mm heads are slightly different between the two. Will investigate model. 35mm mag is not even wired in our booth. In my sweeps prior with that meter I found nothing anywhere except near the upper drive sprocket, which is resolved now, outside of what amount it picks up from the threading light when on. Not sure that is "normal" or not. If it's a DC bulb that makes total sense.

[Ryan Gallagher]

if you have a Teccon head, they generally don't become very magnetized like an iron core head does. The housing is non ferrous brass, the poles are Sendust, but those may give a slight reading on your magnetometer. Also check the mounting hardware as it may or may not be ferrous.

One is Teccon indeed. The other is Ampex.

Our No. 1 projector with the Ampex is the one we have slight crackle/pop noise issues with (with and without film) that have not been traced yet. I have not tried swapping 1 & 2 at the MPU yet but that is on the list for tracing it.

[Mark Gulbrandsen]

One is Teccon indeed. The other is Ampex.

Our No. 1 projector with the Ampex is the one we have slight crackle/pop noise issues with (with and without film) that have not been traced yet. I have not tried swapping 1 & 2 at the MPU yet but that is on the list for tracing it.

You are having noise with the Ampex head because it's a high impedance head... Ampex heads were original to the JJ's. If you can find a good Teccon, I'd replace the Ampex. Teccon are low impedance and also have higher output, as well as better frequency response. The Dolby preamps can accommodate either, but it may take a bit of moving wires around on the back of the preamp depending on how it's wired now. If you happen to have the transformerless preamp cards then all the better, but not sure how many of those actually made it into the field...
If you can't find a Teccon, and if the Ampex isn't too worn, then it'll do for now. But keep in mind they do wear fast.

[Ryan Gallagher]

You are having noise with the Ampex head because it's a high impedance head... Ampex heads were original to the JJ's. If you can find a good Teccon, I'd replace the Ampex. Teccon are low impedance and also have higher output, as well as better frequency response. The Dolby preamps can accommodate either, but it may take a bit of moving wires around on the back of the preamp depending on how it's wired now. If you happen to have the transformerless preamp cards then all the better, but not sure how many of those actually made it into the field...
If you can't find a Teccon, and if the Ampex isn't too worn, then it'll do for now. But keep in mind they do wear fast.

Noted on which is more desirable. Cheers.

We have 8 CAT92C Rev4 Cards for 70mm + Right-Extra installed.. I am not seeing a transformer on these.

IMG_5747.jpg

So at the MPU inputs there is the soldered jumper for low vs high. Is that for impedance matching the heads? Cause all of ours are soldered in the high position. I suppose time to read some manuals.

If that is true and related it's actually correct for the Ampex, and set wrong for the Teccon which I presume was a replacement at some point. Doubt that would solve the noise problem but could be at least one thing that is "wrong".

[Mark Gulbrandsen]

Noted on which is more desirable. Cheers.

We have 8 CAT92C Rev4 Cards for 70mm + Right-Extra installed.. I am not seeing a transformer on these.

IMG_5747.jpg

So at the MPU inputs there is the soldered jumper for low vs high. Is that for impedance matching the heads? Cause all of ours are soldered in the high position. I suppose time to read some manuals.

If that is true and related it's actually correct for the Ampex, and set wrong for the Teccon which I presume was a replacement at some point. Doubt that would solve the noise problem but could be at least one thing that is "wrong".

Yep, those are the good cards. I used those on my dailies projectors at many, many locations and had zero issues. Check with Brad about aTeccon head...

[Paul H. Rayton]

OMG, I can't believe an Ampex head is still actually in service!! The last time I was any of them was back in, like, 1980 or so, and they were surplus from the equipment acquired by D-150. They were sold to some equipment dealer on the east coast (can't remember who), but I had been considering buying the batch. I'm glad I didn't, since the Teccons were vastly superior. But it floors me that the Paramount still had an Ampex head in service, considering all the 70mm that's been run there over the years. They wear down like cheese (to quote the expression from a Dolby tech I knew out here). I could see them still installed in some venue where no 70mm had been run for decades, but now, in 2024? Holy crap! I'd say replace it immediately, during your down-season (budget permitting). Even a used Teccon will be better than a used Ampex head (unless the Teccon is worn down to a nub). (Not to disparage Ampex too much -- they did "okay" for the state of the technology in ... the mid-1950s. But, jeez, times have moved on just a little!!)

[Brad Miller]

I'm sure I have a spare Teccon, but keep in mind it's not plug-and-play. A full A-chain of the mag penthouse must be performed and that means re-calibrating the mag penthouse of the other projector as well to ensure they both match as close as possible so you don't have noticeable frequency response differences between changeovers.

[Mark Gulbrandsen]

OMG, I can't believe an Ampex head is still actually in service!! The last time I was any of them was back in, like, 1980 or so, and they were surplus from the equipment acquired by D-150. They were sold to some equipment dealer on the east coast (can't remember who), but I had been considering buying the batch. I'm glad I didn't, since the Teccons were vastly superior. But it floors me that the Paramount still had an Ampex head in service, considering all the 70mm that's been run there over the years. They wear down like cheese (to quote the expression from a Dolby tech I knew out here). I could see them still installed in some venue where no 70mm had been run for decades, but now, in 2024? Holy crap! I'd say replace it immediately, during your down-season (budget permitting). Even a used Teccon will be better than a used Ampex head (unless the Teccon is worn down to a nub). (Not to disparage Ampex too much -- they did "okay" for the state of the technology in ... the mid-1950s. But, jeez, times have moved on just a little!!)

Paul,
The last Ampex head I saw and worked with was in Suburban Chicago when Back To The Future came out, so 1985... One of the 2nd run theaters played a 70mm print. The projector I found for them was a Century / Cinerama 70mm only machine that had an Ampex head. They ended up running gobs of 70mm stuff and I eventually replaced the Ampex head for a Teccon.

[Steve Guttag]

On the CAT92C...it only works if all of the MPU links on the back are in the "HIGH" position (which was silly, in my opinion, they could have just brought another track to the edge connector to pick up either position). In any event, you have two sets of links on the board. One is the High/Low and the other is for "Fringing Compensation." The High-Low will get you through any significant level issues. If your Ampex head is too far off with post-1983 tracks (too high a level and tough to control HF), put a 330Ω resistor across the input of each channel on the back of the MPU...you'd be amazed how that will bring it into line.

Fringing compensation is for running older head (narrower) on modern tracks. Any by narrower, I don't mean measuring the physical oxide...I mean the pre-1983 record head widths were a little narrower than the post 1983 tracks.
For a short read on the CAT92C. See this download from a shady site:

http://www.film-tech.com/warehouse/w...754&category=2

[Ryan Gallagher]

OMG, I can't believe an Ampex head is still actually in service!!

Haha! Well they have run 70mm here for eons, but not a TON of it. If you consider it was typically booked as a "70mm week" once a year. The Ampex head has probably run less film in it's service here than in the century's previous life where ever they came from. They are pretty remiss to throw anything out... and I don't have another Ampex head lying around... so I'm guessing these units got installed with the mismatch having different histories. They are definitely non-matching JJs in a myriad of ways, more every time I look closer at anything.

Cheers Steve, Mark, and Brad. Slow and steady over here... not diving in over my head just yet, probably won't have a chance to touch the audio tracing for a few weeks yet. Noted on the A-Chain if head is replaced. We'll see what the wallet holders will say, but I feel I need to at least confirm the noise issues follow the Ampex head in order to sell em on it, and then they have to weigh their commitment to 70mm mag capabilities.

I know the A-Chain tuning would not be right, but would it make any sense to also try swapping the set of CAT92C cards for 1 & 2 (keeping note of which order they service). If our noise moved it would be pretty revealing, though I'm fully expecting the noises to follow the head wiring when I swap that.

[Mark Gulbrandsen]

High impedance heads, and high impedance audio systems in general are like an antenna for things such as a brief arc inside a wall switch, turning on the other projectors lamphouse, motor switches, etc. Especially things in close proximity to the head itself. My first dailies outfit had dubbers (The dubbers from hell!)with high impedance heads, and I had similar problems. But in that case both lamphouses were always left on, and any clicks there were came from the ptojector motor and chang over switchs. Later on I switched to Norton low impedance heads and Dolby preamps and all was well. We even had each film's transfer house send us several hundred feet of pink noise to be sure playback eq was good. The location sound mixer always worked with us on speaker eq. The DP ok'd the lamp color temp, levels and eveness of the light. Anyway, I guess you'll have to put up with that Ampex head till it wears out...