AMI Continental 2: RPM Too Fast

[Deadhead Japan]

Greetings all,


My Conti 2 is running a tad fast at 47% RPM.  How do I go about slowing them down?  I have read online that they were designed to operate at this speed.  Any truth to this?

Thanks in advance!

Neal

[jukenorman]

Are you on 50Hz? If so, the turntable motor shaft will be sleeved.

Norman.

[Deadhead Japan]

I am located in the US, but my jukebox came out of Canada many years ago. No sleeve on the shaft.

[jukenorman]

Lol, I thought that maybe you were a Grateful Dead fan from Japan! On 60Hz, I have no suggestions as to how it might run fast. How did you measure the speed? I have always thought that it was an urban myth that jukeboxes were built to run at more than 45rpm, I have never found any evidence. Maybe Marty might have some thoughts on your issue - see what I did there!!

Norman.

[jukeboxmarty]

The motor's speed is fixed by the line frequency, in the USA, it's 60 hertz.

The only other factor that can alter the speed to run faster is the diameter of the motor's output shaft... that's why they add a sleeve for 50 hertz operation.

Apart from changing the diameter of the shaft, there is no way to change the motor's speed.

I have found that many motors do run a little fast, but it's way better to have your records playing a little fast than a little slow.

I have a Continental mechanism sitting around that I recently refurbished, maybe I'll get curious and hook it up to measure the speed.

-Marty

[Deadhead Japan]

Norman and Marty,

I have a RPM app on my phone. It is pretty accurate. I used when I work on turntables. It will give you wow and flutter.

I have read also that the turntables were designed to run fast… don’t know how much truth there is to that.

As far as Deadhead Japan… I am Deadhead that lived in Japan for 10 years.

Neal

[jukenorman]

If you've serviced the motor - and I don't think that I have never known servicing slow down a motor - the only option is to reduce the diameter of the motor spindle to achieve 45rpm. The reason I was asking about how you measured it was because some people use extremely crude methods where errors are likely. 2rpm is too fast, 0.2rpm would be acceptable. Another thought, is your power supply from a grid, it's not some private system that couldn be greater than 60Hz, is it?

Norman.

[Deadhead Japan]

To be honest, I did not notice… my son is the one who noticed that song’ was playing too fast. Power supply is at 60 hz. I will look in to it further.

[e093116]

Hi, I've observed the same thing with my Continental 2.  I have the automix version, which is capable of playing at both 33 and 45 rpm.  This has a sleeved/stepped motor spindle with two different diameters for the different speeds.  I am still trying to sort out the issue on my machine.  I'd like to be able to "buy" my way out of the problem and have a spare motor or two to experiment with, but it seems as though the Continental 2 automix motors are hard to come by...I've looked at adapting a later model motor but it looks like the spindles are all too long and the mounting is different too.

Do you have the single speed motor?

If your motor shaft is "sleeved" you can modify the existing sleeve or create/substitute a different diameter sleeve to correct the speed.  I'd be hesitant to modify the motor shaft itself, if only you find later that the "problem" lies somewhere else and then you have a motor that runs too slow!

Given that the motor design is a fixed speed based on 60 hz operation, I find it hard to believe that when the rest of the associated parts are serviced and "correct", that a speed 2 rpm off was an acceptable production tolerance for 1962, I agree that it should be more like +0.2 rpm max...I have AMI H and K jukeboxes that both run at "exactly" 45 rpm.  To my ear, records that play faster than ~45.5 rpm start to sound shrill/sharp (and I've never liked Alvin and the Chipmunks)...so having this issue on my "flagship" jukebox is rather annoying...

I am using a Sticht tachometer to measure the speed.  It is capable of measuring with direct contact and also non-contact with the use of a small reflective piece of tape added to the item of interest.

Anyway, just some thoughts and comments to add to the discussion.  

I'd be curious to read a report on what Marty's refurbished mech runs at.

Bill 

[Deadhead Japan]

Bill,

My box has the Auto/Mix version. I have verified the speed at 33 RPM and it's slightly higher at 34% and change. I don't recall exactly. As mentioned above, my son is the one is caught the sound off listening to Men at Work: Down Under. I hear it slightly on this selection, but not on other selections. The music does not sound like Alvin... Hence, why I checked it. I really question if it is actually turning at 47%. I have thought about finding a turntable motor that is a non Auto Mix to see the results. I believe the turntables are the same Model K thru Continental (please correct me if I am wrong). As of right now, I only have one jukebox EP and the entire Led Zeppelin single catalog which are all a small hole. I really do not want to "dink" the records.

Let me know what you find out with yours.

Neal

[jukenorman]

Hi Neal,
There's a 60Hz strobe disc here, if you want a second opinion, that will confirm whether it's 47rpm. Print it out and paste on to an old record. Obviously it has to be operated with the original fluorescent tube - LED tubes need not apply!
www.dropbox.com/s/90xft6klj0qigt9/Strobe_45_60hz.pdf?dl=0

Norman.

[e093116]

Hi...thought I'd update with the rest of my story regarding fast running Continental 2.

I ended up buying several later model motors from ebay, with the idea (read: hope) that I'd be able to swap some parts around to solve the problem.

What I found is that the rotor assemblies are more or less interchangeable, but with different details (length, diameter) on the drive aka "business end" of the rotor (motor output) shaft.

The rotor assembly can be disassembled rather easily, the shaft is a light press fit in the rotor.

I found that the more recent motor shafts (MM1 and so on) are long enough that the shaft can be modified to cut off the existing 45/33 rpm drive diameters, and new diameters can be machined on the now shorter shaft to be of appropriate length and diameter to match the Continental 2 shaft, or slightly smaller diameters if you want to tune the speed to be closer to 33.3/45 rpm.  This shaft can then be pressed back into the rotor (to create an assembly) that can drop in to the existing Continental 2 motor (with all of it's specific mounting and other details).  So you don't have to necessarily harm an original Continental 2 rotor assembly if you don't want to.  I made a new rotor assembly this way, but with the diameters changed to (hopefully) correct the speed to be 33.3/45 rpm.

In my case I had initially attempted to "file my way to goodness" on the original Continental 2 rotor, and while I was able to reduce the speed, the filing introduced a ton of wow/flutter. Based on what I now know, I wouldn't recommend attempting to file or sand on the motor shaft...it really needs to be done under more precise conditions.

With the "new" rotor installed, I'm happy to report that the 33.3/45 speeds are appropriate, wow/flutter is much improved/non-existent, and Axl Rose no longer sounds like Daffy Duck.

As the motors do not have a nameplate or any sort of stated rpm on them that I could find, I'm hesitant to state the diameters I used, in the case that someone copies the information and then is upset when "it doesn't work".  Rather I'd suggest that if anyone wants to do the same, carefully measure the turntable speed and use the ratio of the measured speed compared to the target speed, and then reduce the shaft diameter by the same percentage.  In my case I ended up needing to reduce the shaft diameters a little bit less than 3%.

Another comment is that 0.001" of motor shaft diameter is worth roughly 0.3 rpm at 45 rpm (based on calculation), so when machining the shaft you really need to communicate to the machine shop that they need to hold a tight tolerance on the diameter.  I specified +0/-.0005" for tolerance in this regard.

Finally, other things to pay attention to when swapping shafts, check where the "new" shaft contacts the support bushings in the motor housings for wear (I had one shaft that was heavily worn) and to beef up the press fit when you reinstall the shaft, consider adding a couple peen marks with a hammer/punch on the shaft (to create a high spot or two for "tighter" press fit back into the rotor) and/or use some red or green loctite on the shaft.  Generally you lose the tolerance on a light press fit when you push it apart and you want to make sure that your new rotor assembly stays together for the next 50+ years...

[Deadhead Japan]

Bill,

Way above my brain bytes! Like I mentioned earlier... I don't notice the tempo difference. I printed up the strop disc that Norman provided and my box seems to be on speed. 2% faster is a gross error and all songs would sound really off.

Suffering Succotash! its awesome that Axl no longer sounds like Daffy!

Neal

[Deadhead Japan]

All,

My jukebox restoration is finally complete. I loaded the carousel up with records and the majority of the songs were playing fast. It was most notable when playing faster paced songs. It was driving me crazy.

I know the the word on the street is that the dimension of the idler wheel does not influence the speed, but I decided to micrometer the new idler wheel I had installed during the installation. The newly installed idler measured at 44 MM and the old one is 43 MM. When I started the restoration, I replaced the idler because the rubber was hard a rock. I never bothered checking the speed of the table. Well, I put the old idler in after soaking it in dish soap for an hour (read online it softens the rubber), the turntable clocked in at 45 RPM and the songs sound great! Needless to say, I am very happy.

Neal