Any Forsell Air Reference LTD owners here?

Ki Choi

Member Sponsor
May 13, 2010
749
2
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Seattle WA area
#41
Hi Francisco

Got the Takasuki SSP-6GA air pump with Forsell three cylinder silencer air supply setup just for the platter air bearing. Considering the factory drive motor is an AC Synchronous type fixed 600 rpm speed Hurst motor, other than changing the AC supply 60hz frequency, how do you adjust platter speed? Or does your Forsell have rock solid speed performance even with its original factory setup?

Thanks,
Ki
 
May 30, 2010
15,212
561
113
Portugal
#42
Hi Francisco

Got the Takasuki SSP-6GA air pump with Forsell three cylinder silencer air supply setup just for the platter air bearing. Considering the factory drive motor is an AC Synchronous type fixed 600 rpm speed Hurst motor, other than changing the AC supply 60hz frequency, how do you adjust platter speed? Or does your Forsell have rock solid speed performance even with its original factory setup?

Thanks,
Ki
In the standard Forsell you can not adjust speed - the power supply is just a transformer with some resistors that are switched in the circuit after the motor starts to reduce motor vibration. As you say the only way to change speed is using a variable frequency power supply.

I used a simple AC generator - a sinusoidal generator driving an 50W power amplifier feeding a step-up transformer for the synchronous motor. The Air Force One has a DC motor with a tachometer coupled to the flywheel and the LP never ends at the same speed it started. ;) It is why I am using an alternative motor drive while I am modifying the flywheel.
 

Ki Choi

Member Sponsor
May 13, 2010
749
2
18
Seattle WA area
#43
In the standard Forsell you can not adjust speed - the power supply is just a transformer with some resistors that are switched in the circuit after the motor starts to reduce motor vibration. As you say the only way to change speed is using a variable frequency power supply.

I used a simple AC generator - a sinusoidal generator driving an 50W power amplifier feeding a step-up transformer for the synchronous motor. The Air Force One has a DC motor with a tachometer coupled to the flywheel and the LP never ends at the same speed it started. ;) It is why I am using an alternative motor drive while I am modifying the flywheel.
It makes sense... I will source an AC Variable Speed Drive to power the Hurst motor on mine. I have a friend who told me his Forsell has a rock solid speed but even if he can have it at the perfect speed before playing the LP, considering size of the drive motor, friction from the stylus tracking the grooves should introduce enough drag to slow it down, IMO especially with no feedback loop for the speed.

I guess when you refer to Air Force One, it is Forsell kind and not the TechDAS one. ;)

But your setup of having a flywheel driven by a DC servo motor makes even better sense ultimately when you have the tachometer mounted at the platter for actual speed feedback.

Ki
 
May 30, 2010
15,212
561
113
Portugal
#44
It makes sense... I will source an AC Variable Speed Drive to power the Hurst motor on mine. I have a friend who told me his Forsell has a rock solid speed but even if he can have it at the perfect speed before playing the LP, considering size of the drive motor, friction from the stylus tracking the grooves should introduce enough drag to slow it down, IMO especially with no feedback loop for the speed.

I guess when you refer to Air Force One, it is Forsell kind and not the TechDAS one. ;)

But your setup of having a flywheel driven by a DC servo motor makes even better sense ultimately when you have the tachometer mounted at the platter for actual speed feedback.

Ki
Yes. I always referred to my Forsell as the Forsell with the air flywheel, but since the TechDas has shown I recuperated the original designation of the model "Forsell Air Force One Reference". ;)

The main problem of the DC motor is that it has a very low torque and needs to control directly a very large and heavy flywheel - any change in speed needs some tens of seconds to stabilize! I am considering adding a pulley and an higher torque motor, such as a Studer/Revox capstan motor and control circuit.

Having the tachometer built in the platter increases the complexity of the electronic control of the motor - we would have to model and include the belt elasticity in the servo system.
 

Ki Choi

Member Sponsor
May 13, 2010
749
2
18
Seattle WA area
#45
My initial search for AC frequency drive had not been successful. Most them in the market are for industrial three phase type and won't work with single phase Hurst 3203-001 PB motor... I did find the VIP SDS drive and Clearaudio's Synchronous motor drives that should work better.

On the other hand, its speed variance is very small I can see creeping just a little forward and haven't check it with stylus tracking a record yet. My search for perfect speed might be not practical although my ears are very sensitive to pitch changes in records and tape play.

Before I go any further, I will make up another air silencer/filter setup so that I can have dedicated air supplies for tonearm and platter air bearing. As a second pump, I have a pump with same air spec but a bit louder that can be positioned far away.

Have you tried to adjust the airflow to the platter bearing by turning the brass screw on the side of the air bearing body under the turntable? I have tried to restrict the airflow to slow down the platter just a little but it isn't making any difference beyond floating or no floating of the platter. Without the belt the platter and with its own air supply, it spins for a long time on its own after just a slight push. It might be too much air. The manual says to let it stop and see if it will just try to reverse rotation but haven't reach it yet.
 

ddk

Industry Expert
May 19, 2013
3,897
407
83
Utah
#46
My initial search for AC frequency drive had not been successful. Most them in the market are for industrial three phase type and won't work with single phase Hurst 3203-001 PB motor... I did find the VIP SDS drive and Clearaudio's Synchronous motor drives that should work better.

On the other hand, its speed variance is very small I can see creeping just a little forward and haven't check it with stylus tracking a record yet. My search for perfect speed might be not practical although my ears are very sensitive to pitch changes in records and tape play.

Before I go any further, I will make up another air silencer/filter setup so that I can have dedicated air supplies for tonearm and platter air bearing. As a second pump, I have a pump with same air spec but a bit louder that can be positioned far away.

Have you tried to adjust the airflow to the platter bearing by turning the brass screw on the side of the air bearing body under the turntable? I have tried to restrict the airflow to slow down the platter just a little but it isn't making any difference beyond floating or no floating of the platter. Without the belt the platter and with its own air supply, it spins for a long time on its own after just a slight push. It might be too much air. The manual says to let it stop and see if it will just try to reverse rotation but haven't reach it yet.
I've been using a Chroma 61501 unit for my tables for many years without any problems. It's very flexible and has a very extended range of frequecy and voltage control. The only downside is the fan noise, it needs to be in another room. I just run a long 10 gauge wire from the unit to the tables.

http://www.chromausa.com/acpowersources.php#61500lo

david
 

Ki Choi

Member Sponsor
May 13, 2010
749
2
18
Seattle WA area
#47
I've been using a Chroma 61501 unit for my tables for many years without any problems. It's very flexible and has a very extended range of frequecy and voltage control. The only downside is the fan noise, it needs to be in another room. I just run a long 10 gauge wire from the unit to the tables.

http://www.chromausa.com/acpowersources.php#61500lo

david
David:

Thanks for the info. it looks like a good one. My other option was old PS Audio P300 with variable frequency control. Which turntable you use needs speed variation via frequency control?
 

ddk

Industry Expert
May 19, 2013
3,897
407
83
Utah
#48
David:

Thanks for the info. it looks like a good one. My other option was old PS Audio P300 with variable frequency control. Which turntable you use needs speed variation via frequency control?
I had a PS Audio it doesn't work well for this purpose. I have several tts that I use this with. EMT 927 & Thorens Reference, both are 220v/50hz, two American Sounds, they use the Micro Seiki RY-2000 motors that are synchronous with no controllers.

david
 
May 30, 2010
15,212
561
113
Portugal
#49
David:

Thanks for the info. it looks like a good one. My other option was old PS Audio P300 with variable frequency control. Which turntable you use needs speed variation via frequency control?
The PS Audio P300 was great for this application, although as far as I remember the variation in frequency was in large steps, not enough to correct speed of a turntable. Mine is powering a friend Sirius turntable bought from the US at 60 Hz.

If you want to build your own power supply you can find the schematics in Paul Horowitz "The Art of Electronics" - the Wien Oscillator, using a small lamp to create the nonlinearity in the feedback loop is very stable.
 

KennyG

New Member
Dec 13, 2018
3
0
1
63
#51
Thanks Fransico for your reply and the photos.

The airline connectors are non-industrial type as you said. It might be for serious aquariums I guess. the difficult part for us in US is that the tubing is 6mm OD that is very close to 1/4" OD (and I've confirmed it worked as replacements) but the airline female fitting turned out to have M10 thread that is unobtainable here.

I am rebuilding the platter air bearing and separate air systems for the platter and the arm as you have done. Since I have the factory system with one air supply to be splitted into two for the turntable, i would need to source additional air setup.

However, the air bearing idea is very intriguing to me for both the platter and the arm if they work properly to float and to be isolated from all mechanical vibrations. Guess it is easier said than practically done though.
Greetings! I am new to this forum, but once I registered I did a search for Forsell and found this thread. I currently have my Forsell TT apart and have been doing some modifications to the tonearm to stop that whistling that happens when one of the guide tube's holes get half way covered up.
Now that that is complete I'd like to take the air bearing apart and rebuild it...or at least clean & inspect it. However I can't figure out how to get the platter off! Would you please give me instruction on how to get the platter off?
Thanks a Lot, Kenny
 

KennyG

New Member
Dec 13, 2018
3
0
1
63
#52
I think I've somehow merged my reply with Ki Choi's...I was trying to quote his post about rebuilding the air bearing on his Forsell Table...been a long day.
 

Ki Choi

Member Sponsor
May 13, 2010
749
2
18
Seattle WA area
#53
The Kenny G?

I have rebuilt the air bearing on my Forsell after talking my wife into visiting Stockholm during our vacation one year so that I can meet the man,
Lennart Bergqvist in person. Lennart was Dr. Forsell's right hand person who ran manufacturing. He is also a jazz drummer. I bought parts to the Forsell and got a lesson on how to disassemble the air bearing for repair from him.

Basically, the air bearing wasn't designed for service. I guess it was meant to last forever...

So in order to get to the critical part of the air bearing, you have to remove as much of the glue that holds the aluminum round housing to the plate as possible and use force to remove the housing.

there's a one Delrin machined part that regulates airflow. More than likely, you need to take the worn part to your local machine shop and make a new one.

Pieces_of_Delin_found[1].jpg

Hope it helps.

Ki
 
May 30, 2010
15,212
561
113
Portugal
#54
The Kenny G?

I have rebuilt the air bearing on my Forsell after talking my wife into visiting Stockholm during our vacation one year so that I can meet the man,
Lennart Bergqvist in person. Lennart was Dr. Forsell's right hand person who ran manufacturing. He is also a jazz drummer. I bought parts to the Forsell and got a lesson on how to disassemble the air bearing for repair from him.

Basically, the air bearing wasn't designed for service. I guess it was meant to last forever...

So in order to get to the critical part of the air bearing, you have to remove as much of the glue that holds the aluminum round housing to the plate as possible and use force to remove the housing.

there's a one Delrin machined part that regulates airflow. More than likely, you need to take the worn part to your local machine shop and make a new one.

View attachment 46520

Hope it helps.

Ki
Thanks Ki, but how did you remove the platter? With the platter in position we can not access anything.
 

Ki Choi

Member Sponsor
May 13, 2010
749
2
18
Seattle WA area
#55
You will see the shaft in the far right of the photo. This is after I had removed the retainer ring. But to get to the retainer ring, you have to remove the glued cap. I will see if I can add another photo.

DSC_0055.JPG
 

KennyG

New Member
Dec 13, 2018
3
0
1
63
#56
You will see the shaft in the far right of the photo. This is after I had removed the retainer ring. But to get to the retainer ring, you have to remove the glued cap. I will see if I can add another photo.

View attachment 46524
No, the "other" KennyG! Thank You very much, this sheds light on the whole thing. I don't think there's anything wrong with mine at this time, so I think I'll wait to take it apart, now that you've shown me the way.
I Thank You VERY much!
 

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