(To conform with file uploading limits a reduced size PDF file containing the figures is attached).
Purpose
* To share hints and lessons learned from the service I performed on my B790's turntable motor.
* An important focus of this lessons learned is how to separate the motor's rotor and stator assemblies safely and easily.
History
* My ReVox B790 Linatrack turntable serial number 004778 I purchased new in July 1978 from REW Audio Visual Co. Ltd, London, England.
* I enjoyed using my B790 throughout multiple worldwide assignments, however in 1983 when I purchased a new Oracle Premiere turntable my ReVox turntable went into storage for some 30 years.
* Before powering up after long storage, I purchased a comprehensive 'Full Monty' B790 overhaul kit from Nagravox http://www.nagravox.com and completed the replacement of all parts included in this excellent kit.
* However, after installing the kit and powering up, the platter spin was erratic and could not be controlled.
* Actually (and fortunately), there was nothing wrong with either the overhaul kit or the motor, but I learned my lesson the hard way. Investigating the Motor Control Board 1.179.152 found its operation was unstable causing the motor to lose control and behave erratically - this I have fixed and will document and share in a separate post.
* Please find my comments and suggestions, how to separate the B790 motor's rotor and stator assemblies safely and easily, and some straightforward motor checks.
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Special Tools and Jigs
* Small circlip pliers suitable for circlips size 7mm.
* Homemade wooden jig to support the turntable assembly when turned upside down.
Materials
* New circlips size 7mm (not a common size).
* PDP 65 lubricant or synthetic lubricant for turntable spindle bearings, as required.
* Moly grease for thrust bearing.
Dis-assembly
* Unless you really need to, try to avoid removing the whole motor from the top chassis. In my case I had to first remove the motor and tonearm to have the metal work professionally re-painted at an industrial paint shop (the original Nextel paint had become faded and was flaking off). Pulling out the full motor risks losing one or more of the 6 thin spacer shims between motor and chassis, furthermore these thin spacer shims are frustrating to reinstall - as I found out the hard way.
* By separating only the rotor from the stator, with the stator still attached to the top chassis will allow sufficient inspection of the stator coils.
* On the underneath of the motor, remove the three slot-head screws from the bearing thrust plate. Be careful not to break or remove the tiny wire strand(s) in the bearing well (Fig. 8).
* Using circlip pliers carefully remove the top 7mm circlip and flat washer.
* Next the tacho-toothed plate can be removed (Fig. 2).
Figure 2. B790 motor with tacho-toothed disc installed (left) and with disc removed (right) .
* While the 7mm circlips can be discarded and replaced with new, you need to retain the Belleville conical washers and low-friction shims.
* Important, take a photo showing the sequence order and orientation of all parts as they must be reinstalled in the same way otherwise the motor won't operate properly - as I found out the hard way with a Belleville conical washer reinstalled facing the opposite direction.
Figure 3. Order of dis-assembly, left to right; Outer (top) 7mm circlip, top washer, toothed tacho-disc, spacer plate, two Belleville conical washers, inner (bottom) 7mm circlip, three low-friction shims (one is hidden underneath)
* To safely separate the rotor and stator assemblies avoid using a pulley removal tool or levers - no matter how hard I tried with this method I was not successful! I believe the reason is because a significant amount of effort is required to overcome two strong forces:
a) The heavy weight of the rotor assembly.
b) Added to the strong magnetic pull of the rotor's magnet.
* I came up with a simpler method which I believe is quicker, requires no pulling tool or levers, and is gentler on the motor's parts with no damage inflicted from inserting levers. This method worked well.
* As mentioned, do not remove the motor from the top chassis. This brings two benefits:
1. The chassis acts as a frame holding the stator, providing a larger surface area to grip the stator.
2. No need to remove the thin spacer shims - these thin shims are easy to lose and frustrating to align during re-assembly.
* Place the chassis upside down on the wooden support jig. This way, gravity is helping to pull the heavy weight of the rotor assembly away from the strong magnetic pull of the rotor's magnet.
Figure 4. Top chassis placed upside down with rotor in place - note bare spindle is exposed
* With one hand under the wooden support jig to catch the rotor assembly when it falls, and the other hand above, simply push on the exposed spindle from above and the rotor assembly should drop down.
Figure 5. Rotor assembly pushed down below the wooden support jig.
* The top chassis can then be turned right-side up to inspect the stator coils. While I was worried about finding a broken conductor or scratched coil, fortunately this was not the case with all coils intact.
Figure 6. Inspection of stator coils.
* What I did observe though was the internal plastic locating plug had become loose.
ReVox B790 Service Note #67.18
"If an audible clank occurs when starting, stopping or changing the speed of the turntable, the plastic locating lug of the coil assembly inside the motor has become worn with the result that it has too much play in the slot in which it engages. To correct this condition, the motor has to be dis-assembled to gain access to the coil assembly. (Carefully observe the sequence of circlips and washers etc. for later re-assembly.) Once access is gained to the coil assembly apply a few drops of quick setting cement (Cyano Acrylate, Loctite etc.) so that it will seep in between plastic and metal parts. Carefully re-assemble the motor and watch out not to pinch your fingers because the powerful magnet pulls the two sections together with great force.
On late production units the parts were cemented together during assembly already."
Figure 7. Inspection of the internal plastic (black-coloured) locating plug - fixed with a small drop of epoxy glue
Re-assembly
* Using new circlips 7 mm size, re-assemble all parts in the reverse order from dis-assembly. Ensure the side of each Belleville washer faces in the same direction as before - this is important to maintain the spring characteristic to keep all spindle parts in compression.
* When fitting the top circlip, 7 mm size, make sure that the circlip fits correctly inside the spindle's groove - you need to push down on the toothed-tacho disc assembly to compress the Belleville washers.
* When applying a small amount of Moly grease to the thrust bearing well be careful not to break or remove the tiny wire strand(s) (Fig. 8).
* When fastening the thrust bearing plate, it must lie flat against the stator assembly, with the three slot-head screws fully tightened so that the spindle, and hence the rotor is pushed up to rotate clear from the stator.
Figure 8. Thrust bearing with wire strand in-place
Electrical Checks after Motor Re-assembly
* After re-assembly, recommend checking the electrical continuity of the coils.
* There are four (4) Motor Coils - For the B790, typically 55-60 ohms DC resistance measured between Connector Pins; #1 and #6, #1 and #8, #1 and #10, #1 and #12.
* Tachometer Coil - For the B790, typically 24-28 ohms DC resistance between Connector Pins #14 and #15.
Figure 9. DC resistance check of tachometer coil - rotor stationary
* Spinning the rotor by hand should generate a small varying AC voltage at wire harness pins #14 and #15.
Figure 10. Tachometer AC signal while spinning the rotor by hand
* From this experience I believe the motor can be considered 'ready to go' if all electrical checks are good and the rotor can be spun freely by hand with no noisy or scratching sounds.
Operational Checks after Turntable Re-assembly
* With the B790's phono leads connected to input of preamp and the preamp's line outputs connected to the AnalogMagik test package showed:
a) Exceptionally stable speed of 33 1/3 rpm corresponding to measured test frequency of 3,150 Hz.
b) Measured Wow & Flutter under 0.1%.
* Considering this is a 45-year-old turntable I was very pleased with these results.
Figure 11. After re-assembly, Wow & Flutter results - measured December 2022
Final comments
* The reason for this writing is to share my experience servicing a ReVox B790 turntable motor. At time of this writing the turntable has been operating without issue for 7 months now.
* As mentioned, there was nothing wrong with either the overhaul kit or the motor, it was the Motor Control Board 1.179.152 where I found operation was unstable - this I fixed and will document and share in a separate post.
* Pleased to report my ReVox B790 turntable now tracks and plays LPs flawlessly, just like it used to!
Purpose
* To share hints and lessons learned from the service I performed on my B790's turntable motor.
* An important focus of this lessons learned is how to separate the motor's rotor and stator assemblies safely and easily.
History
* My ReVox B790 Linatrack turntable serial number 004778 I purchased new in July 1978 from REW Audio Visual Co. Ltd, London, England.
* I enjoyed using my B790 throughout multiple worldwide assignments, however in 1983 when I purchased a new Oracle Premiere turntable my ReVox turntable went into storage for some 30 years.
* Before powering up after long storage, I purchased a comprehensive 'Full Monty' B790 overhaul kit from Nagravox http://www.nagravox.com and completed the replacement of all parts included in this excellent kit.
* However, after installing the kit and powering up, the platter spin was erratic and could not be controlled.
* Actually (and fortunately), there was nothing wrong with either the overhaul kit or the motor, but I learned my lesson the hard way. Investigating the Motor Control Board 1.179.152 found its operation was unstable causing the motor to lose control and behave erratically - this I have fixed and will document and share in a separate post.
* Please find my comments and suggestions, how to separate the B790 motor's rotor and stator assemblies safely and easily, and some straightforward motor checks.
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Special Tools and Jigs
* Small circlip pliers suitable for circlips size 7mm.
* Homemade wooden jig to support the turntable assembly when turned upside down.
Materials
* New circlips size 7mm (not a common size).
* PDP 65 lubricant or synthetic lubricant for turntable spindle bearings, as required.
* Moly grease for thrust bearing.
Dis-assembly
* Unless you really need to, try to avoid removing the whole motor from the top chassis. In my case I had to first remove the motor and tonearm to have the metal work professionally re-painted at an industrial paint shop (the original Nextel paint had become faded and was flaking off). Pulling out the full motor risks losing one or more of the 6 thin spacer shims between motor and chassis, furthermore these thin spacer shims are frustrating to reinstall - as I found out the hard way.
* By separating only the rotor from the stator, with the stator still attached to the top chassis will allow sufficient inspection of the stator coils.
* On the underneath of the motor, remove the three slot-head screws from the bearing thrust plate. Be careful not to break or remove the tiny wire strand(s) in the bearing well (Fig. 8).
* Using circlip pliers carefully remove the top 7mm circlip and flat washer.
* Next the tacho-toothed plate can be removed (Fig. 2).
Figure 2. B790 motor with tacho-toothed disc installed (left) and with disc removed (right) .
* While the 7mm circlips can be discarded and replaced with new, you need to retain the Belleville conical washers and low-friction shims.
* Important, take a photo showing the sequence order and orientation of all parts as they must be reinstalled in the same way otherwise the motor won't operate properly - as I found out the hard way with a Belleville conical washer reinstalled facing the opposite direction.
Figure 3. Order of dis-assembly, left to right; Outer (top) 7mm circlip, top washer, toothed tacho-disc, spacer plate, two Belleville conical washers, inner (bottom) 7mm circlip, three low-friction shims (one is hidden underneath)
* To safely separate the rotor and stator assemblies avoid using a pulley removal tool or levers - no matter how hard I tried with this method I was not successful! I believe the reason is because a significant amount of effort is required to overcome two strong forces:
a) The heavy weight of the rotor assembly.
b) Added to the strong magnetic pull of the rotor's magnet.
* I came up with a simpler method which I believe is quicker, requires no pulling tool or levers, and is gentler on the motor's parts with no damage inflicted from inserting levers. This method worked well.
* As mentioned, do not remove the motor from the top chassis. This brings two benefits:
1. The chassis acts as a frame holding the stator, providing a larger surface area to grip the stator.
2. No need to remove the thin spacer shims - these thin shims are easy to lose and frustrating to align during re-assembly.
* Place the chassis upside down on the wooden support jig. This way, gravity is helping to pull the heavy weight of the rotor assembly away from the strong magnetic pull of the rotor's magnet.
Figure 4. Top chassis placed upside down with rotor in place - note bare spindle is exposed
* With one hand under the wooden support jig to catch the rotor assembly when it falls, and the other hand above, simply push on the exposed spindle from above and the rotor assembly should drop down.
Figure 5. Rotor assembly pushed down below the wooden support jig.
* The top chassis can then be turned right-side up to inspect the stator coils. While I was worried about finding a broken conductor or scratched coil, fortunately this was not the case with all coils intact.
Figure 6. Inspection of stator coils.
* What I did observe though was the internal plastic locating plug had become loose.
ReVox B790 Service Note #67.18
"If an audible clank occurs when starting, stopping or changing the speed of the turntable, the plastic locating lug of the coil assembly inside the motor has become worn with the result that it has too much play in the slot in which it engages. To correct this condition, the motor has to be dis-assembled to gain access to the coil assembly. (Carefully observe the sequence of circlips and washers etc. for later re-assembly.) Once access is gained to the coil assembly apply a few drops of quick setting cement (Cyano Acrylate, Loctite etc.) so that it will seep in between plastic and metal parts. Carefully re-assemble the motor and watch out not to pinch your fingers because the powerful magnet pulls the two sections together with great force.
On late production units the parts were cemented together during assembly already."
Figure 7. Inspection of the internal plastic (black-coloured) locating plug - fixed with a small drop of epoxy glue
Re-assembly
* Using new circlips 7 mm size, re-assemble all parts in the reverse order from dis-assembly. Ensure the side of each Belleville washer faces in the same direction as before - this is important to maintain the spring characteristic to keep all spindle parts in compression.
* When fitting the top circlip, 7 mm size, make sure that the circlip fits correctly inside the spindle's groove - you need to push down on the toothed-tacho disc assembly to compress the Belleville washers.
* When applying a small amount of Moly grease to the thrust bearing well be careful not to break or remove the tiny wire strand(s) (Fig. 8).
* When fastening the thrust bearing plate, it must lie flat against the stator assembly, with the three slot-head screws fully tightened so that the spindle, and hence the rotor is pushed up to rotate clear from the stator.
Figure 8. Thrust bearing with wire strand in-place
Electrical Checks after Motor Re-assembly
* After re-assembly, recommend checking the electrical continuity of the coils.
* There are four (4) Motor Coils - For the B790, typically 55-60 ohms DC resistance measured between Connector Pins; #1 and #6, #1 and #8, #1 and #10, #1 and #12.
* Tachometer Coil - For the B790, typically 24-28 ohms DC resistance between Connector Pins #14 and #15.
Figure 9. DC resistance check of tachometer coil - rotor stationary
* Spinning the rotor by hand should generate a small varying AC voltage at wire harness pins #14 and #15.
Figure 10. Tachometer AC signal while spinning the rotor by hand
* From this experience I believe the motor can be considered 'ready to go' if all electrical checks are good and the rotor can be spun freely by hand with no noisy or scratching sounds.
Operational Checks after Turntable Re-assembly
* With the B790's phono leads connected to input of preamp and the preamp's line outputs connected to the AnalogMagik test package showed:
a) Exceptionally stable speed of 33 1/3 rpm corresponding to measured test frequency of 3,150 Hz.
b) Measured Wow & Flutter under 0.1%.
* Considering this is a 45-year-old turntable I was very pleased with these results.
Figure 11. After re-assembly, Wow & Flutter results - measured December 2022
Final comments
* The reason for this writing is to share my experience servicing a ReVox B790 turntable motor. At time of this writing the turntable has been operating without issue for 7 months now.
* As mentioned, there was nothing wrong with either the overhaul kit or the motor, it was the Motor Control Board 1.179.152 where I found operation was unstable - this I fixed and will document and share in a separate post.
* Pleased to report my ReVox B790 turntable now tracks and plays LPs flawlessly, just like it used to!
