Preface
* This is a supplement to the earlier report "ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned", where it was stated "Unless you really need to, try to avoid removing the whole motor from the top chassis. 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."
* The motor/chassis thin spacer shims are needed to maintain a correct height and evenness of level, however the different sizes of shims are frustrating to align and can fall out of place when trying to reinstall the motor to the chassis. Furthermore, you might find that once these spacer shims are removed it may be troublesome to achieve the same height profile as before, as this can depend on the different shim thickness and chassis to motor surface finish.
Purpose
* To provide an emergency workaround or alternate solution in the unfortunate event of:
1. Losing one or more of the motor's thin spacer shims.
2. Unsatisfactory result trying to reinstall the thin spacer shims.
Special Tools and Materials
* Vernier caliper to measure shim and washer thickness.
* Standard M5 flat washer as replacements.
Procedure
* The motor is fastened to the chassis with three (3) Allen-hex screws. With my turntable, each Allen-hex screw had two (2) thin spacer shims (6 shims in total). An example of measured thickness of two original shims is shown in Fig. 1.
Figure 1. Pair of original fitted shims, measured thickness 0.6 mm.
* As a workaround, the original six shims can be replaced with three standard M5 flat washers. Using a vernier caliper measured a thickness of 0.6 mm for two original shims (Fig. 1), vs. thickness of 0.8 mm for a single M5 flat washer as replacement (Fig. 2).
Figure 2. Single M5 flat washer thickness 0.8 mm.
* While this is not an exact replacement, it's unlikely a 0.2 mm increase in washer vs. shims thickness corresponding to a 0.2 mm increase in platter height will affect performance.
* In any case, small differences in platter height should be compensated for when adjusting the cartridge maximum lowering depth, as described in report "Nagaoka MP-200 Cartridge with ReVox B790 Turntable - Hints and Lessons Learned", section "Adjusting the Maximum Drop Vertical Height".
Important Checks
* To avoid imbalances in platter height, all M5 washers must have a clean and flat surface and be of the same height and profile.
* The B790 turntable has 4 levelling springs, one in each corner of the chassis, adjusted from underneath the turntable base to achieve a level surface of the top chassis. Suggest using a small circular level purchased at hardware shop to check the level at each corner of the chassis surface (Fig. 3).
Figure 3. Circular level placed on one corner of chassis.
Final notes
* As mentioned in Purpose, this content is to provide an emergency workaround or alternate solution in the unfortunate event of:
a. Losing one or more of the motor's thin spacer shims.
b. Unsatisfactory result trying to reinstall the thin spacer shims.
* So even if the original shims get mixed up or become lost, it is not the end. In my case, I tried these standard M5 flat washers between motor and chassis, followed by adjusting the cartridge vertical drop height and leveling the surface of the top chassis, and pleased to report this method worked very well.
References
1. ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned
2. Nagaoka MP-200 Cartridge with ReVox B790 Turntable - Hints and Lessons Learned
3. ReVox B790 Turntable Erratic Speed Control - Hints & Lessons Learned
* This is a supplement to the earlier report "ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned", where it was stated "Unless you really need to, try to avoid removing the whole motor from the top chassis. 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."
* The motor/chassis thin spacer shims are needed to maintain a correct height and evenness of level, however the different sizes of shims are frustrating to align and can fall out of place when trying to reinstall the motor to the chassis. Furthermore, you might find that once these spacer shims are removed it may be troublesome to achieve the same height profile as before, as this can depend on the different shim thickness and chassis to motor surface finish.
Purpose
* To provide an emergency workaround or alternate solution in the unfortunate event of:
1. Losing one or more of the motor's thin spacer shims.
2. Unsatisfactory result trying to reinstall the thin spacer shims.
Special Tools and Materials
* Vernier caliper to measure shim and washer thickness.
* Standard M5 flat washer as replacements.
Procedure
* The motor is fastened to the chassis with three (3) Allen-hex screws. With my turntable, each Allen-hex screw had two (2) thin spacer shims (6 shims in total). An example of measured thickness of two original shims is shown in Fig. 1.
Figure 1. Pair of original fitted shims, measured thickness 0.6 mm.
* As a workaround, the original six shims can be replaced with three standard M5 flat washers. Using a vernier caliper measured a thickness of 0.6 mm for two original shims (Fig. 1), vs. thickness of 0.8 mm for a single M5 flat washer as replacement (Fig. 2).
Figure 2. Single M5 flat washer thickness 0.8 mm.
* While this is not an exact replacement, it's unlikely a 0.2 mm increase in washer vs. shims thickness corresponding to a 0.2 mm increase in platter height will affect performance.
* In any case, small differences in platter height should be compensated for when adjusting the cartridge maximum lowering depth, as described in report "Nagaoka MP-200 Cartridge with ReVox B790 Turntable - Hints and Lessons Learned", section "Adjusting the Maximum Drop Vertical Height".
Important Checks
* To avoid imbalances in platter height, all M5 washers must have a clean and flat surface and be of the same height and profile.
* The B790 turntable has 4 levelling springs, one in each corner of the chassis, adjusted from underneath the turntable base to achieve a level surface of the top chassis. Suggest using a small circular level purchased at hardware shop to check the level at each corner of the chassis surface (Fig. 3).
Figure 3. Circular level placed on one corner of chassis.
Final notes
* As mentioned in Purpose, this content is to provide an emergency workaround or alternate solution in the unfortunate event of:
a. Losing one or more of the motor's thin spacer shims.
b. Unsatisfactory result trying to reinstall the thin spacer shims.
* So even if the original shims get mixed up or become lost, it is not the end. In my case, I tried these standard M5 flat washers between motor and chassis, followed by adjusting the cartridge vertical drop height and leveling the surface of the top chassis, and pleased to report this method worked very well.
References
1. ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned
ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned
(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...
audiokarma.org
Nagaoka MP-200 Cartridge with ReVox B790 Turntable - Hints and Lessons Learned
Nagaoka MP-200 Cartridge with ReVox B790 Turntable - Hints and Lessons Learned (To conform with file uploading limits a reduced size PDF file containing the figures is attached). History * My ReVox B790 Linatrack turntable serial number 004778 I purchased new in July 1978 from REW Audio Visual...
audiokarma.org
ReVox B790 Turntable Erratic Speed Control - Hints & Lessons Learned
(To conform with file uploading limits a reduced size PDF file containing the figures is attached). Purpose * To share hints and lessons learned from fixing my B790 turntable's speed fluctuations. * This report complements the earlier report, "ReVox B790 Turntable - Motor Servicing Hints &...
audiokarma.org