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 Co. Ltd, London, England.
* At time of purchase, I paid for the optional upgrade to have the Linatrack tonearm fitted with a Mission 773 cartridge.
* 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 undertaking a full restoration (Fig 1.). For the recent restoration I used an excellent overhaul kit purchased from Nagravox, https://nagravox.com
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Cartridge Installation and Setup - Hints and Lessons Learned
* As mentioned I had the optional upgrade of Mission 773 cartridge installed (Fig. 2.). While Mission is a UK company well noted for their loudspeakers, their Mission 773 cartridges were manufactured in Japan (manufacturer’s name not known). The 773 was an excellent choice of Moving Magnet (MM) cartridge for the B790 tonearm, being lightweight (6-gram body, total 6.7-gram with 3mm spacers and mounting screws), it also had high compliance.
Figure 2. Original Mission 773 MM cartridge
* The Mission 773 was installed in my B790 turntable's tonearm without brass front spacer, but a brass 8.1-gram rear counterweight (ReVox p.n. 1.179.115.06) - I believe this was a good match of all relevant parameters, cartridge weight, compliance, and size of counterweight.
Figure 3. Underside of Mission 773 cartridge showing: Without front brass spacer, Rear brass counterweight of 8.1 grams.
* I was extremely pleased with the sound of the Mission 773, but after 44 years the stylus was damaged, product information was gone, so it needed to be replaced with a modern high-grade MM cartridge.
* The ReVox documentation recommends cartridges having high compliance.
* I chose the Nagaoka MP-200 for reasons of:
a) Favourable reviews about its audio clarity and detail.
b) Mechanical suitability for the Linatrack's lightweight tonearm; body weight 6.5-grams, static compliance of 20 x 10-6 cm/dyne, height stylus tip to top of body 18 mm (Fig. 4.), and standard half-inch (12.7 mm) mounting configuration allowing machine screw fastening from underside.
Figure 4. MP series common cartridge dimension diagram - courtesy Nagaoka K.K.
* All the MP series of Nagaoka phono cartridges use standard half-inch mount fastening, allowing them to be readily attached to the underside of the Linatrack tonearm.
* Something I observed, the cartridge alignment jig that I had could not be used with the MP-200 as the width of the MP-200 body is wider than my jig allowed (Fig. 5.). The MP-200 is a Moving Iron (MI) design, possibly the size of the internal permalloy magnet contributes to the wider width. Without the right alignment jig I could only make a rough visual alignment, fortunately I was able to make final adjustments to the cartridge alignment using the AnalogMagik test program, described later in section Operational Checks after Turntable Re-assembly.
Figure 5. MP-200 body wider than the legs of my cartridge alignment jig allowed - photo taken with turntable upside down.
* The commercial literature recommends an overall height of 20 mm from stylus tip to top of cartridge body. To achieve an overall height of 20 mm from stylus tip I first used a 2 mm spacer (18 mm + 2 mm). However, this 20mm height proved too much resulting in the bottom of the cartridge body rubbing against the record surface when lowered. Furthermore, the front face of the tonearm body was sitting too high relative to the front facing small circuit board (Fig 6.).
Figure 6. MP-200 cartridge with initial fitting of 2mm spacer - note insufficient clearance to record surface.
* Changing to a 1 mm spacer was a much better fit with overall height 19mm (18mm + 1 mm). Critically, the mid-point of the front face of the tonearm body is now at the optimum position aligned with the lower edge of the front facing small circuit board (Fig 7.).
Figure 7. MP-200 cartridge with final fitting 1mm spacer - correct position with respect to front facing circuit board.
* Interesting side note, retrieving the original Mission 773 cartridge measured the height from stylus tip to top of body was 16 mm, and the 773 cartridge was fitted to the tonearm using a 3mm spacer. So, an original overall height of 19mm (16mm + 3 mm) agreed with the same overall height dimension of 19 mm that worked for me.
* Next, to balance the increase in cartridge weight (6-g for Mission 773, vs. heavier 6.5-g for Nagaoka MP-200), added a small lead strip of 2.8 grams to increase the overall counterweight. This lead strip was glued onto the base of the original brass counterweight (Fig 8.), for a total counterweight of 10.9-g (8.1-g + 2.8-g).
Figure 8. Lead strip glued to the underside of original counterweight - photo taken from underneath tonearm.
* Although this was the easiest way to add counterweight and worked well, if I was to do it again, I would fully remove the tonearm assembly from the chassis so that the lead strip could be glued to the backside (not the underside) of the original counterweight. Advantage of gluing the lead strip to the backside vs. the underside makes the original fitted fastening screw accessible should one need to remove the original brass counterweight, plus it allows for additional clearance to the record surface.
* On that note, if fitting a heavier cartridge be careful of gluing too many lead strips underneath the counterweight, if you do so and swing the tonearm back to rest position, the added weights could knock over the stylus cleaning brush, and in worse case scrape the record surface.
* To set both the cartridge stylus to the correct maximum lowering depth and vertical tracking force is a bit tricky, because as soon as the arm is swung into position the platter starts spinning. This is how I performed it, although others may have a better method:
1) With the power cord disconnected, remove the platter, raise up the plinth from the chassis and carefully disconnect the motor harness connector from the Motor Control Board - this is also tricky!
2) Lower the plinth back in position, replace the platter, and with power turned on the arm can now be swung into position atop the (non-moving) platter. The cartridge can now be positioned back and forth, up and down as often as needed.
3) Position the stylus over the inner ridge of the turntable mat and adjust the lowering depth turn screw until the stylus almost, but not quite touches the top of the inner ridge of the mat when the cartridge is lowered.
4) Using a stylus weight gauge I set the initial stylus force to 1.9 grams - for the MP-200 cartridge Nagaoka recommends 1.5-g to 2.0-g.
5) With the power cord disconnected, remove the platter, raise up the plinth from the chassis and carefully reconnect the motor harness connector to the Motor Control Board.
6) Finally lower the plinth back in position and replace the platter.
Adjusting the Vertical Tracking Force
* Turning the side-mounted rear adjust screw CW reduces force, turning CCW increases force (Fig. 9).
* Initial force was set to 1.9-g, this was later adjusted using the AnalogMagik test program to obtain the lowest distortion measurement.
Adjusting the Maximum Drop Vertical Height
* Turning the top-mounted adjust screw CW raises the cartridge, turning CCW lowers the cartridge.
* The MP-200 cartridge stylus when lowered was set to just barely above (not touching) the turntable mat's innermost groove.
Figure 9. Tracking force adjustment by side-mounted rear screw, vertical height adjustment by top screw.
Operational Checks after Turntable Re-assembly
* This was an interesting experiment and a great learning experience for me.
* With the B790's phono leads connected to the input of my AGI-511 preamp (another 1970's vintage restoration project) and the preamp's line outputs connected to the AnalogMagik test package, I conducted numerous measurements to optimize: Distortion vs. Vertical Tracking Force, Lateral Resonance, Stylus Azimuth and Zenith, Phono-Stage Loading etc. The following are some of the more interesting findings to share with you.
* For Speed Control, Wow & Flutter and Anti-skating I was very pleased there was no need to optimize any of these measurements as the results met or exceeded ReVox’s published specifications for the B790.
Optimization of Tonearm Resonance
* Commercial literature gives guidelines for tonearm resonance to ideally fall between 8 Hz and 12 Hz. Their explanation is that resonances lower than say 6~7 Hz may be amplified by record warps, while resonances higher than say 14 Hz may interact with low frequency music such as organ notes.
* My first measurements showed a peak resonance frequency of ~ 15 Hz (Fig 10.), obviously this was not ideal and needed investigating.
Figure 10. B790 with MP-200, initial Resonant Frequency results with lightweight mounting hardware
* In simple terms, tonearm resonant frequency is inversely proportional to the square root of (Cartridge Compliance x Total Tonearm System Mass). To lower the resonant frequency requires:
a) Increasing the cartridge's compliance - not possible with a given cartridge type.
b) Increasing the total tonearm system mass - some changes in mounting hardware are possible.
* Initially I was concerned about the increased weight of the MP-200 cartridge, so I first attached the MP-200 cartridge to the tonearm using the lightweight aluminum screws originally supplied with the Mission 773 cartridge. Resonant frequency results shown in Fig 10.
* To increase the effective mass, I changed from aluminum screws to stainless-steel screws supplied with the Nagaoka MP-200 cartridge, and from nylon spacer washers to a 1mm flat spacer bar. Finally, with these changes I could lower the dominant resonant frequency down to 10~11 Hz, results shown in Fig 11.
Figure 11. B790 with MP-200, final Resonant Frequency results with increased weight mounting hardware.
Cartridge initial fastening with light weight hardware
* One pair of nylon washers 1mm, 0.5-g.
* Two aluminum screws supplied with original Mission 773, 0.375-g.
* Measured total for above = 0.875-g.
Cartridge final fastening with increased weight hardware
* Single flat spacer 1mm, 0.1-g.
* Nagaoka supplied stainless steel screws and thin nylon washers, 1.03-g.
* Measured total for above = 1.135-g (Fig. 12.).
Figure 12. Measured weight of final fastening hardware used - display in grams.
Observations and Discussions
* Unsure what is the cause of the double resonance peak unless it's a characteristic of the B790’s small low-mass tonearm.
* In the final setup, changing to slightly heavier fastening hardware for the MP-200 cartridge (initial 0.875-g., increased to 1.135-g.) moved the two resonance peaks down in frequency by approx. one half-Hz, and significantly changed the lower frequency peak to become the dominant peak frequency (from 15.0 Hz down to 10~11 Hz). This follows from the relationship of resonant frequency inversely proportional to the square root of the total Tonearm Mass x the Cartridge Compliance, i.e., increasing either the total Tonearm Mass or the Cartridge Compliance will lower the resonant frequency.
* Although I can only describe my experience with the MP-200 cartridge, consulting Nagaoka's product brochure the four cartridges MP-100, MP-110, MP-150 and MP200 have the same published body weight of 6.5-g, hence if choosing any of these cartridges I expect the procedures described in this document could be applied. On the other hand, Nagaoka’s MP-300 and MP-500 cartridges have a published body weight of 7.8-g and would likely require an extra counterweight be attached to the tonearm, being mindful of a practical limit to the maximum size and weight that could be fitted.
Optimization of Stylus Zenith Angle
* Stylus zenith angle is like Yaw in an airplane, that is the sideways rotation around a vertical axis.
* As discussed earlier, without a cartridge alignment jig to fit the MP-200’s body, I did my best to make a visual alignment, with results displayed in the intermodulation distortion measurements (Fig 13.).
Figure 13. B790 with MP-200, initial Stylus Zenith Angle.
* By slightly rotating the cartridge body counterclockwise (when viewed from above) had a positive effect on reducing the intermodulation distortion (Fig. 14.).
Figure 14. B790 with MP-200, final Stylus Zenith Angle.
Phono Stage Loading
* Nagaoka's MP-200 product brochure states a capacitance loading of 100 pF. However, this value looks low and is not representative of most phono-stage's loading, typically 47k ohm in parallel with up to 470 pF capacitance.
* The phono cabling provided with the B790 turntable has an internal capacitance specified at 220 pF, while the AGI-511 preamplifier has fixed 47k ohm internal loading, with open sockets to insert loading capacitance of user's choice.
* Here's what the program displayed for frequency balance using three different settings of capacitance loading: 220 pF (phono cable only), 320 pf (phono cable + 100 pF in pre-amp), and 440 pf (phono cable + 200 pF in pre-amp).
* From these results the optimum frequency balance was observed with the standard phono cabling in parallel with 100 pF in the AGI-511 preamplifier’s phono stage (Figs. 16. and 18.) - please note your results may be different depending on equipment type.
Figure 15. Example 220 pF loading - phono cable only
Figure 16. Final 320 pF loading - phono cable with 100 pF in pre-amp phono stage and final chosen setting
Figure 17. Example 420 pF loading - phono cable with 200 pF in pre-amp phono stage
Figure 18. One channel of AGI-511 phono stage with 100 pF WIMA capacitor (rectangular, red object in center of photo).
Distortion vs. Vertical Tracking Force
* As mentioned previously, the initial vertical tracking force was set to 1.9 grams. Small adjustments to the vertical tracking force altered the measured Total Harmonic Distortion (THD). One example of optimizing the tracking force for lowest THD is displayed in Fig 19.
Figure 19. Example of Vertical Tracking Force optimized for lowest THD.
Other Maintenance Tasks
* While not specific to the MP-200 cartridge, to share some basic maintenance tasks I performed that can be helpful during the fitting of any new phono cartridge.
Arm Electronic Board
* Reference ReVox B790 Service Instruction Manual, 1.179.150 Arm Electronic Board.
https://www.reeltoreel.nl/studer/Public/Products/Revox/Revox_B790/Revox_B790_Serv.pdf
* All electrolytic capacitors changed out with new, installed factory updates up to level 3 of 9.1.79.
Tonearm Guide Rails
* After a thorough cleaning using isopropyl alcohol, applied a very thin application of light synthetic oil. Then to prevent dust accumulation, with a clean paper towel carefully wiped off all traces of oil from the guide rails.
Tonearm Pulley Cord
* As my turntable had been in storage for several years the original pulley cord had become stretched and deformed, replaced with a new pulley cord (1.179.126.00) and spring (1.010.110.37).
Figure 20. New pulley cord and spring (rear), tonearm lowering dashpot (front).
Tonearm Lowering Dashpot
* As mentioned, my turntable had been in storage for several years resulting in the tonearm lowering dashpot becoming stuck in position.
* Removing the dashpot's piston from the cylinder and cleaning with isopropyl alcohol both the piston and inside of the cylinder wall made some improvement, however the movement remained sticky, erratic.
* Earlier series of B790 turntables were fitted with a bellow, whereas my B790 had the later type of cylinder / piston damper, allowing the amount of air escaping from the dashpot to be adjusted by rotating the cylinder.
* However, the cylinder could not be rotated due to stiction from years in storage. To access the cylinder locking screw required the tonearm assembly (held in place by 3 mounting screws) be removed from the plinth.
Figure 21. Underside of tonearm, showing LED and cylinder locking screw.
* With the tonearm assembly removed, the screw on the LED bracket can be loosened to allow the cylinder to be rotated slightly.
* Hint, when cleaning with the piston removed, suggest pouring a small amount of isopropyl alcohol into the well of the cylinder. If fluid flows freely out the bottom of the air escape hole it's a good indication the small escape hole is open and not clogged.
* Then wipe the inside of the cylinder wall with Q-Tips to thoroughly clean - I had to repeat this cleaning step several times as the tiniest amount of foreign material can jam the piston's movement.
* After this thorough cleaning the dashpot lowering motion now operates smoothly.
* Hint, a thorough cleaning of the piston and cylinder should take priority over adjusting/rotating the cylinder, i.e., rotating the cylinder if the parts are not thoroughly cleaned will not fix a damping problem.
Other Maintenance - B790 Turntable
* Prior to mounting the MP-200 cartridge, important maintenance tasks I performed on the B790 turntable are documented in reports:
* ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned
https://audiokarma.org/forums/index...otor-servicing-hints-lessons-learned.1025344/
* ReVox B790 Turntable Erratic Speed Control - Hints & Lessons Learned
https://audiokarma.org/forums/index...-speed-control-hints-lessons-learned.1025346/
Final Comments and Discussion
* The reason for this writing is to share my experience installing a Nagaoka MP-200 cartridge in the ReVox B790 turntable. At the time of writing this setup has been operating without issue for 4 months now.
* I am very pleased with the sound quality of my B790 with the MP-200 cartridge, it's better than I can ever remember, and it performs extremely well with the rest of my audio equipment setup (AGI-511 pre-amp, Threshold PCX crossover feeding a pair of Bryston power amps into Acoustat electrostatic speakers and woofer).
* From the test results, I believe that with proper alignment and adjusting the B790 turntable can accept a variety of MM/MI cartridges, provided:
1. Cartridge mounting is standard half-inch (to insert fastening screws from the underside).
2. Body weight is reasonable, say under 7~8-gram (to limit counterweight mass and size).
3. Static compliance >20 x 10-6 cm/dyne (to achieve ideal resonant frequency of tonearm system).
* A topic I am still investigating is cartridge Azimuth (like Roll in an airplane, i.e., rotation CW and CCW when viewed on axis from the front). The AnalogMagik program displays some difference in crosstalk between L & R channels, but apart from inserting a shim on one side between cartridge and tonearm I don't know if there's an easy way to adjust cartridge azimuth with the B790 turntable - pending further investigation.
---------------
(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 Co. Ltd, London, England.
* At time of purchase, I paid for the optional upgrade to have the Linatrack tonearm fitted with a Mission 773 cartridge.
* 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 undertaking a full restoration (Fig 1.). For the recent restoration I used an excellent overhaul kit purchased from Nagravox, https://nagravox.com
Figure 1. Refurbished B790 Linatrack turntable - serial number 004778
Cartridge Installation and Setup - Hints and Lessons Learned
* As mentioned I had the optional upgrade of Mission 773 cartridge installed (Fig. 2.). While Mission is a UK company well noted for their loudspeakers, their Mission 773 cartridges were manufactured in Japan (manufacturer’s name not known). The 773 was an excellent choice of Moving Magnet (MM) cartridge for the B790 tonearm, being lightweight (6-gram body, total 6.7-gram with 3mm spacers and mounting screws), it also had high compliance.
Figure 2. Original Mission 773 MM cartridge
* The Mission 773 was installed in my B790 turntable's tonearm without brass front spacer, but a brass 8.1-gram rear counterweight (ReVox p.n. 1.179.115.06) - I believe this was a good match of all relevant parameters, cartridge weight, compliance, and size of counterweight.
Figure 3. Underside of Mission 773 cartridge showing: Without front brass spacer, Rear brass counterweight of 8.1 grams.
* I was extremely pleased with the sound of the Mission 773, but after 44 years the stylus was damaged, product information was gone, so it needed to be replaced with a modern high-grade MM cartridge.
* The ReVox documentation recommends cartridges having high compliance.
* I chose the Nagaoka MP-200 for reasons of:
a) Favourable reviews about its audio clarity and detail.
b) Mechanical suitability for the Linatrack's lightweight tonearm; body weight 6.5-grams, static compliance of 20 x 10-6 cm/dyne, height stylus tip to top of body 18 mm (Fig. 4.), and standard half-inch (12.7 mm) mounting configuration allowing machine screw fastening from underside.
Figure 4. MP series common cartridge dimension diagram - courtesy Nagaoka K.K.
* All the MP series of Nagaoka phono cartridges use standard half-inch mount fastening, allowing them to be readily attached to the underside of the Linatrack tonearm.
* Something I observed, the cartridge alignment jig that I had could not be used with the MP-200 as the width of the MP-200 body is wider than my jig allowed (Fig. 5.). The MP-200 is a Moving Iron (MI) design, possibly the size of the internal permalloy magnet contributes to the wider width. Without the right alignment jig I could only make a rough visual alignment, fortunately I was able to make final adjustments to the cartridge alignment using the AnalogMagik test program, described later in section Operational Checks after Turntable Re-assembly.
Figure 5. MP-200 body wider than the legs of my cartridge alignment jig allowed - photo taken with turntable upside down.
* The commercial literature recommends an overall height of 20 mm from stylus tip to top of cartridge body. To achieve an overall height of 20 mm from stylus tip I first used a 2 mm spacer (18 mm + 2 mm). However, this 20mm height proved too much resulting in the bottom of the cartridge body rubbing against the record surface when lowered. Furthermore, the front face of the tonearm body was sitting too high relative to the front facing small circuit board (Fig 6.).
Figure 6. MP-200 cartridge with initial fitting of 2mm spacer - note insufficient clearance to record surface.
* Changing to a 1 mm spacer was a much better fit with overall height 19mm (18mm + 1 mm). Critically, the mid-point of the front face of the tonearm body is now at the optimum position aligned with the lower edge of the front facing small circuit board (Fig 7.).
Figure 7. MP-200 cartridge with final fitting 1mm spacer - correct position with respect to front facing circuit board.
* Interesting side note, retrieving the original Mission 773 cartridge measured the height from stylus tip to top of body was 16 mm, and the 773 cartridge was fitted to the tonearm using a 3mm spacer. So, an original overall height of 19mm (16mm + 3 mm) agreed with the same overall height dimension of 19 mm that worked for me.
* Next, to balance the increase in cartridge weight (6-g for Mission 773, vs. heavier 6.5-g for Nagaoka MP-200), added a small lead strip of 2.8 grams to increase the overall counterweight. This lead strip was glued onto the base of the original brass counterweight (Fig 8.), for a total counterweight of 10.9-g (8.1-g + 2.8-g).
Figure 8. Lead strip glued to the underside of original counterweight - photo taken from underneath tonearm.
* Although this was the easiest way to add counterweight and worked well, if I was to do it again, I would fully remove the tonearm assembly from the chassis so that the lead strip could be glued to the backside (not the underside) of the original counterweight. Advantage of gluing the lead strip to the backside vs. the underside makes the original fitted fastening screw accessible should one need to remove the original brass counterweight, plus it allows for additional clearance to the record surface.
* On that note, if fitting a heavier cartridge be careful of gluing too many lead strips underneath the counterweight, if you do so and swing the tonearm back to rest position, the added weights could knock over the stylus cleaning brush, and in worse case scrape the record surface.
* To set both the cartridge stylus to the correct maximum lowering depth and vertical tracking force is a bit tricky, because as soon as the arm is swung into position the platter starts spinning. This is how I performed it, although others may have a better method:
1) With the power cord disconnected, remove the platter, raise up the plinth from the chassis and carefully disconnect the motor harness connector from the Motor Control Board - this is also tricky!
2) Lower the plinth back in position, replace the platter, and with power turned on the arm can now be swung into position atop the (non-moving) platter. The cartridge can now be positioned back and forth, up and down as often as needed.
3) Position the stylus over the inner ridge of the turntable mat and adjust the lowering depth turn screw until the stylus almost, but not quite touches the top of the inner ridge of the mat when the cartridge is lowered.
4) Using a stylus weight gauge I set the initial stylus force to 1.9 grams - for the MP-200 cartridge Nagaoka recommends 1.5-g to 2.0-g.
5) With the power cord disconnected, remove the platter, raise up the plinth from the chassis and carefully reconnect the motor harness connector to the Motor Control Board.
6) Finally lower the plinth back in position and replace the platter.
Adjusting the Vertical Tracking Force
* Turning the side-mounted rear adjust screw CW reduces force, turning CCW increases force (Fig. 9).
* Initial force was set to 1.9-g, this was later adjusted using the AnalogMagik test program to obtain the lowest distortion measurement.
Adjusting the Maximum Drop Vertical Height
* Turning the top-mounted adjust screw CW raises the cartridge, turning CCW lowers the cartridge.
* The MP-200 cartridge stylus when lowered was set to just barely above (not touching) the turntable mat's innermost groove.
Figure 9. Tracking force adjustment by side-mounted rear screw, vertical height adjustment by top screw.
Operational Checks after Turntable Re-assembly
* This was an interesting experiment and a great learning experience for me.
* With the B790's phono leads connected to the input of my AGI-511 preamp (another 1970's vintage restoration project) and the preamp's line outputs connected to the AnalogMagik test package, I conducted numerous measurements to optimize: Distortion vs. Vertical Tracking Force, Lateral Resonance, Stylus Azimuth and Zenith, Phono-Stage Loading etc. The following are some of the more interesting findings to share with you.
* For Speed Control, Wow & Flutter and Anti-skating I was very pleased there was no need to optimize any of these measurements as the results met or exceeded ReVox’s published specifications for the B790.
Optimization of Tonearm Resonance
* Commercial literature gives guidelines for tonearm resonance to ideally fall between 8 Hz and 12 Hz. Their explanation is that resonances lower than say 6~7 Hz may be amplified by record warps, while resonances higher than say 14 Hz may interact with low frequency music such as organ notes.
* My first measurements showed a peak resonance frequency of ~ 15 Hz (Fig 10.), obviously this was not ideal and needed investigating.
Figure 10. B790 with MP-200, initial Resonant Frequency results with lightweight mounting hardware
* In simple terms, tonearm resonant frequency is inversely proportional to the square root of (Cartridge Compliance x Total Tonearm System Mass). To lower the resonant frequency requires:
a) Increasing the cartridge's compliance - not possible with a given cartridge type.
b) Increasing the total tonearm system mass - some changes in mounting hardware are possible.
* Initially I was concerned about the increased weight of the MP-200 cartridge, so I first attached the MP-200 cartridge to the tonearm using the lightweight aluminum screws originally supplied with the Mission 773 cartridge. Resonant frequency results shown in Fig 10.
* To increase the effective mass, I changed from aluminum screws to stainless-steel screws supplied with the Nagaoka MP-200 cartridge, and from nylon spacer washers to a 1mm flat spacer bar. Finally, with these changes I could lower the dominant resonant frequency down to 10~11 Hz, results shown in Fig 11.
Figure 11. B790 with MP-200, final Resonant Frequency results with increased weight mounting hardware.
Cartridge initial fastening with light weight hardware
* One pair of nylon washers 1mm, 0.5-g.
* Two aluminum screws supplied with original Mission 773, 0.375-g.
* Measured total for above = 0.875-g.
Cartridge final fastening with increased weight hardware
* Single flat spacer 1mm, 0.1-g.
* Nagaoka supplied stainless steel screws and thin nylon washers, 1.03-g.
* Measured total for above = 1.135-g (Fig. 12.).
Figure 12. Measured weight of final fastening hardware used - display in grams.
Observations and Discussions
* Unsure what is the cause of the double resonance peak unless it's a characteristic of the B790’s small low-mass tonearm.
* In the final setup, changing to slightly heavier fastening hardware for the MP-200 cartridge (initial 0.875-g., increased to 1.135-g.) moved the two resonance peaks down in frequency by approx. one half-Hz, and significantly changed the lower frequency peak to become the dominant peak frequency (from 15.0 Hz down to 10~11 Hz). This follows from the relationship of resonant frequency inversely proportional to the square root of the total Tonearm Mass x the Cartridge Compliance, i.e., increasing either the total Tonearm Mass or the Cartridge Compliance will lower the resonant frequency.
* Although I can only describe my experience with the MP-200 cartridge, consulting Nagaoka's product brochure the four cartridges MP-100, MP-110, MP-150 and MP200 have the same published body weight of 6.5-g, hence if choosing any of these cartridges I expect the procedures described in this document could be applied. On the other hand, Nagaoka’s MP-300 and MP-500 cartridges have a published body weight of 7.8-g and would likely require an extra counterweight be attached to the tonearm, being mindful of a practical limit to the maximum size and weight that could be fitted.
Optimization of Stylus Zenith Angle
* Stylus zenith angle is like Yaw in an airplane, that is the sideways rotation around a vertical axis.
* As discussed earlier, without a cartridge alignment jig to fit the MP-200’s body, I did my best to make a visual alignment, with results displayed in the intermodulation distortion measurements (Fig 13.).
Figure 13. B790 with MP-200, initial Stylus Zenith Angle.
* By slightly rotating the cartridge body counterclockwise (when viewed from above) had a positive effect on reducing the intermodulation distortion (Fig. 14.).
Figure 14. B790 with MP-200, final Stylus Zenith Angle.
Phono Stage Loading
* Nagaoka's MP-200 product brochure states a capacitance loading of 100 pF. However, this value looks low and is not representative of most phono-stage's loading, typically 47k ohm in parallel with up to 470 pF capacitance.
* The phono cabling provided with the B790 turntable has an internal capacitance specified at 220 pF, while the AGI-511 preamplifier has fixed 47k ohm internal loading, with open sockets to insert loading capacitance of user's choice.
* Here's what the program displayed for frequency balance using three different settings of capacitance loading: 220 pF (phono cable only), 320 pf (phono cable + 100 pF in pre-amp), and 440 pf (phono cable + 200 pF in pre-amp).
* From these results the optimum frequency balance was observed with the standard phono cabling in parallel with 100 pF in the AGI-511 preamplifier’s phono stage (Figs. 16. and 18.) - please note your results may be different depending on equipment type.
Figure 15. Example 220 pF loading - phono cable only
Figure 16. Final 320 pF loading - phono cable with 100 pF in pre-amp phono stage and final chosen setting
Figure 17. Example 420 pF loading - phono cable with 200 pF in pre-amp phono stage
Figure 18. One channel of AGI-511 phono stage with 100 pF WIMA capacitor (rectangular, red object in center of photo).
Distortion vs. Vertical Tracking Force
* As mentioned previously, the initial vertical tracking force was set to 1.9 grams. Small adjustments to the vertical tracking force altered the measured Total Harmonic Distortion (THD). One example of optimizing the tracking force for lowest THD is displayed in Fig 19.
Figure 19. Example of Vertical Tracking Force optimized for lowest THD.
Other Maintenance Tasks
* While not specific to the MP-200 cartridge, to share some basic maintenance tasks I performed that can be helpful during the fitting of any new phono cartridge.
Arm Electronic Board
* Reference ReVox B790 Service Instruction Manual, 1.179.150 Arm Electronic Board.
https://www.reeltoreel.nl/studer/Public/Products/Revox/Revox_B790/Revox_B790_Serv.pdf
* All electrolytic capacitors changed out with new, installed factory updates up to level 3 of 9.1.79.
Tonearm Guide Rails
* After a thorough cleaning using isopropyl alcohol, applied a very thin application of light synthetic oil. Then to prevent dust accumulation, with a clean paper towel carefully wiped off all traces of oil from the guide rails.
Tonearm Pulley Cord
* As my turntable had been in storage for several years the original pulley cord had become stretched and deformed, replaced with a new pulley cord (1.179.126.00) and spring (1.010.110.37).
Figure 20. New pulley cord and spring (rear), tonearm lowering dashpot (front).
Tonearm Lowering Dashpot
* As mentioned, my turntable had been in storage for several years resulting in the tonearm lowering dashpot becoming stuck in position.
* Removing the dashpot's piston from the cylinder and cleaning with isopropyl alcohol both the piston and inside of the cylinder wall made some improvement, however the movement remained sticky, erratic.
* Earlier series of B790 turntables were fitted with a bellow, whereas my B790 had the later type of cylinder / piston damper, allowing the amount of air escaping from the dashpot to be adjusted by rotating the cylinder.
* However, the cylinder could not be rotated due to stiction from years in storage. To access the cylinder locking screw required the tonearm assembly (held in place by 3 mounting screws) be removed from the plinth.
Figure 21. Underside of tonearm, showing LED and cylinder locking screw.
* With the tonearm assembly removed, the screw on the LED bracket can be loosened to allow the cylinder to be rotated slightly.
* Hint, when cleaning with the piston removed, suggest pouring a small amount of isopropyl alcohol into the well of the cylinder. If fluid flows freely out the bottom of the air escape hole it's a good indication the small escape hole is open and not clogged.
* Then wipe the inside of the cylinder wall with Q-Tips to thoroughly clean - I had to repeat this cleaning step several times as the tiniest amount of foreign material can jam the piston's movement.
* After this thorough cleaning the dashpot lowering motion now operates smoothly.
* Hint, a thorough cleaning of the piston and cylinder should take priority over adjusting/rotating the cylinder, i.e., rotating the cylinder if the parts are not thoroughly cleaned will not fix a damping problem.
Other Maintenance - B790 Turntable
* Prior to mounting the MP-200 cartridge, important maintenance tasks I performed on the B790 turntable are documented in reports:
* ReVox B790 Turntable - Motor Servicing Hints & Lessons Learned
https://audiokarma.org/forums/index...otor-servicing-hints-lessons-learned.1025344/
* ReVox B790 Turntable Erratic Speed Control - Hints & Lessons Learned
https://audiokarma.org/forums/index...-speed-control-hints-lessons-learned.1025346/
Final Comments and Discussion
* The reason for this writing is to share my experience installing a Nagaoka MP-200 cartridge in the ReVox B790 turntable. At the time of writing this setup has been operating without issue for 4 months now.
* I am very pleased with the sound quality of my B790 with the MP-200 cartridge, it's better than I can ever remember, and it performs extremely well with the rest of my audio equipment setup (AGI-511 pre-amp, Threshold PCX crossover feeding a pair of Bryston power amps into Acoustat electrostatic speakers and woofer).
* From the test results, I believe that with proper alignment and adjusting the B790 turntable can accept a variety of MM/MI cartridges, provided:
1. Cartridge mounting is standard half-inch (to insert fastening screws from the underside).
2. Body weight is reasonable, say under 7~8-gram (to limit counterweight mass and size).
3. Static compliance >20 x 10-6 cm/dyne (to achieve ideal resonant frequency of tonearm system).
* A topic I am still investigating is cartridge Azimuth (like Roll in an airplane, i.e., rotation CW and CCW when viewed on axis from the front). The AnalogMagik program displays some difference in crosstalk between L & R channels, but apart from inserting a shim on one side between cartridge and tonearm I don't know if there's an easy way to adjust cartridge azimuth with the B790 turntable - pending further investigation.
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