All decks that use the **Sankyo mechanism** share a common weak point: the **Sankyo VA700BO2 cam motor**. These motors are prone to eventual failure regardless of cassette deck brand or usage pattern. This is caused by a set of compounding problems: - **Harsh duty cycle**: runs only for a second or two at a time, always stopping in the same spots. - **Uneven brush wear**: repeated stop/start leads to dead spots and poor contact. - **Self-cleaning prevented**: brushes never get long continuous motion to polish the commutator. - **Galvanic corrosion**: brush holders use a different metal than the solder lugs, causing corrosion at the joints. ![[IMG_0788.jpg | center |600]] *Figure 1: Armature of a VA700BO2, showing the dirty and worn surface of the commutator .* Even after re-soldering the brushes and performing the various cleanings recommended by the community, I’ve found that these motors still don’t deliver the kind of reliability I’m comfortable putting my name on. --- ### Replacement Options The community’s go-to replacements have long been the **Mabuchi RF-370CA** and the **Mabuchi RF-310T**. #### Mabuchi RF-310T ![[Pasted image 20250923114942.png|center|300]] This motor has the **correct bolt pattern**, which makes mounting straightforward. However, it has several limitations: - The **22 mm shaft is too long** and must be cut down. - Its **torque is undersized** compared to the original motor, causing dual-capstan decks to struggle. - It operates at a **higher RPM**, which can lead to overshoot in the mechanism. #### Mabuchi RF-370CA ![[Pasted image 20250923115230.png|center|300]] The RF-370CA is **more powerful than the RF-310T** and offers torque and speed much closer to the original motor. Its drawbacks are mainly mechanical: - A **different bolt pattern** requires grinding and cutting for installation. - The 10.5mm **shaft is slightly shorter** than ideal, though usually workable. #### Brush Type Limitation Both of these motors use **precious metal brushes**, which are poorly suited to the cam motor’s duty cycle. In this application, the motor is almost never run continuously.it is constantly stopping, starting, and reversing in short bursts. ![[Pasted image 20250923111829.png | center |600]] *Figure 2: Carbon brushes (left) compared with precious metal brushes (right).* Precious metal brushes wear quickly under this cycling, as they cannot withstand repeated arcing as effectively as carbon brushes. ![[IMG_0779.jpg | center |600]] *Figure 3: Armature of a precious metal brush motor. The commutator shows material transfer and buildup from the brushes, along with grooving and erosion along the contact path. The grooves are formed by repeated arcing during stop-start operation* Recently, after purchasing more RF-370CA motors, I noticed an increasing number arriving with rust inside. The seller’s advice was simply to run them at 12V until they “work properly.” Needless to say, I’ve grown tired of gambling on AliExpress motors of unknown origin, or spending hours rebuilding tired originals for customer decks. --- ### Solution After months of searching, I was able to work with a company willing to manufacture small batches of a modified Mabuchi 370CA. The specifications were tuned to more closely match the original Sankyo VA700BO2, with improvements aimed at extending lifespan. ![[NewMotorImage (6).jpg| center ]] ![[NewMotorImage (8).jpg| center ]] *Mabuchi RF-310T-11400, Sankyo VA700BO2, Mabuchi RF-370CA-15370, Custom 370CA* To understand why different replacement motors do or don’t fit, it’s useful to compare their mounting bolt sizes, patterns, and shaft lengths. The | Motor | Bolt Size | Bolt Pattern Ø (mm) | Shaft Length (mm) | |----------------|-----------|----------------------|-------------------| | VA700BO2 | M2 | 15 | 11 | | RF-310T | M2 | 16 | 22 | | RF-370CA | M3 | 17 | 9 | | Custom 370CA | M3 | 17 | 10.5 | The VA700BO2 uses the smallest M2 bolts and a compact 15 mm pattern. The RF-310T shares the M2 bolt size but its 22 mm shaft is much too long, requiring modification. The RF-370CA steps up to M3 bolts with a wider 17 mm pattern and a slightly short shaft. The Custom 370CA strikes the best balance, with the same 17 mm pattern as the RF-370CA but a shaft length much closer to the original motor. Mechanical fit is only half the story. The electrical characteristics of each motor also vary significantly | Motor | Nominal Voltage (V) | No Load Speed (rpm) | No Load Current (A) | Rated Load Speed (rpm) | Rated Load Current (A) | Rated Load Torque (g·cm) | Stall Torque (g·cm) | Stall Current (A) | | -------------- | ------------------- | ------------------- | ------------------- | ---------------------- | ---------------------- | ------------------------ | ------------------- | ----------------- | | RF-310TA-11400 | 2.5-6 | 2800 | 0.017 | 2190 | 0.061 | 3.3 | 15 | 0.22 | | RF-370CA-15370 | 12 | 5600 | 0.026 | 4840 | 0.17 | 25.3 | 187 | 1.06 | | Custom 370CA | 6 | 6000 | 0.042 | 5090 | 0.235 | 39.3 | 259.6 | 1.31 | The custom 370CA runs at 6 V and delivers more torque at lower speeds, which makes it closest to the original VA700BO2 in performance. The RF-310T, by contrast, is underpowered and better suited to lighter loads. The standard RF-370CA is stronger than the RF-310T, but since it’s a 12 V motor it does not match the original’s 6 V behavior as closely when down-volted. Beyond specs, internal construction also dictates how well these motors survive in real-world use. | Motor | Shield Fit | Brush Type | Bushings | Armature | Shaft Length | Current Behavior | | -------------- | -------------------- | --------------------------- | -------------- | -------- | -------------- | ------------------- | | RF-310TA-11400 | Shield does not fit | Precious metal (wears fast) | Sintered iron | Standard | Too long | Higher fluctuations | | RF-370CA-15370 | Original shield fits | Precious metal (wears fast) | Sintered iron | Standard | Correct length | Higher fluctuations | | Custom 370CA | Original shield fits | Graphite carbon (durable) | Sintered brass | Balanced | Correct length | Lower, more stable | Both the RF-310T and RF-370CA rely on precious metal brushes and sintered iron bushings, which makes them less durable under the cam motor’s stop-start duty cycle. The custom 370CA has graphite carbon brushes, brass bushings, and a balanced armature, resulting in smoother operation and greater reliability. > [!info] Motor No-Load Current Draw at 6V > Comparison of the old and new motors running at no load with a 6 V supply. > > | Old Motor | New Motor | > |-----------|-----------| > | ![[NewMotorGif(1).gif\|300]] | ![[NewMotorGif (2).gif\|300]] | The old motor averages about 83 mA with a fluctuation of 0.9 mA, while the new motor averages 67 mA with only 0.4 mA of variation. This indicates the new motor runs not only more efficiently but also with a noticeably steadier current profile. This can be attributed to better brush contact and a more balanced armature, along with the use of larger carbon brushes that handle the stop-start duty cycle more effectively. The addition of isolators on the brush holders further reduces vibration, contributing to the smoother and more stable current profile you see in the new motor. --- ### Final Design Hurdle The catch is that the new motors still use the wider mounting pattern. Installing them requires drilling and some Dremel work to make the fasteners fit. My goal is to produce a version with the correct bolt spacing that accepts the stock screws, but this would require a minimum order of 500 units, too much of a risk for such a low-volume market. For now, the workaround is a **3D-printed drilling jig** that snaps onto the motor bracket. It positions the bit with hardened drill bushings, ensuring the two new holes are placed accurately every time. Because of the bushings, the jig can be reused indefinitely, making the modification both quick and consistent. ![[NewMotorImage (9).jpg|300]] ![[NewMotorImage (10).jpg|300]] ![[NewMotorImage (11).jpg|300]] ![[NewMotorImage (12).jpg|300]]