Dual 704 turntable is......
I recently acquired a Dual 704 turntable from a renowned tonearm designer. Despite being in good shape, particularly the tonearm, the turntable required some special attention. I have identified the following issues that need to be addressed:
- Motor - Requires assistance to start running at times.
- Missing tonearm rest.
- Nearly mint Grado FCE+1 cartridge missing the needle (I plan to replace it with a Shure V15III).
After spending a delightful time chatting with the owner, I took the turntable home and immediately began the restoration process.
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| the heart of the EDS900 - four induction coils |
704 has a 4-coil Hall Effect motor EDS900 as the driving unit. The design of this motor is very primitive. However, the specification of this turntable is still very impressive. You can't hear any motor noise and feel no vibration at all. It seems that the workmanship is so good even the design is not modern.
Speeds: 33.33 and 45rpm
Platter: 1.2kg, 300mm, dynamically balanced
Wow and flutter: 0.05% WRMS
Rumble: Weighted less than 67dB / Unweighted less than 46 dB
Tonearm effective length: 222mm
Tangential tracking error: 0.16 degree/cm
Tonearm bearing friction: vertical less than 0.07mN / horizontal less than 0.15mN
Tonearm: Torsionally rigid tubular aluminum in low friction four points gimbal suspension, counterbalance with two mechanical anti-resonance filters
The jerky motor might be caused by the motor driver board. The test show that the driver board is in perfect condition. So the motor itself should be the problem. The rotation was not smooth but the rotation rate is accurate. This symptom showing one or two of the coils in the motor was not working. The scenario is just like a 4-cylinder engine with one dead spake. The car can run but lack of power and is not running smooth.
The EDS 900 has four coils in total and you can see the picture of the internal. The coils are glued on to the housing by epoxy. Had a quick check on the connector and bingo! one of the connection to the coil was opened. Look closer to the printed circuit board and two of the traces were corroded. Spent an hour to fix the circuit, lubricate the whole motor unit and also changed all the E-caps in the driver board. The turntable was now running at its should be.
The signal wires were also changed to Belden Teflon 83284 wire ( the capacitance of the original wire is 140pF and the new wire is about 90pF. it is a very important to know the value. The change of the capacitance will affect the frequency response. 83284 is a RG316 coaxial wire and even it is specified for high frequency use up to 3GHz but it also can be used for audio.)
The next thing to be tackled was the tonearm rest. Buying an arm rest is easy but why not to make one. I still have some beech wood stocks so I took a small piece and sanded it to a desired shape. The best height of the rest is about 1.2cm with a 2.5cm sideboard which is used to prevent the overshot of the tonearm return. After soaking the finished wood parts in stain conditioner and then the stain, the arm rest was then polished with furniture wax.
All tasks have been done. It is the time to sit down and listen music. 704 is a semi-automatic turntable. You need to place the head onto the record and set the needle down manually. The needle is always in UP position when you put the head onto the record so it is fault-proofed. The turntable will be automatically off when the head reaches the end. The head is now in the UP position again. When you put the head back to the arm rest the head will be in down position. Amazing simple but a very smart design.
The Vertical Tonearm Control allows the cartilage is in optimal playing position without using spacer in the headshell.
Finally here is the pic to show the coil connection with the resistance value. If you found your 704 having a jerky motor, check it out first.
Enjoy
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