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phono By Ron Tipton Restoration of a Vintage Turntable Some simple modifications that bring new life to classic gear. ing the motor current showed an increase to 58mA (from the normal current of 50mA) that corresponded, in time, to the drag. RESTORATION PHOTO 1: Unmodified Netronics 350D with the original Shure tonearm. The power cable to the wall-wart transformer and the stereo output cable came out from under the base unit. T he Netronics1 model 350D, shown in Photo 1, was apparently available in kit form only during the 1970s, although I remember buying mine earlier than that (I’ve lost the paperwork on this turntable). The 350D is a “bare-bones” turntable featuring direct drive and two speeds: 33 1/3 and 45 RPM. All of the speed control is done inside the motor2 housing, so the external circuit, shown in Fig. 1, was both very simple and somewhat deficient. Netronics has long since gone out of business. Although I spent quite a bit of time on a web search, I was unable to find any information on the 350D, but I did locate a review of the later, three-speed version, the 350F, in a review published by the Boston Audio Society in May 1978³. The 350F included pitch lock as well as a meter to indicate the exact setting. Both models used the same 12V AC wall transformer and both incorporated Netronics’ “dual-resonant anti-feedback suspension” (more on this later). Both the two- and three-speed motors use an oil suspension bearing that is extremely rugged, as evidenced by the fact that mine still ran after about 35 years. However, it didn’t run too well! There was a noticeable drag at one point in each revolution, and a meter measur- FIGURE 1: Original Netronics 350D power supply and speed control. 20 audioXpress 2/11 tipton3202.indd 20 My first step was to disassemble the motor and clean the bearings. Using an eyedropper, I put a few drops of lacquer thinner into the bearing well. Then with a cotton-tipped swab, I cleaned the bearing. I repeated this a few times to be sure of dissolving and removing all of the old oil. The cotton swabs did a good job of absorbing and removing the oilthinner mix. I re-lubricated the bearing with a few drops of “Nye” oil (high-quality, synthetic SAE 20)4 and reassembled the motor. There was considerable “compression” when I put the spindle into the bearing well, indicating a “good” bearing (very little wear). After putting the platter on the spindle (for its added weight), I needed to wait about ten minutes for the spindle to settle to be able to put the top cap back on the motor. Then I re-mounted the motor on the motorboard. The cyclic drag was gone and the motor current showed just slight flutter around its 50mA nominal current. Also, the motor was now very quiet as I listened to the motorboard with a stethoscope. (This is an excellent tool for working on turntables. You can buy a new one for a very reasonable price from American Science & Surplus, catalog number 92082P15.) I found the much-touted (see B.A.S. review) “dual-resonant anti-feedback” suspension system to be exasperating! Just walking around the room (carpeted wood floor) was enough to cause a stylus skip. This suspension system used four conical springs (Photo 2), one at each corner of the motorboard, which sat on the base unit. Then there were an additional four conical springs, one at each www.audioXpress .com 1/3/2011 8:21:42 AM PHOTO 2: Original suspension springs. Four springs with plastic caps were used to support the base unit. Four springs without caps supported the motorboard on the base unit. PHOTO 3: The new base unit feet were salvaged from a discarded modern turntable. PHOTO 4: Four stacks of art-gum erasers now support the motorboard on the base unit. FIGURE 2: Regulated 18V power supply for the restored Netronics 350D. This supply is part of the “Multiple Wall-Wart Power Supply” described in the June 2008 issue of audioXpress. corner of the base unit. The springs were terribly under-damped—it was a very “bouncy” system. I removed all the springs. To support the base unit, I used four feet salvaged from a defunct Stanton6 turntable (Photo 3). To support the motorboard on the base unit, I used three art-gum erasers stuck together with rubber cement at each corner (Photo 4). I also glued each eraser stack to the top surface of the base unit with a dab of rubber cement. In this application, the rubber cement joints are always under compression so the adhesive just keeps the erasers in place. (The number of erasers needed depends on their size, the stack needs to be between 1" and 1 1/8" high.) This suspension seems to work well. PROBLEM-SOLVERS The circuit diagram in Fig. 1 shows that the motor power supply is not very “robust.” The half-wave rectifier with single filter capacitor supplies 15 to 16V DC (depending on the mains voltage), and the AC ripple is, in my opinion, excessive. The motor is rated at 18V DC, so the original power supply was marginal audioXpress February 2011 tipton3202.indd 21 21 1/4/2011 3:01:57 PM PHOTO 5: Aluminum angle added to the motorboard for all input and output connectors. The binding post is insulated from the angle bracket. at best. A stroboscope revealed a “not quite steady” rotational speed, so a better supply seemed in order. A regulated 18V DC supply takes care of any variation in mains voltage and also steadies the speed even without the pitch lock found in the 350F. Rather than building a new regulated supply, I changed one of the outputs from 24V to 18V (see “A Multiple Wall-Wart Power Supply,” June ’08 aX). For completeness, I’m including a circuit diagram of the 18V supply here (Fig. 2). An external power supply takes care of another potential problem: hum pickup. With no AC power into the turntable unit, noise pickup from the power mains should be minimized. I added an aluminum bracket on the lower rear edge of the motorboard (Photo 5). The 2.5mm DC power connector is shown on the right. The adjacent binding post connects to the motor frame only. I added it just in case connecting it to the preamplifier enclosure would lower the noise level (I can’t see any difference in my system). The two RCA connectors are the stereo outputs from the pickup cartridge, which leads to the next modification: a new tonearm. In Photo 1 you can see a vintage Shure tonearm type M2327. This item didn’t come with the kit from Netronics. I purchased it somewhere and installed it along with a Shure M3D cartridge. Both served a long and use- PHOTO 6: The restored turntable with tonearm and cartridge salvaged from a Stanton STR8-80. 22 audioXpress 2/11 tipton3202.indd 22 www.audioXpress .com 1/4/2011 3:03:25 PM PHOTO 7: Motorboard underside. The motorboard is ¾″ thick particle board. The aluminum panel used to mount the toggle switches and fine speed control is 3″ wide by 0.1″ thick and also serves to stiffen the motorboard. ful life and deserved replacement. I salvaged the tonearm with a Stanton 500E MK II cartridge (0.4 × 0.7 mil elliptical stylus) from the same defunct turntable that contributed its feet (Photo 6). The cast aluminum platter weighs 50 oz. and is about an inch thick at the rim. I needed to add the quarter-inchthick brushed aluminum plate under the tonearm to make the arm level with the platter, rubber mat, and recording. The plate is electrically connected to the output audio common. One final modification: the speed control potentiometer. Netronics used a 5K audio taper pot with a 4700Ω resistor from wiper to the CCW end. This combination produces a somewhat linear 2500Ω pot, which I replaced with a dual 5kΩ linear pot with the two sections in parallel. You can see the simplicity of the motorboard wiring in Photo 7. The two small holes that are visible on the bottom of the motor are the coarse speed adjustments. The 5K pot is the fine speed control, about ±5%. FINAL THOUGHTS Was this restoration worth the time and effort? Yes, I think so for at least a couple of reasons. Except for the suspension system, this is a good design; it is simple and it works well. It seemed a serious waste to scrap it just because it didn’t have the latest “bells-and-whistles” such as speed lock and dynamic breaking. I have other LP turntables, all of them direct drive. This vintage model, after restoration, sounds as good as the others given that the rest of the audio system is the same. aX NOTES 1. Netronics, 333 Litchfield Road (Route 202), New Milford, CT 06776 (out of business). 2. DC motor type MKL-15SI-T, Matsushita Electric (now Panasonic). 3. The Boston Audio Society, PO Box 260211, Boston, MA 02126,www.bostonaudiosociety.org. The review is available online: 06-08-7805.pdf 4. Small quantities of this, and other highquality oils, are available from Don Walizer, ecg@ conwaycorp.net. 5. American Science & Surplus, PO Box 1030, Skokie, IL 60076, www.sciplus.com, 888-724-7587. 6. Stanton Magnetics, 772 S. Military Trail, Deerfield Beach, FL 33442.www.stantonmagnetics.com, 954-949-9600. 7. Shure, Inc., 5800 W. Touhy Avenue, Niles, IL 60714, www.shure.com, 847-600-2000. audioXpress February 2011 tipton3202.indd 23 23 1/4/2011 3:03:54 PM