What's a PRVC (Powered Remote Volume Control)?

     Funny you should ask! Prior to the Y100/Y160 of 1960, Remote Volume Control (or RVC as it was called) for Seeburg jukeboxes was accomplished using a relatively short, shielded cable ending in a single of dual (for stereo) volume control in the familiar ‘keyhole’ shaped plastic housing.  The cable was kept short and shielded to minimize the hum pickup a long, unshielded cable would introduce into the audio.

     With the introduction of the Y100 and Y160 using the SHFA3 amplifier, the volume control stayed mounted to the amplifier, with a DC motor attached to its rear. The motor drove a planetary drive consisting of a special volume control housing containing three ball bearings, which would rotate the control when either the motor shaft revolved, or the potentiometer shaft protruding out the rear of the cabinet was manually turned. However, the installation of the PRVC made it more difficult to manually rotate the potentiometer, since you were now also working against the motor shaft. The advantage of using the planetary drive was that neither the motor nor the potentiometer would be damaged if the motor continued to turn once either stop on the potentiometer had been reached. The shaft would just continue to turn, but the ball bearings would not, the motor shaft simply rotated against the bearings. In addition, a rubber link (9 in the exploded PRVC2 drawing above) was used to connect the motor to the shaft, permitting a fair amount of misalignment between the two. On the motor bracket, a small terminal strip with screw connections served to connect the cabling to the motor for up/down volume control. A similar terminal strip on the amplifier made connections for motor power and selection reject. The small components (6, 7, and 8 in the drawing) connecting the terminal strip to the motor comprise a filter, used to insure that the electrical noise generated when the motor turns is not coupled into the jukebox audio.

     For the new, now (Powered) Remote Volume Control (now called a PRVC), the same plastic control housing was used as the earlier non-powered units, with the potentiometer replaced with a switch. The multi-conductor shielded cable was replaced with a 5-wire unshielded cable, which could be any reasonable length, since it no longer carried audio. The same motor and planetary drive volume control was carried over into the new family of solid-state amplifiers, starting with the TSA1, introduced in 1962 with the model LPC1 stereo console. This machine, by the way, was the beginning of the console design era for Seeburg, in which the mechanism was totally hidden from view. The other manufacturers soon followed suit, introducing models in which the mechanism could not be seen.

     For all of the TSA-series amplifiers, excluding the last (TSA10), the volume control was physically mounted to the amplifier chassis, which in turn placed limitations on where the amplifier could be located in the machine, since the volume control shaft had to be accessible from the rear of the machine.

     It should be noted that TSA10 was not the last of the TSA-series amplifiers in production. TSA9 amplifiers continued to be built for several years after the TSA10 was replaced by the SHP1, which was introduced in the SPS160 model of 1972. The reason for this is that the lower-powered TSA9 was used in the 100-selection machine, while the TSA10 and later SHP was used in the 160-selection machines. The TSA9 was only made obsolete in 1975 with the introduction of the SB100 jukebox using the SHP2, a lower powered-version of the SHP1/3. Another reason for the obsolescence of the TSA-series was that they used power output transistors made of Germanium, which were starting to get expensive since the introduction of the Silicon output transistor a few years before. In fact, replacement Germanium output transistors for these amplifiers are getting hard to obtain nowadays.

     The TSA10 introduced a volume control physically identical to those used on the earlier amplifiers, except that the potentiometer and housing were connected to the amplifier using a shielded cable assembly, which was about three feet long. This volume control, VC1, gave the designers much more freedom to place the amplifier where there was adequate room and ventilation in the cabinet. The first machine to use this new set up was the USC1, having a cabinet design that would not have permitted an older-style amplifier to be located without major changes. The newer SHP series continued the practice of having the volume control on a separate cable assembly.

     With the introduction of the SHP amplifier came a new volume control, VC2.  The difference is that the older VC1 also routed the pickup audio from the mechanism to the amplifier, permitting removal of the mechanism without ripping out the entire cabinet cable. In the SPS160 and later machines, a connector on the Service Switch Assembly performed the same duty. From a mechanical point of view, PRVC1 will work for all amplifiers used in machines between the Y100/160 and DS100/160. The PRVC2 and PRVC3 should be compatible with the entire TSA series, and those machines equipped with the SHP amp and having VC2 installed. The photo at left shows a VC2/PRVC3 installation.

    PRVC-1 through PRVC-3 motors are interchangeable. PRVC-1 motors were mounted directly onto the VC, with screws connected directly to the motor, and apparently had no noise suppression, or terminal strip to connect the red/green wires. It had 'pigtails' which needed to be soldered to the wires. PRVC-2 added the terminal strip, a mounting bracket, along with the rubber coupler/removable pin, and the suppression circuit. There were two PRVC-2's, type 'A' and type 'B'. There were two different motors used. One mounted sideways and the other the same as the one pictured at left. Thanks to Ron Rich for pointing this out.
 

     At the left is a scanned schematic of a PRVC3, showing the color-coded wires Seeburg provided in the installation kit, surrounded by a large, flat oval. This is the cable connection between the control unit (at right) and the motor itself (lower left). The spade lug connections are for power to the motor and reject to the amplifier mute/trip relay.

      With the SPS2 jukebox of 1973 came the introduction of the VC3 volume control. This new model used a completely different motor and volume control, probably originating as a cost reduction. The new motor was a simple low-voltage DC type, very similar to motors found in some model Radio-Controlled cars. The volume control was replaced with a standard 25 KOhm unit, without the planetary drive. The simple sheet metal housing of VC1 and VC2 was replaced with a two-part molded plastic piece. The plastic part mounting the volume control was quite simple, having the control itself and the reject switch, plus the cable and a terminal strip used for the PRVC reject and motor power connections to the amplifier. The other plastic part mounted the motor, with an internal set of reduction gears, used to reduce the motor speed to a more reasonable rate to drive the volume control. Rather than using the molded rubber link between the drive and the potentiometer, the new unit used a pair of plastic disks, with Velcro attached (see photo at left of an early PRVC4 motor assembly). One disk mounted on the motor end, using a tension spring to keep it engaged with the other disk, which simply mounted on the potentiometer shaft. The Velcro disks performed the same function as the older planetary drive, rotating the potentiometer and slipping when either end of the potentiometer rotation had been reached. However, there is a major difference between this new PRVC (now called the PRVC4) and the older generation. When the PRVC is actually installed, the VC3 must be removed from the jukebox rear panel, the motor unit attached, and the combination reinstalled using the same threaded weld studs that mounted the VC3 originally. But to do so meant that the assembly had to be rotated, so that it was no longer possible to adjust the volume from the rear of the machine. You could only do so from the remote unit. While the motorized units and the volume controls are different, the control unit is identical for all versions of the PRVC

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     The drawing above is a manual scan of the VC3/PRVC4 schematic. Again, the sender is at the right, and the motor unit is at left.

     The first PRVC-4 motor assembly came with a two wire (3 pin) interconnect plug for the motor, and about a two foot long, 5-wire cable with a six connector plug, with a five connector terminal strip that was to be mounted inside the cabinet, at the rear. Later versions (using VC4) eliminated the wire, the 6-pin plug, and mounted the terminal strip directly to the motor housing. Pictured at left is a VC4 installed on a PRVC4. Also, a third wire was added to the interconnect between the Volume Control and the motor section.

     The last version, shown at left, featured a new-style clutch, replacing the Velcro pads with three steel balls, glued to the disc that attached to the Volume Control, and three smaller mating holes drilled into the other disc. However, the balls refused to remain in place. The last change was to lessen the spring pressure forcing the balls into the holes, to prevent the balls from digging into the opposite side and being dislodged.

     The drawing above is an exploded parts diagram of the later version of the PRVC4 motor assembly. Note the disc with the three balls (item 16).

     The drawing above is a manual scan of the VC4/PRVC4 schematic. Again, the sender is at the right, and the motor unit is at left.

    There were three versions of the PRVC remote unit. All are interchangeable with any PRVC. The first version had a spring loaded rotary switch that controlled the volume.  This version had no ‘Up-Down’ markings. To control the volume, the switch was twisted left for down, and right for up. The next version replaced the rotary switch with a center-off, spring loaded toggle switch, shown in the exploded diagram at left as item 2). It was mounted so that an upwards push would increase the volume, and a downwards push decreased the volume. It was marked on an over-sized washer labeled ‘increase-decrease’. The last version, which came out sometime in the STD series phonos was a ‘cost-cutting’ version of the previous one. Eliminated was the metal washer with the silk-screened instructions and some of the switch mounting hardware. This version featured a round plastic sign which stated Up/Down. This type is prone to problems since the switch can twist and/or force its way through the plastic housing, due to the fact that the supporting metal parts were left off. Another problem, common to all versions is ‘operator error’. This happens when the person hooking it up does not use the insulator (item 7 in the diagram). On SHP amps this will cause the 2 amp fuse, located under the amp chassis to blow. For these amplifiers, do NOT replace the two amp fuse with a bigger one, even if it only blows once in a while!  Find the problem and fix it! If you don't, the larger fuse won't blow, but the amp WILL.  For the TSA amps, it can cause the internal series resistor to burn up. In either case, the control will not operate. On the TSAs, this can cause intermittent operation of the control.

     Thanks and a great big tip-o'-the-hat to Ron Rich, who provided most of the photos, all of the PRVC4 modification info, the PRVC remote unit info, and the tips regarding problems when not using the insulator. Also, thanks to the guy who did most of the Seeburg drawings during the 60s, 70s, and 80s, Gene Beyak for doing such a great job on all the manuals.