I originally posted this article on line about 10 Years ago.
I wrote this more as a reminder for myself as to what I did, as opposed to being overly detailed instructions.
Modifying the Sansui FR-5080 to play 78's
BE SURE YOUR TURNTABLE IS UNPLUGGED BEFORE YOU BEGIN!
While the majority of belt drive turntables can be easily converted to play 78's, direct drive models are usually more involved. The control circuitry used in Sansui's SR-5080, SR-525, etc however are an easy modification that anyone with a little electronics knowledge can perform in an evening. The principle can be applied to the majority of belt drive turntables which use DC (as opposed to AC) motors as well.
Before I opened up the case to look at the electronics, the separate pitch controls for 33 & 45 rpm were a giveaway that this would probably be an easy turntable to work with.
Before getting started now is a good time to remove the counterweight.
Looking from this perspective the
counterweight will unscrew by turning clockwise. Put it aside. We won't need to put it back on until the
job is complete.
Now secure the tone-arm to the rest. I use a piece of yarn tied with a slip knot but a wire 'twist tie' would do as well. Because the tone-arm will have to be secured and unsecured several times during the adjustment process I prefer using yarn.
Next remove the turntable mat...
Then grasp the turntable securely....
and lift it straight off. This is usually a snug fit. Avoid the temptation to wiggle it side to side.
The turntable must be turned upside down in order to get to the electronic circuit we need to modify. If your very careful you can put the dust cover back on and flip the turntable upside down, using a towel between it and your workbench to prevent scratches. Alternatively you can support the turntable with several books placed around the perimeter. Make sure there will enough clearance for the tone-arm.
Now we're ready to flip the unit over...
and the fun begins!
A screwdriver bit in a cordless (reversible) drill will make short work out of removing all screws from the bottom plate.
Once your sure you have found all the screws holding the bottom plate, try and shift it loose. If it doesn't move freely check for any screws you may have overlooked. Do NOT try and lift the plate off yet. There should be a grounding wire attached to the plate (see next picture) which may not be obvious when first attempting to remove the bottom plate.
Remove the screw holding the ground wire.
Once the cover is removed, look for the 2 potentiometers that are the pitch controls. From the back they look like little tin cans. The one on the right goes to the 45rpm circuit. This is the one to modify. If you don't want to put any holes in the case of your turntable and have no need for the 45rpm speed there are two other options available:
1. Add a 18Kohm resister between either of the yellow wires and the potentiometer. If you go this route the pitch control will still function and you should have a usable range of around 70 to 90 rpm - perfect for nearly every vintage of '78' you may have in your collection. In this case a digital tachometer will be helpful if you plan on adjusting the speed frequently.
2. Bypass the potentiometer with a 22Kohm resistor. The easiest way to do this would be to solder the resistor to the top lug of the potentiometer and to the 45 rpm switch where the yellow wire connects. You will need to remove this yellow wire completely (the one which connects the middle lug of the potentiometer to the 45rpm switch). Final speed calibration will be the same as if you install the 45/78 switch.
Be sure that the resistor leads cannot short out on any metal parts.
If you want to keep your 45 rpm capabilities then you will need to add a switch to select between 45 & 78. Determine the best location for the switch and drill through the top plate. If possible it is best to drill a small hole from the rear so your sure not to hit any of the circuitry and then drill the final size from the front. We chose to install ours in the notch located on the lower left hand corner of plate to which the potentiometers are mounted. The lower ear had to be removed to accommodate the lever switch we used. You can install the switch anywhere you like, but it would be best to keep it as close to the rest of the circuit as possible. If you prefer it would be possible to mount the switch to the bottom of the turntable as well, however this will be a slightly more involved installation.
You can see our finished speed selector switch installed at the lower left hand corner. We used a double pole double throw switch, but only used one side. A single pole double pole switch would have been less expensive, but I prefer the look of a 'lever' switch and my local Radio Shack only had them available in the double pole variety.
What we are doing with the switch is bypassing the potentiometer and replacing it with a 22Kohm resistor for 78, and putting the potentiometer back into the circuit for 45.
1. unsolder the yellow wire from the top lug  of the potentiometer
and solder it to the center lug of the switch.
2. solder one end of a 22Kohm resistor to the middle lug of the potentiometer 
3. solder a wire to the other end of the 22Kohm resistor, then cover the resistor and its leads with heat shrink tubing.
4. solder the other end of this wire to top lug of the switch 
5. Connect a wire from the top lug of the potentiometer  to the bottom lug of the switch 
Now its time to test the turntable. Carefully turn the unit upright and support the front (the back feet should still be attached but the front ones are on the plate we removed). Look underneath and make sure there is nothing touching any of the electronics. We've left the circuit exposed underneath and portions of the circuit will have 110 volts running through them when we test the turntable. Replace the platter and remove the yarn or wire securing the tone-arm. The FR-5080 is a fully auto turntable and when you start it up it will move the tone-arm over where record. Keep the lift lever set in the 'up' position so that the tone-arm doesn't come down on the spinning platter.
Make sure the newly installed 78 rpm switch is flipped towards the front of the turntable (this should be the 45 setting) and the speed selector is set to 33. Plug the turntable in and push the start lever. Check the strobe markings on the turntable platter and adjust the pitch control if necessary for 33. Now depress the 45 switch and again adjust as necessary. If both of these settings operate properly you can continue to the next step. If not, unplug the turntable, secure the tone-arm and turn it back over to check your wiring.
Flip your newly installed switch and the turntable should speed up to close to 78. At this point ours ran at 76. The turntable has an additional pitch adjustment located underneath the motor (pictured below) that we will need to set to get the correct speed. For this step it's best to have an extra set of hands so that you can pull the turntable forward off the table enough so you can reach the 45 adjustment.
A slight turn of our screwdriver and we were running at the proper speed of 78.2rpm for standardized 78's. (I'm no longer sure if that was the standard speed, or if it was 77.8) We used a kit built Tach for speed calibration (no longer available), however you can get as accurate results using a strobe disk you can print yourself. I got mine at Nauck's Vintage Records Website
Now that we have the speed control set we're still not ready to play a record. Unplug the turntable, remove the platter, secure the tone-arm and flip the turntable over.
If your working with a SR-525 or other fully manual turntable all that is left to do bridge the ground wires on the phono output jacks and put the cover back on. If your modifying the SR-5060 you will also need to disable the auto-return feature. The groove on some 78's continues past the sensor for the auto-return mechanism.
With the turntable upside, the tone-arm pivot should be at the lower right. There is a small magnet attached to an arm that swings with the tone-arm. When the magnet gets near the relay (which is enclosed in a small white plastic case) it activates the return mechanism. We need to break the magnet off. Do this carefully by bending it back and forth with a pair of pliers until the metal support breaks off.
We only had to bend the tab with the magnet attached 5 times before it snapped off. Alternatively you could have used a large pair of side cutters to cut the magnet support bracket.
Note: we are breaking the metal mounting tab and not the magnet itself. If you break the magnet you will have tiny, sharp shards all over the inside of your turntable.
TIP: Instead of removing the magnet, two other options are:
[detailed instructions not provided for these options]
An option you may wish to consider is the addition of a mono / stereo switch. All you will need to do is connect wires from each of the 'hot wires' from the phono cartridge to a single pole, double throw switch.
If you're not sure which wires are 'hot', remove the 2 screws holding the panel with the phono jacks. Carefully pull it away from the turntable base and check which colored wires go to the center of the jacks. Then locate these wires at the tone-arm base (see previous picture).
If your going to be using your turntable with a computer, you may wish to consider bridging the ground wires on the phono jacks. Doing so will minimize the need for any additional grounding. All it takes is 2 pieces of wire to connect the 3 ground lugs together. If you're using a conventional stereo system with phono inputs you may wish to omit this step and ground your turntable in the normal fashion.
Now put everything back together, hook your turntable up and enjoy your record collection!