eb2jim
Super Member
A lot of people wish that their two speed turntables were actually capable of playing 78 rpm discs. Some have large collections, and some have a small handful that got handed down or acquired. I have been lucky enough to have a stack that I got from my grandparents, plus a decent amount of early rock and some jazz 78’s. Over the years I have been also lucky enough to have a nice Dual 1219 that can spin them, and unlucky enough to come across rumbly, noisy cheap or ancient stuff that either does as much damage as playing, or produces a sound so bad it is neither listenable to or worth transferring to tape or digital file. Buying a turntable that has 78 can be a crapshoot of good to go, needs lots of work, or sounds like a train is in the background.
What I wished for was a way to take a nicer modern quiet turntable – belt or direct drive – and play my regular discs and then play my 78s when I was in the mood, with minimal effort. And with minimal rumble. Since you need to have a 78 cart and needle to do it correctly in the first place, a simple spare headshell swap would be the best option for me, so I thought of a popular Technics as a format. Headshells are available loads of places, and the deck often has a holder socket in back for a spare headshell and cartridge. For an example I selected the venerable SL-D2 with its solid build and its design being better suited for home use vs. club use. I was also aware that several places offered mods to Sl-1200’s and figured if it were possible for those, then maybe also for the cheaper and more plentiful used D2. I searched around and found the answer already existed. Popular Electronics had published an article in their April 1982 issue telling how to replace 45 rpm in favor of 78. I did not want to lose 45, so I altered the idea a bit, and here is how. This modification involves adding a simple 56k resistor, two lengths of wire, a bit of soldering, and drilling a discreetly hidden hole in the back for an added mini toggle switch – all available at your nearest Radio Shack. Don’t like any holes? Then you could have the switch exit the bottom and be not mounted or just sacrifice 45 like the original.
CAVEAT EMPTOR: First, if you have never figured out how to use a soldering iron safely or well, don’t do this. Find someone who is skilled, and ask them. Second if you do not have a decent workspace, and can be left alone for a bit, wait till you do. Thirdly, think ahead to how and where you will support, protect and work on the machine. Wear goggles when using solder. If you wreck your turntable, don’t blame me. If you get a blob of hot solder on your skin or cornea, blame yourself. Impatient, reckless and hasty? Buy one from somebody that does this.
I have done this with several, as have other AK’ers. The automatics from that period that have a 10” arm setting will be great candidates. I did this to an MCS 6700, which is my favorite Technics (so far) as it had 10” and could also stack, so I have a modern quiet direct drive automatic changer. The DD is a nice option as typically early belt drives used a mechanical lever to move the belt to a different position on the motor spindle. The change in diameter achieved 45 from 33. To add 78 would require a machine shop and not allow you to keep 45. Later belt drives can be modified as they changed speed by changing the motor speed. But that is for another thread. For this we modify a Technics DD. You can use SL-D1, D2, D5, 1650, 1950, MCS 6700, and many others. They all seem to follow the same pattern. You will have to investigate yours to see if it works. When you remove the bottom you will be able to figure it out.
The modification works in this manner: There are two sets of coils within the platter area. One applies torque to the platter; the other generates a position frequency signal. That signal is compared to a reference frequency set in the IC. Current to the drive coils is adjusted until the two frequency signals lock the speed. A direct current feedback path is part of this set up, and external to the IC. Different resistance is switched in and out of this feedback path to determine 33rpm or 45rpm. Typically 45 is dictated by R7 on the circuit board. The main pitch control zeroes speed in, and under the platter you can access the variable resistors which set the speeds in range for 33 and 45 by using a small screwdriver. When you solder a 56k resistor in parallel to R7 you get 78 rpm. You are just piggybacking it.
So, get your SL-D2 or similar era Technics direct drive and UNPLUG it. Remove the mat and pull the platter up and off, and set it aside. Lock the tonearm in place. On a nice clean towel (to protect the dustcover), flip over the unit on its dustcover and set it where you can work on it upside down. Remove the bottom panel, and keep track of all your screws and which feet go where – be neat and organized.
Next, locate R7 on the printed circuit board. Take one length of hookup wire and solder an end to one end of R7. Take a second equal length and to the same to the other end of R7. These two lengths of wire will be soldered to the center terminals of a DPDT mini toggle switch. Be careful - you aren't de-soldering R7 - just tapping a wire onto each end.
Some people prefer mini slide switches, as they can be hidden more discreetly on the bottom front, but are accessible by your fingertip. I prefer to locate a mini toggle on the back behind the arm, so I can reach over the machine and hit the switch when I want 78. In both cases the switch is hidden. You may not care, and want it right on top or on the front. Pick your location and style of switch for your own preferences, and just be sure that it doesn’t get in the way of something like the record, tonearm, platter, something already inside, etc.
On one end of the DPDT have your 56k resistor bridge the two end terminals. So, in one setting, the 56k resistor is completely out of the circuit and the turntable works as it always did. And in the other it gets thrown in parallel to R7, which speeds 45 up to 78rpm. Set the wires away from anything they may bind up on or get caught on – give yourself a good amount to find the side of the housing and feed it along there. Twist ties can help keep it where it belongs. Screw the bottom cover back, and flip it back over. You now simply have to adjust VR2 and VR1 to adjust the speeds general settings(* you can and may want to adjust VR1 and 2 from underneath so maybe hold off on the bottom replacement).
Setting the pitch control (VR3) to its midpoint, dial in 33 using a strobe disc (buy one, or print one out) and adjusting the under-platter speed adjusting variable resistor with a screwdriver. Then set the speed to 45 and do the same with the 45 speed adjusting vr. This is a trial and error process, and you can adjust the hidden vr's from top or bottom, so you can zero this in while watching a strobe disc. Typically if 45 is within range of the external pitch controls midpoint, you should be close on 78. Return the tonearm to shut off the machine, hit the toggle, and turn it on to spin the platter, which should now be spinning close to 78rpm. Using a strobe disc with 78rpm, you should be able to dial in 78 with the external pitch control.
Take your 78-rpm cartridge and headshell and swap them onto the tonearm, and play your 78s. HAVE FUN!!!
I used Radio Shack’s stranded hook up wire, a 275-614 DPDTmini toggle, and a 56 kilohm resistor from one of their packs. For the cart, it is subjective: I chose the Shure M78s as they were available brand new cheaply on Amazon as well as from many dealers and ebay. It is similar in size and weight to an M35c I had, so no worries about adjusting tracking. I mounted this in a standard headshell, and found Musicians Friend offered an almost identical Technics knockoff headshell for around $8. You can also find 78 needles for a surprising number of cartridges from over the years so certain classics have them available. If you wish a true mono vintage cart, they are still out there old and new, or you can bridge pins, buy a stereo-to-mono RCA plug adapter, etc.
I have done this several times, and I know other AK’er have as well. I have had one running 78 for quite a while, with no negative results. Let us know how it worked out for you.
Below is a pic to help you along.
What I wished for was a way to take a nicer modern quiet turntable – belt or direct drive – and play my regular discs and then play my 78s when I was in the mood, with minimal effort. And with minimal rumble. Since you need to have a 78 cart and needle to do it correctly in the first place, a simple spare headshell swap would be the best option for me, so I thought of a popular Technics as a format. Headshells are available loads of places, and the deck often has a holder socket in back for a spare headshell and cartridge. For an example I selected the venerable SL-D2 with its solid build and its design being better suited for home use vs. club use. I was also aware that several places offered mods to Sl-1200’s and figured if it were possible for those, then maybe also for the cheaper and more plentiful used D2. I searched around and found the answer already existed. Popular Electronics had published an article in their April 1982 issue telling how to replace 45 rpm in favor of 78. I did not want to lose 45, so I altered the idea a bit, and here is how. This modification involves adding a simple 56k resistor, two lengths of wire, a bit of soldering, and drilling a discreetly hidden hole in the back for an added mini toggle switch – all available at your nearest Radio Shack. Don’t like any holes? Then you could have the switch exit the bottom and be not mounted or just sacrifice 45 like the original.
CAVEAT EMPTOR: First, if you have never figured out how to use a soldering iron safely or well, don’t do this. Find someone who is skilled, and ask them. Second if you do not have a decent workspace, and can be left alone for a bit, wait till you do. Thirdly, think ahead to how and where you will support, protect and work on the machine. Wear goggles when using solder. If you wreck your turntable, don’t blame me. If you get a blob of hot solder on your skin or cornea, blame yourself. Impatient, reckless and hasty? Buy one from somebody that does this.
I have done this with several, as have other AK’ers. The automatics from that period that have a 10” arm setting will be great candidates. I did this to an MCS 6700, which is my favorite Technics (so far) as it had 10” and could also stack, so I have a modern quiet direct drive automatic changer. The DD is a nice option as typically early belt drives used a mechanical lever to move the belt to a different position on the motor spindle. The change in diameter achieved 45 from 33. To add 78 would require a machine shop and not allow you to keep 45. Later belt drives can be modified as they changed speed by changing the motor speed. But that is for another thread. For this we modify a Technics DD. You can use SL-D1, D2, D5, 1650, 1950, MCS 6700, and many others. They all seem to follow the same pattern. You will have to investigate yours to see if it works. When you remove the bottom you will be able to figure it out.
The modification works in this manner: There are two sets of coils within the platter area. One applies torque to the platter; the other generates a position frequency signal. That signal is compared to a reference frequency set in the IC. Current to the drive coils is adjusted until the two frequency signals lock the speed. A direct current feedback path is part of this set up, and external to the IC. Different resistance is switched in and out of this feedback path to determine 33rpm or 45rpm. Typically 45 is dictated by R7 on the circuit board. The main pitch control zeroes speed in, and under the platter you can access the variable resistors which set the speeds in range for 33 and 45 by using a small screwdriver. When you solder a 56k resistor in parallel to R7 you get 78 rpm. You are just piggybacking it.
So, get your SL-D2 or similar era Technics direct drive and UNPLUG it. Remove the mat and pull the platter up and off, and set it aside. Lock the tonearm in place. On a nice clean towel (to protect the dustcover), flip over the unit on its dustcover and set it where you can work on it upside down. Remove the bottom panel, and keep track of all your screws and which feet go where – be neat and organized.
Next, locate R7 on the printed circuit board. Take one length of hookup wire and solder an end to one end of R7. Take a second equal length and to the same to the other end of R7. These two lengths of wire will be soldered to the center terminals of a DPDT mini toggle switch. Be careful - you aren't de-soldering R7 - just tapping a wire onto each end.
Some people prefer mini slide switches, as they can be hidden more discreetly on the bottom front, but are accessible by your fingertip. I prefer to locate a mini toggle on the back behind the arm, so I can reach over the machine and hit the switch when I want 78. In both cases the switch is hidden. You may not care, and want it right on top or on the front. Pick your location and style of switch for your own preferences, and just be sure that it doesn’t get in the way of something like the record, tonearm, platter, something already inside, etc.
On one end of the DPDT have your 56k resistor bridge the two end terminals. So, in one setting, the 56k resistor is completely out of the circuit and the turntable works as it always did. And in the other it gets thrown in parallel to R7, which speeds 45 up to 78rpm. Set the wires away from anything they may bind up on or get caught on – give yourself a good amount to find the side of the housing and feed it along there. Twist ties can help keep it where it belongs. Screw the bottom cover back, and flip it back over. You now simply have to adjust VR2 and VR1 to adjust the speeds general settings(* you can and may want to adjust VR1 and 2 from underneath so maybe hold off on the bottom replacement).
Setting the pitch control (VR3) to its midpoint, dial in 33 using a strobe disc (buy one, or print one out) and adjusting the under-platter speed adjusting variable resistor with a screwdriver. Then set the speed to 45 and do the same with the 45 speed adjusting vr. This is a trial and error process, and you can adjust the hidden vr's from top or bottom, so you can zero this in while watching a strobe disc. Typically if 45 is within range of the external pitch controls midpoint, you should be close on 78. Return the tonearm to shut off the machine, hit the toggle, and turn it on to spin the platter, which should now be spinning close to 78rpm. Using a strobe disc with 78rpm, you should be able to dial in 78 with the external pitch control.
Take your 78-rpm cartridge and headshell and swap them onto the tonearm, and play your 78s. HAVE FUN!!!
I used Radio Shack’s stranded hook up wire, a 275-614 DPDTmini toggle, and a 56 kilohm resistor from one of their packs. For the cart, it is subjective: I chose the Shure M78s as they were available brand new cheaply on Amazon as well as from many dealers and ebay. It is similar in size and weight to an M35c I had, so no worries about adjusting tracking. I mounted this in a standard headshell, and found Musicians Friend offered an almost identical Technics knockoff headshell for around $8. You can also find 78 needles for a surprising number of cartridges from over the years so certain classics have them available. If you wish a true mono vintage cart, they are still out there old and new, or you can bridge pins, buy a stereo-to-mono RCA plug adapter, etc.
I have done this several times, and I know other AK’er have as well. I have had one running 78 for quite a while, with no negative results. Let us know how it worked out for you.
Below is a pic to help you along.
