Copyright July 21, 2018, published at stereo.toups.info All rights reserved.
Warning, use this information at your own risk. I cannot guarantee that this modification is without risk since it uses the turntable beyond the original marketed design.
Note this modificaton should work with the SL-1300 MK2 family of turntables including the SL-1300 MK2, SL-1310 MK2, SL-1400 MK2, SL-1400 MK2, SL-1410 MK2, SL-1500 MK2 and SL-1510 MK2.
As I previously reported on a thread, SL-1300 MK2 78 rpm mod, at Audio Karma I've been able to modify a Technics SL-1300 MK2 to add a 78 rpm mode. (I am referring to the MK2 and not the original model, the original model has a modification that is well known and involves an added resistor.)
The SL-1300 MK2 uses the DN860 chip to generate the reference frequencies needed for 33 and 45 rpm speeds. Note that this is the same chip that is used in the SP-10 MK3 with the variable speed control that does 33, 45 and 78 rpm. From the service manual for the SP-10 MK3, pin 12 can be used to select the chip mode that generates the frequency needed for 78 rpm. So to activate the 78 rpm mode, all one has to do is temporarily short pin 12 of the DN860 chip to ground.
When in 78 rpm mode, frequency pitch control still works as originally designed.
The simple modification is to add a momentary switch as another speed select switch that can temporarily ground pin 12 to trigger 78 rpm mode. Note this should only be a momentary switch, after a ground pulse, the IC remembers the mode and stays in that mode until another speed switch (33 or 45) is pressed.
The good point is that this is quite simple and involves no external circuits and only the addition of a switch and 2 wires to the turntable. Of course, one has to figure where to mount the switch. Also while in 78 mode the display will show this odd display.
The "full" modification at this point consists of two modifications both put on a small board inserted into the turntable. The first modification enables the pin 12 78 mode when both the 33 and 45 speed buttons are pressed and then released simultaneously. This solves the problem of mounting another switch. The existing 33 and 45 speed selection are used. The second modification uses diode logic to turn on the proper digits to display 78 in the 78 mode. In order to implement, some traces need to be cut that select certain segments of the 7 segment displays and new wires soldered to the appropriate segments. Also some resistors are replaced with different values to keep the drive current at the appropriate levels.
A schematic of the modification and test circuit in LTSpice format is available here.
In the following circuit, Q1 implements a NOR gate for the 33 and 45 signals. The output (collector of Q1) is high only if both buttons are pressed, i.e., connected to ground. When the output is high, C1 is rapidly charged through D1. When C1 is high, Q2 and Q3 both conduct and the 78 output line is pulled low. When the either of the buttons is released, the capacitor C1 discharges through Q2 via R4. This allows for a delay in the turn off of Q2 and Q3 once a button is released. Note that Q2 serves as an emmitter follower to allow a smaller current and smaller C1 to add delay to the turn off. Q3 is an inverting buffer that drives the 78 rpm pin on the DN860 as well as the appropriate segments on the LED to display the number 78. The idea is that if you release both buttons in a time less than the delay, the last low signal the DN860 sees is the 78 signal and it then latches into that mode.
Below is the circuit used to change the display to allow it do display a 78 when in that mode. It requires 4 cuts to the original circuit board and additional wiriing. See the circuit board layout in a succeeding diagram for more details.
(R9 may not be needed? To be updated later.)
In the following diagram, items in red are new wires that are added to connect to the added circuit board along with a jumper. The connections to the new circuit board are labelled with the (in some cases a short form) connection name used in the previous schematics. Also in red are notes regarding changes in resistor values. Note also that R210 is removed from the original circuit. The green blue and yellow highlights trace the path of the 33, 45 and 75 selection logic. Also shown are 4 wire cuts in light brown that are needed as part of the LED modification.
These cuts are shown below on the circuit board and are circuled in red.
The above circuits were implemented on a small piece of prototyping board. Wires were routed to the front and rear of the original turntable circuit and some resistors were replaced on the board also. The board was then wrapped in electrical tape and carefully placed inside the truntable near the display during reassembly.
However, I am aware of two potential issues.
1) One is that on the SL-1300 MK2 the 33/45 selection modifies the auto-return time constant. (Although there should be no impact if manual return is used.) I have not investigatee which time constant the modified system uses in 78 mode. I do no know what impact this could have on operation.
2) The other is that the SP-10 series uses different divider resistors for 33, 45 and 78 rpm selections on the AN-660 control circuit (IC203 on the SP-10 and IC202 on the SP-1300 MK2.) The SP-1300 MK2 uses a single resistor configuration for both 33 and 45 rpm. At the moment, this appears adequate at 78 mode although probably not optimal.