Logic DM101 Electronic Reverse engineered "to fix mine" by; Kelvin Unsworth 21st October 2006
Note: Some component values may be wrong due to colour fade. This is how I think the circuit works. There are NO guarantees. Part numbers/values and circuit connections could be wrong. Use at your own risk. 240 VAC enters via a 100ma anti surge fuse (not shown on scematic) then passes through a 100 ohm resistor. There is then an anti surge varistor to prevent mains spikes. The 240 VAC is then recified and filtered with two 22uF electrolytic capacitors giving approx' +150 and -150 VDC with a 0V in between. On the -150VDC side the -ve voltage is supplied to the -ve out put transistors Q5 and Q6. It also supplies a 15k wire wound resistor that feeds on to two 7.5v zenner diodes. These make the -15v supply rail. On the +150VDC side the +ve voltage is supplied to the +ve out put transistors Q3 and Q4. It also supplies a 15k wire wound resistor that feeds the LED circuitry. Current will flow through either Q1 or LED1 depending on the state of the two flip flops in IC2. If the 45RPM button has been pressed then out puts QA (IC2a) and QB(IC2b) will be on ( at +15v) and D1 and D2 will not conduct and Q1 will turn off. The current will then flow through LED1 lighting it. I either QA or QB are off then Q1 will be turned on and there will not be enough voltage across LED1 to light it. The current flow will continue through to Q2. Current will flow through either Q2 or LED2 depending on the state of the two flip flops in IC2. If the 33RPM button has been pressed then out puts NOTQA (IC2a) and QB(IC2b) will be on ( at +15v) and D3 and D4 will not conduct and Q2 will turn off. The current will then flow through LED2 lighting it. I either NOTQA or QB are off then Q2 will be turned on and there will not be enough voltage across LED2 to light it. QB = turn motor on QA = 45RPM NOTQA = 33RPM Pressing SW3 (45RPM button) puts 15v onto the set input of flip flop a and b latching QA and QB on via D9 and D8. Pressing SW1 (33RPM button) puts 15v onto the set input of flip flop b via D7 and reset flipflop b latching NOTQA and QB on. Pressing SW2 resets flipflop b turning QB off. The current flowing via Q1/LED1 Q2/LED2 feeds through to two 7.5 v zenner diodes making the +15v supply. The flipflip a out put QA is used to operate three solid state switches in DG308ACJ these short out R29,R30,R26 and R25 in the oscilator circuit changing it's frequency from 50 Hz (33RPM) to 67.5 Hz (45RPM) Flipflop b out put QB is used to switch the oscilator out put from IC3b into the drive opamp IC1. The DG308ACJ ic is only shown as the four solid state switches on the scematic. IC3a/b is an LF353N my CAD program did not have this IC so it is shown as an LM348N.
