A customer brought in a Casio SK1 this week with two requests:
1) Replace the 1/8" headphone out with a 1/4" mono line out. Customer said he tried just using an adapter but the audio was too hot to comfortably drive a mixer or guitar, so some attenuation is needed.
2) Keep the SK1 from powering off after 7 minutes.
I decided to tackle #2 first.
The schematic is freely available, and Casio goes into some detail about the Auto Power-Off (APO) function.
My first instinct is to remove the connection from the cpu to T5 and keep that line held low (0V). This is also the method often shared on electro-music.com and other forums. Easy to do, just remove one jumper (right above the mounting hole) and wire the right side to a ground point.
This method does not work.
It keeps the analog circuit on, however the cpu goes to sleep after 7 minutes. It also prevents the sk-1 from ever turning off. Double whammy.
I decided to dive right into designing a keep-awake circuit. I want to keep the SK1 awake by periodically "pressing" a button to reset that 7 minute timer. I decided to use the 'Vibrato' button and press it twice in quick succession every 5 minutes. This would be the least obtrusive, as it would just engage (or disengage) the effect for 200ms every 5 minutes. Much less obtrusive than playing a key or messing with any other setting.
I decided to do this with a small 8-pin microcontroller. Yes, this could be done with an analog circuit, however I would need two timers - one for 5 minutes, and another two shot timer with 200ms pulses. I would also likely need a large capacitor to allow for a 5 minute interval. There's not too much real estate available in the SK1 housing, and I already had some PIC12F510 8-bit microcontrollers available.
The PIC turns on a 2N7000 N-channel MOSFET which perfectly mimics the button press. A resistor on the gate of the MOSFET, and another to tie MCLR to 5V means that this is a 4 component solution. The MCLR resistor could even be removed with a config adjustment, but I had already put away my programmer (lazy, I know).
It took some time to find a voltage source that didn't remain on when the slide switch is set to OFF. I found VA (JB-4) that does just that, provides 5V when the SK1 is on, and 0V when it's off. This way my little circuit board doesn't draw power when the SK1 is switched off, and the battery will last a little longer.
That wraps up the APO mod. The 1/4" line out was fairly straight forward. It's important that you take your output signal from before the power amplifier IC1 (LA4138). I picked it off right at pin 7, sent through a 100nF film capacitor, and to the tip of a Neutrik 1/4" jack. Because the headphone jack is connected to the speaker through it's internal switch, I disconnected the speaker entirely. The speaker in this unit was malfunctioning anyway, so it's not too much of a loss.
One could likely send the tip switch of the new 1/4" jack out and back into the power amp to allow use of the speaker.