DRO Electronics

I have breadboarded the DRO electronics now. I've settled on three 8digit LED displays for axis position and three 0.96" OLED displays as labels for the axes. These will show whether the axis is X,Y, Z or whatever, plus summed axes and any other information for the axes that may be needed.

I'm also having a 2.4" TFT touchscreen LCD as the main keypad instead of separate keys. This means I can change the layout of the keys and have firmware controlled keypads as required. I've added one hardware key per axis though as an axis zero button.

The TFT LCD can also be used to display various more complicated modes like centre finding and so on.

This is a couple of the displays breadboarded as I tried to get the TFT displays to work. This is a bit of a nightmare as I've had boards that look identical to each other but which seem to have completely different controller chips on them. The touch screen functionality isn';t the most advanced int he world either, requiring voltages to be set up with GPIO lines and then and ADC reading to determine each axis position. Finally got it working though. The OLED display was easy, I just ported the OLED watch code...

The PIC at the moment is a 16F18875. I think I may end up running out of flash space, well, maybe, but that's not a huge problem as there are PICs with the same pinout and more flash space. I'll move to one of them if I need to.
I'm in the middle of routing a PCB with the main displays and big PIC on it. The circuits for each gauge I've decided to put on separate daughter boards. It means I have to lay out just one PCB and I can also adjust the gauge circuit to match any future gauges I find that have slightly different interfaces. Each of these interfaces has it's own PIC12F1822 which captures the gauge digital output and acts as an I2C slave which the main processor then polls. I have done most of that code already. This means that there are seven processors in total and an idle loop for polling each gauge, the main processor doesn't then have to worry about polling six data streams. It should all work...

The main display PCB being routed:

Well, the start of it anyway.

Sony 7035 Volume Control Replacement

I have been asked to look at a crackly volume control on a Sony 7035 tuner amplifier. This is a nice unit from the 80s or so. The volume control is a ganged potentiometer along with balancecontrols. There's no way to get a replacement after all these years, so the only option is to attempt a repair or replace some parts with something else.

Looking at the circuit diagram, the volume pots are centre tap versions as well, which is used for the LOUDNESS function. Not having managed to find any four gang pots, one option was to use a processor and digital pots. Unfortunately the voltage on the pots is +/- 27V which is too much for the digital pots I coukld find. So, using a dual gabg pot for volume I decided to fix the banalnce to mid position and remove the LOUDNESS function.

I made a PCB on the CNC router

then removed the unwanted copper due to the voltages

Once fitted into the Sony, the volume control works with no crackling.

The original pots had an extended stem which the volume knob attaches to, so I need to machine an extender for the shorter pot I fitted. I hope the extender is solid enough once the knob is attached and that it won't wobble about too much.

The balance is mid position, the LOUDNESS and MUTE don't work, and I fitted a linear pot as the log pot I fitted didn't feel very good. I'll see how this solution works, but a second version may be in order.

I'm wondering if I can put a processor on the PCB and use FETs to effectively create a high voltage set of digital pots. One problem with this is that the processor may cause noise due to switching which the radio may pick up, as it is quite sensitive and will be in the same box.