Hohner Bass 3 (not really a pedal)

[FUZZZZzzzz]

I keep counting 30 keys on the video, but only seem to find 29 resistors for the designated keys. maybe number 0 is the first one?

[anotherjim]

There is the fixed resistor + tuning trimpot for the missing end note.
Note: for this oscillator type, the lowest resistance in the feedback from the inverter output to the timing cap at its input sets the highest frequency.

[Strategy]

Cool project. I'm a keyboard player and have a few organs (similar tech) but have never DIYed a keyboard of this type (done some synth projects and loads of pedals to run the organs through). Have always wanted one of these Hohner basses! They pop up on Reverb a surprising amount but majority of them are in Europe. Rheem made a model called Kee Bass (yes that's the spelling) that also sounds good. There were many more makes and models of this type of bass tone keyboard as a standalone floor pedal unit like organ pedals. Following this !
Strategy

[Rob Strand]

Have always wanted one of these Hohner basses

I had a poke around on some youtube videos and it seemed to me the sound varies from unit to unit quite a bit.  Both the basic tone and the attack.  Some had an unnatural decay.   I can only guess the trimpots have been tinkered with over the years or they weren't adjusted consistently at the factory.    The service manual is a bit light on details.   Keyboard guys like yourself will probably know what it is supposed to sound like.

[Rob Strand]

I keep counting 30 keys on the video, but only seem to find 29 resistors for the designated keys. maybe number 0 is the first one?

In the spreadsheet I called the 33k2 R0 but it's not called '0' on the schematic.  The resistors have numbers 1 to 29 and the corresponding key is the switch *below* the resistor.

One small thing about the spreadsheet.  There's some stuff in the top right.  That stuff was me working out a range of thresholds consistent with the 74C14 datasheet.    It's not really finished.   I started to do that when the frequencies didn't work out but then I worked out the schematic had a bug.

[FUZZZZzzzz]

Im going to make some layouts and worry about the keys last. I really like the idea to etch a stylophone like keyboard. I wonder if the trimpot per key is easy to do. The smaller it is the easier the decision to take it with you on projects and jams

[FUZZZZzzzz]

I know some of you dislike vero board / perf board, but this project works well on this format. I will build all the parts and then arrange them inside the enclosure when done. It will make it easier to troubleshoot. I hope I got it right. Easy to misplace a component. Not yet verified. When it is I will delete the ones that are not.

[FUZZZZzzzz]

I'm not sure about the PITCH knob. It doesnt make sense to me. Maybe someone can enlighten me.

[FUZZZZzzzz]

Also, it says D910 on the schematic. What do they mean? 1n910? What would be a widely available alternative?

http://pdf.datasheetcatalog.com/datasheet_pdf/bkc-international-electronics/1N695_to_1N933.pdf


And what kind of switches are we looking at? This can be much simpler? right?

[anotherjim]

I'm not sure about the PITCH knob. It doesnt make sense to me. Maybe someone can enlighten me.

The frequency of that kind of CMOS oscillator is highly dependent on supply voltage, so the voltage regulator is hacked to make it variable.
In practice, I wouldn't expect the scaling of the keyboard resistor control to be the same at say 6v as at 5v.

[Rob Strand]

Also, it says D910 on the schematic. What do they mean? 1n910? What would be a widely available alternative?

I think D910 actually is the part number.   I saw a reference to a Sharp D910 but I wasn't convinced that was it.  The weird thing is they use it on the power rails as well, so it could be power diode.   A few other references on the web imply it is a Schottky diode.

I suspect a 1N4148/1N914 type would work fine for the signal parts of the circuit.  You could try a 1N4004 to see if it sounded better/worse.  You could also try Schottky.