Sunday, July 14, 2013

A-110 Tuning Knob Modification

Triggered by a photo by Scott Rogers that popped up on Flickr a few weeks ago, I decided to re-create the A-110 Oscillator modification that he made.
Scott simply added a resistor in parallel with R5 to increase the tune knob range to a bit more than an octave up and down.

Locating R5 on the A-110 VCO
On Scott's blog he writes:
" I’ve already made my first modification to my Doepfer A-110 Oscillators.
For some reason they have set the Tune knob range to only a single tone up or down. 
One thing I do a lot is offset the oscillators by a fifth so this wasn't going to do!

So I contacted Doepfer and they told me (more or less) what to do.
So what I did was this: located resister R5 on the circuit board and added a second resister in parallel with it to reduce it’s value.

Locating R5 on the A-110 VCO
on the other side
I added a 100K resister and this now gives me a range of an octave and a third up and a bit more than an octave down. 
So now it’s a bit more difficult to fine tune, but now I have a huge tuning range that lets me do a lot more! "

The modification is also described in the A-110 service manual on the Doepfer website:
" From the factory the range is about 2 semitones to enable a fine tuning of the VCO.
If you are a bit familiar with soldering you may modify the A-110: shorten R9 (this increase the range about by a factor 3) or reduce the value of R5 (1M).
You may solder e.g. a second resistor in parallel to the existing R5 (1M). The smaller the value of the resistor the higher the range."

A 100K, 5% resistor in paralel with R5
Re-creating this modification was not difficult, after locating the R5 resistor, soldering the 100K, 5% resistor in parallel to it was a piece of cake.

I don't have any test-equipment, but after listening to the result, it changed the range of the tune knob on my A-110 from around 3 or 4 semitones up/down to a little bit over 1 octave /around 15 semitones, so the results seem to vary a little bit per A-110... which probably has to do with different internal potmeter settings on the PCB.

Thanks to Nicholas Keller for additional support, info and links!

The mods described on this site will most likely void any warranty and, if not done carefully, can damage the circuit board, IC chips, and faceplates.