Pimp Up The Volume

In the Pedal Power post I briefly outlined how the Volume Foot Controller worked. Here's a more thorough description. If I am to use it as a control voltage input I will need a special cable.

I don't like this schematic in the main Service Manual page

TRRS??

There's a better view in the Block Diagram

TRS... I think?

It's easier to see the contacts in this diagram and it's clearer that pins 2 and 7 both connect to the Tip of the TRS jack. The Ring connects to pin 4, carrying the FC signal back to the SS-30. Note that 2 and 3 are unused.

This is the original connector

TRS! And two connections to the Tip.

For the SS-30M I used a new connector, a Switchcraft 114BPCX

Switchcraft 6.35 mm Chassis Mount PCB Mount Jack Socket 114BPCX

The Tip Spring and Tip Shunt are equivalent to pins 7 and 6 on the original, with the shunt disconnecting the +15V supply from the Ring Spring when the jack in inserted.

To recap, the TRS connector is wired like this.
T - Tip - +15V
R - Ring - FC signal
S - Sleeve - Ground.

When using a foot pedal, the Tip and Sleeve are connected to either end of a variable resistor with the FC signal coming from the wiper. Thus the FC signal is swept between 0 and 15V.

Therefore, to drive this input from a control voltage only the Ring and Sleeve are needed and as the Tip is normally used to carry the signal a normal cable isn't going to work.

To use this input with a control voltage a convertor cable will be need to only connect the FC signal and ground. Perhaps a diode would good to block negative voltages too.

This cable will be a 6.35mm TRS at one end and 3.5mm TR at the other for connection to Eurorack. The RS of the TRS will be connected to the TR at the other end.

Note, I have tried this CV input by simply holding some wires on to a jack plug and it does work.
Also, when I was playing around with different Eurorack LFO inputs I noticed that the sound was similar to a recording by a band with a close relationship with the SS-30. I may come back to that and possibly solve a 40 year old mystery.

Edit

Instead of making a cable I decided to go a slightly different way. As soon as anything is plugged into the Volume Foot Control the connection between +15V and FC is broken, so it must be made again to get any sound out of the SS-30. As this connectiion is on the back panel and largely inaccessible leaving anything plugged in is going to be a problem - even more so if I've removed the +15V.

I need to be able to a) keep a cable connected b) default to the +15V connected to FC and c) occasionaly connect FC to a control voltage from elsewhere. d) still connect an actual foot controller if required.

Right now d) is low priority, but I have a plan for a,b, and c.

Firstly, this is going to be connected to the SS-30

Hosa YPP-117

As this splits the signal in two I can either connect just the FC to a control voltage or connect both together and regain the +15Vto FC link.

My plan is to run two TR-TR cables from this adaptor to the back of my patch-panel - A Behringer PX1000.
 This panel is configured such that if two cables are connected to both A and B sockets of the same port on the back and nothing is inserted on the front, they are shorted together.

This doesn't quite work though, because only inserting two connections at the front will not disconnect this pair.

Trying to use the patch panel to manage the situation will only work if I use two different ports' connections (not two connections on the same port) on the back and manually patch them together at the front. Not as neat but will work.

There's one more thing though. The +15V to FC connection is normally current limited by a 2.7K resistor.  If I bypass this what will happen?
To avoid the potential problem a special cable incorporating the resistor could be made to link the to two connections back together.
Then pulling this cable will be required to access the FC connection

i.e.
At the back:
Port X - +15V connected to rear A
Port Y - FC connected to rear A

At the front, either:
PortXA patched to PortYA to short the connection and keep Volume at a maximum - requires special cable?
OR
PORT YA patched to input from modular, or whatever source of CV

Also:
Port X' and Y's rear B sockets are both shorted to the front connectors. These should never be connected to anything. The +15V is only for looping back and FC can only take one input. I have ordered some caps to block the B sockets on the front of the patchbay and prevent any accidents.

Edit II

I receieved the splitter cable and by patching it's two ends back togethr the volume level was restored. I then set about cabling the patch bay. Unfortunately I immediately found that the +15' supply line from this circuit had gone and there was no sound.

Power Supply (for Foot Controller)

I made a guess that TR7 was the fault and, sure enough, swapping it out restored the supply.
I assume that the line was shorted to ground at some point in the wiring and that was what did for it. Quite why though is less clear. What is certain, is that I had been running the supply line back to the FC input without the 2.7K resistor I mentioned above. My best guess is that I must ensure this resistor is in line with the power-supply at all times, but also ensure that it is not shorted. I will need a special cable for this.