Smooth Drawbars (H100) Question

[jdeacon]

Sorry if this has been asked before, but can anyone give me or point me at an explanation of how the smooth, make-before-break drawbars work, please?

I'm doing a VB3/MIDI controller (having failed to track down a Native Instuments B4D) using a Raspberry Pi as the brains. I have an old set of H100 smooth drawbars, but I can't for the life of me figure out how they work. I understand that the sliding contact is always connecting a pair of busbars (make before break). But every busbar is connected to every other busbar, according to my multimeter.

I think I read somewhere that what is actually connecting the busbars, on the little circuit board at the end, are one ohm resistances. Is that right? Does that mean that the busbar assembly is a "stepped potentiometer" using very low resistances? And that my multimeter just isn't capable of measuring resistances that small?

I've found a thread from someone doing a similar thing with older break-before-make busbars, but that's all so far.

[Guitarbro]

You are correct. If I remember correctly the total resistance of the thing is less than an ohm.

[indianajo]

Yes. it takes a special voltmeter to make accurate measurements under 2 ohms. DVM are so sensitive to the battery condition, I use a fixed 4.7 ohm resistor as a calibrator every time I measure a speaker. Subtract the difference. My 200kohm/volt Simpson 260 VOM could make a better approximation, but at that low resistance, a current measurement with an accompaning measurement of the source voltage usually is more accurate. R=V/I where I is current in amps.

[flatrockmobile]

Ok. Here's what I did with some DBs from an E series. There are 18 bus bars. I cut them off flush with the TOP of the little pcb on the end. Then sanded off the pcb traces. On the under side, I jumpered the first pair then soldered a 1k resister between the jumpered pair to the next bar. Repeat. You'll have a set of 8k ohm stepped pots.

I used the Midi Gadgets MDBC 24 which wants to see 10k pots.

These came out very accurate with GSI VB3.

I made 2 sets for use with my homemade controller cobbled out of a BC upper manual.

Now I need to learn how to play!!!

[jdeacon]

Thanks.

I can't see any way with the kind of circuit I have in mind to use the tiny resistances of the original setup (and can't see how that could have worked for Mr Hammond anyway), so I think I'll be desoldering and removing the original little resistance board. What puzzled me is that no-one until now mentioned doing that when converting drawbar sets to controllers. Perhaps they mostly use the non smooth kind.

flatrockmobile, did you add extra resistance between the power rails and the first or last busbar? Otherwise, with 22 (or 18) drawbars and 16 (or 8) busbars there would be an extreme configuration with a ramp pattern to the drawbar registration of forming one continuous connection from top to bottom with no resistance and, in my ciruit at least shorting the power rails.

[flatrockmobile]

Think of the 2 outside rails as the 2 outside terminals of a pot and the drawbar as the wiper. I was originally going to remove the pcb, but the way the bars hooked up through it,it provided the only stability. They will roll. I also had to make sure I had each connection ready before I touched it with the soldering iron. Think hot and quick so the plastic doesn't melt.

[jdeacon]

That was lucky then. I've just done the first bank. I used a solder sucker and then desoldering braid. I couldn't get the little circuit board to lift, but I could see the resistance strip (Nichrome I think someone said) lifting. So I left the little board in place and eventually lifted off the strip undamaged.

Going back to the short-circuit question, each drawbar wiper (smooth variety) not only connects that drawbar's output to a busbar, it can connect one busbar to the adjacent busbar because the make-before-break contact point is wide enough to touch two busbars at once. If you picture a bank of drawbars arranged just so, each busbar could become connected to its neighbour, creating a short from one outside rail to the other.

I'd still love to know how all this works in the Hammond.

[Guitarbro]

Here is the schematic LINK

[jdeacon]

Wow! Thanks. Mr Hammond was one clever chap.

My electronics isn't anywhere near good enough to say Aha! yet. But I suspect that that transformer is important. It all goes to make me even more impatient for the day when I have the money and the space to try for a real B3/C3.

[Guitarbro]

Going back to the short-circuit question, each drawbar wiper (smooth variety) not only connects that drawbar's output to a busbar, it can connect one busbar to the adjacent busbar because the make-before-break contact point is wide enough to touch two busbars at once. If you picture a bank of drawbars arranged just so, each busbar could become connected to its neighbour, creating a short from one outside rail to the other.

There is a tiny piece of resistance wire between the contacts.

I'd still love to know how all this works in the Hammond.

Depending on which tap of the resistor ladder is selected more or less current flows through the transformer.
The transformer converts the high current low voltage signal from the tone generator into a higher voltage signal for the preamp
older hammonds use a multi tap transformer for this.
This circuit is pretty much a cheaper version of the one used on earlier models.

[flatrockmobile]

Going back to the short-circuit question, each drawbar wiper (smooth variety) not only connects that drawbar's output to a busbar, it can connect one busbar to the adjacent busbar because the make-before-break contact point is wide enough to touch two busbars at once. If you picture a bank of drawbars arranged just so, each busbar could become connected to its neighbour, creating a short from one outside rail to the other.

This is why I chose to short the pair together then place a resister between the shorted pair and the next position. Electrically, it acts like the non sooth, 9 bus DBs. In VB3, you cannot get in between positions with this arrangement. As you slide the DB slowly, it will go from one position directly to the next on the screen.

On one outside rail of the assembly, I connected the 5v from the mdbc24 and ground on the other such that the further they are pushed in, the less resistance to ground, eventually shorting to ground when pushed all the way in. Works just like a series of stepped sliders or stepped rotary pots.

Before I did all this, I experimented with clip jumpers, resisters, and an ohm meter.

This would not work in regular Hammond circuitry, but it worked well with the card I purchased.