Amongst a recent purchase was an unassembled instrument set. I managed to sort through it and assemble it. It is however missing one small circuit board as outlined in the picture. If anyone has a damaged instrument set with this part or just the missing circuit board please contact me.
Regards Martin.

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I have a broken speedometer, I am sure the boards are working.

I am guilty (like others) of raiding a spare set of instruments for the speedometer circuit board - so's I can drive an aftermarket speedo with the OE sender.

Surely it's just an amplifier/driver circuit that's needed to do this - isn't the OE sender just a two wire inductive proximity switch?

Any electronically-inclined brains out there who can step forward and help save the world's stock of K instruments from being killed for their ivory?

I'm off to google.

With my 48 cans of beer.

(Edit, none of the links seem to work anymore on any of the old posts that might feature a suitable circuit)

Ahaaa Chris you have enlightened me, I now know why it's missing. I couldn't work out why the pod had been pulled apart. Stumpy looks like he might be willing to supply the board I am missing.
Regards Martin.

Well I raided a crashed set of instruments (my fault) for a board for my K75 Madass, but I'm reluctant to raid one of my spare, good instruments for the K75k. So (with 48 cans of beer to hand) I'm currently trying to decipher the OE circuit board - with help of a historical schematic courtesy of a Motobricker called RBM (whom I strongly suspect ot be our own robmack) in one hand:



And a circuit board in the other..

Thanks Chris, it looks like it's sorted, Stumpy has come to the rescue.  You need to manufacture the circuit and advertise them, it would save an endangered species.
Regards Martin.

I am always looking on eBay, when I see parts for sale, I try to buy them. I have two broken and one working instrument cluster, I have a full fairing that cost £15.00 although it has broken mounts, it came with a decent headlight and that alone was worth the money.

I am rapidly amassing spares after not getting anything much in the way of spare bodywork. I recently scored a complete set of 75s panels and luggage. The instrument pod was part of that purchase and although totally disassembled it was too good to scrap..
Regards Martin.

Well there's always a spare Krauser fairing in my attic if you're that desperate for bodywork...next to the other 2-3 fairings (can't remember)

It turned out that it was a lot more sensible to make this simple adaptor, than it was to try and build a circuit to amplify the signal from the OE sensor so's it could drive some aftermarket instruments.
The sensor is just a NPN inductive sensor with an M8 x 1.0 body (about six quid on Oohbay*). So, unlike the sensor that was supplied with the instruments, it doesn't need those tiny magnets gluing to the wheel to drive it - it'll work fine with the teeth on the crown wheel inside the bevel drive.

So no need to raid a good set of instruments for the K75k....so hopefully no more 'missing' speedometer boards. 

*which is about a sixth of the price of a used OE sender.

I should maybe have made the point of this a bit clearer.

The pic shows the type of commonplace aftermarket instruments that I'm on about. At the bottom of the pic from left to right is the speedometer sensor that is supplied in the box with the instruments (along with three little magnets that you're supposed to stick on the wheel spokes or something - the supplied sensor needs a magnet to actuate it) Next to it is a cheapo sensor from oohbay, the reason for using this instead of the supplied item is twofold: it is slightly different in function in that it doesn't require a magnet to actuate it - it will sense the OE toothed ring inside the bevel drive and, it's in a smaller package (M8 threaded body) which means it is small enough to shove inside the sensor hole in the bevel box. The black thing to the right of it is a homemade adaptor which kind of converts the oohbay sensor into something like the OE sensor.


On the technical side of things, the Oohbay sensor is a NPN type, which just means that it has a similar output to the type supplied in the box with the instruments. No need to worry any further (note: the 'other' type of output - PNP - doesn't work with the instruments - I've tried!) Anyway, here's a pic to show it working in the high tech testing facility aka my shed - the lathe is doing 65 kph!

Completely off-topic Chris: are those lathes any good? I've been thinking about buying one, c/w screw cutter.

They are....okay.

The variable speed is really handy and works well, ditto the electronic reverse. I inherited mine, so I don't know what it was like new, but I did have to strip and set the jib strips and slides to get it something like. Now it's fine, certainly accurate enough for everyday stuff. It'll take a reasonable cut on plastics and aloominum, but on steel it can soon struggle. 

What kind of stuff do you think you'll be making?


Edit: should have added. They come as standard with an imperial leadscrew - the metric leadscrew conversion is about forty quid.

Probably not much more than spacers, but I'm sure I'll find something else to do. Like converting all my bikes to hydraulic clutches just for something to do

I would say it’ll do or adapt a slave cylinder just fine. You can still hear the radio when it’s running too -  very important.

Well, seeing as I'd ordered a small bucketful of capacitors and transistors and...stuff, I thought I might just as well explore Plan A and come up with a diy circuit to obviate any more thefts of the OE speedometer boards (Things To Do In Lockdown When You're Bored, remember)

And...seeing as my new oscilloscope needed an outing ('new' = 40 years old, compared to its predecessor which was 50+ and recently died  )

...I set about this circuitry lark, with zero expectations of success, armed with a couple of speculative diagrams courtesy of the interweb. Turns out I was overestimating things. All it takes to get the OE speedometer sender to drive the Oohbay instruments of doom, is this:


Which costs an eye-watering thirtyone pee, or five for one pound fiftyfour pence (one to use, one to lose, and three to flick at your friends). Nothing else required (apart from a socket and a little bit of veroboard I suppose).

I set off with a breadboard full of, things, mostly randomly connected as is my style. And the thing worked (OE speed sensor clamped in the lathe, again, and a spare set of instruments rolling about the bench with the vibration) One by one I took out the various clamping resistors (who needs 'em) the capacitors (don't understand what they do anyway), and finally, the diode (probs because I've never sussed whether the little painted ring at one end means positive or...the other one). Everything carried on working, which is maybe because the instruments have a reasonable (i.e bombproof) circuit for the input.

Here's the proof. Little mister thirtyone pee is the tiny thing in the middle:


Here's the not very technical (or accurate) explanation:


The above trace is just the raw output from the OE sender - the raw voltage it generates as the toothed ring whizzes past. It's about 2 volts peak to peak, gets a bit higher if you open the taps, drops to about 1 volt peak to peak at walking pace, but mostly its about 2 volts.

The OE speedometer circuit board takes this raw signal and does (I guess) a top-quality job of smartening this up into a BMW-quality output that is good enough to go into the fine, old, analogue speedometer. You don't need this quality of work for those cheapo Oohbay instruments, so leave that circuit board alone, do you hear!

All you need is mister thirtyone pee. Here's what that little fellow can do:


The top trace is the output from the OE sender (okay the bottom bit of the wave is a little bit clipped off, but that's because it's now connected to one of thirtyone pee's legs). The bottom trace is the output from thirtyone pee. Which is a nice clean 12volts, dropping (nice and cleanly) to 0volts everytime the sender coughs. This output from thirtyone pee drives the Oohbay instruments just fine -at walking pace, at GLF, and everything inbetween. (Tested for fifteen minutes or so until the plastic change gears on the lathe melted a bit - forgot to put the leadscrew into neutral, oh well, eggs and omelettes and all that)

I did precede this with a circuit that cleaned up the OE sender output a bit (top trace shown below) but this bit of refinement wasn't necessary in the end - mister thirtyone pee can sort all that out.


The above pic also shows that inverting the input to the instruments isn't necessary either, all it needs to see is a drop from 12v to 0v somewhere along the line (hah, 'inverting the input', I mean swapping the sender output to a different one of thirtyone pee's legs, sorry)

The supply voltage for this was coming from a nearly-prehistoric power supply (on the back there's a sticker with a scribbled diagram showing how to attach the crocodile clips to the 'bolts though the neck' or something)  I'd like to bet it can teach a motorbike about messy spikes and crap. So, I'm confident that this will work on the actual bike.

LM358s... I remember them costing a bloody fortune to the point that I'd use two 741s instead.

That's the thing - there's two of the buggers in there - one just sits and watches the other one working!

I can remember the physics lesson boringly titled 'the gain of the 741', as if that was supposed to captivate us teenage twits - it didn't. I wish I could tell our teacher that forty odd years later his efforts weren't wasted.

Great find Chris. It hopefully will save OEM pod mutilation. I've ordered a set of LED's to replace the missing lights, 
I've got blue backlighting on my current pod I'm going to try red on the back up pod. Once up and running I've decided to fit it to my Brick which has a couple of slight problems, then try to sort out it's minor problems. I'm hoping that they might eventually ease the ban on motorcycle rides. They're having the  Easter police blitz on steroids trying to stop people travelling over Easter.
Regards Martin.

I have the board packed ready for sending, I just don’t have the time to queue for an hour or two to post it yet.

chris846 wrote:That's the thing - there's two of the buggers in there - one just sits and watches the other one working!

I can remember the physics lesson boringly titled 'the gain of the 741', as if that was supposed to captivate us teenage twits - it didn't. I wish I could tell our teacher that forty odd years later his efforts weren't wasted.

agree
it wasn't for at least 10 years that our poor old tech teacher (during the electrical trades course ) wise words got used by myself at least

No worries Stumpy I really appreciate it. I managed to get a very short 2Km ride in, and the tacho wouldn't go over 3000 RPM. I'm hoping it's minor but at least I've now got a backup.
Regards Martin.

Any issues with the board - I'll shortly have an unused spare that would love a proper, forever home. Happy to send it to Oz.

Thanks Chris I appreciate the offer everything's under control Stumpy has offered to send me one.
Regards Martin.

MartinW wrote:No worries Stumpy I really appreciate it. I managed to get a very short 2Km ride in, and the tacho wouldn't go over 3000 RPM. I'm hoping it's minor but at least I've now got a backup.
Regards Martin.

could be that the zenner references have become blown martin
I have found that if not sticking to the reference voltage  say 7.5 volts but feeding it with maybe 9 or 12 volts  that the meter movements get swamped and fail to register above about 1/3 to 1/2 full scale
normally the zenners go short circuit with issues occurring to them ....but if the earth reference is lost ...all sorts of things can start happening ...the earth is particularly important

I would be testing the paths on the film circuit board BEFORE proceeding any further , especially putting in something new , sometimes you only get one chance at that .

Thanks Charlie I'll have a look at that.
Regards Martin.

Pulled apart the Brick, as in removed the whole fairing including the headlight to remove the instrument pod. I had a closer look and suspected maybe the tacho needle was hitting the glass. I pulled it apart and adjusted the needle and screwed it roughly back together and lo and behold working tacho. The last time I used it, it had sat in the sun for quite awhile and I suppose it may have warped slightly. 

Anyway while I was there I took the opportunity to hook up the spare early instrument pod and check it over. Apart from not a flicker from the tacho and speedo everything else works. So I suspect that something shared between the two is at fault. I'll have to chase up a wiring diagram and have a good look. I also took the opportunity to change the back lighting from blue LED's to red LED's just for something different.
Regards Martin.

Hi

Im on the lookout for a replacement speedometer circuit board.
The long thin board.
I noticed a faint burn mark underneath capacitor 3.  10μf 6.3v electrolytic capacitor.
The copper tracks underneath are a mess.
If anyone has a replacement board, I’m happy to negotiate a price.

Regards

Gabriel

Gabriel that particular circuit board is the one that some of the café racer conversion crowd remove . Some of the aftermarket speedos require that circuit to get them to work with the BMW sensor. Somebody good with electronics may be able to replace the capacitor and repair the tracks.
Regards Martin.

Hi Martin, I picked up a 1986 k on the weekend and it needs a fuel pump and a fuel cap, vic said you may have a contact for these.
Chat soon Ian Drinky

Ian I've been running a Chinese Fuel Pump since July 2017 with no problems. Though they are no longer the flavour of the month. http://www.motobrick.com/index.php/topic,10054.msg86054.html#msg86054
Quantum US do fuel pumps and mountings kits but they are also believed to be Chinese. https://www.bing.com/aclick?  EME US also do pumps and kits. https://www.euromotoelectrics.com/ Locally you can try The BM shop at Stafford.http://www.bmshop.com.au/ or K model Beemer bits in NSW.  https://kmodelbeamerbits.com.au/
The fuel cap that I got off Vic was only the shell and I was going to use it to replace mine which had to be drilled when the lock jammed. 
Regards Martin.

Thanks Martin chat soon