"Dead" Luna 750C Display

[ITI160]

Hello. I am new to the e-bike world. My build is on a full suspension frame -- BBS02 with a 48V side shark custom-mounted battery on the down tube and I'm using a DPC-10 display. I finished the build in May. It has been a ton of fun riding it since.
A month later, two friends purchased kits and built BBSHD's on spare bikes because I was racing past their commercial e-bikes.
The question I have is, can a 750C display be troubleshot and fixed? One of the friend's display units failed on a ride last week. It was intermittently turning on and off. After the ride, we unplugged it to check the pins. They all looked fine. Plugging it back in, we couldn't get it to turn on. He had just purchased a new DPC-18, so he installed it and was back up and running again.
I plugged the 750C into my set up, but it wouldn't power up.
Is there a way to trouble shoot the cause and possibly fix the unit? It is 5 months old and never crashed. Or are displays done when they stop working.
Thanks for any expertise that you can provide.

[joel ruttenberg]

I am having a similar issue with my stock BBSHD display. after it turns on and runs for a few seconds it cancels out and flashes motor and battery icons. power still works, (pedal assist and throttle). If anyone knows anything I would be grateful. been working with the factory but don't have an answer yet.

[JPLabs]

The usual means of diagnosing a bad display is simply a parts swap. To figure out why the display failed, you could open it up and look for broken connections, check continuity of the wiring harness, and look for other mechanical issues which may exist. You could look for burned components, test components like diodes and capacitors, too, I suppose.

I'm not aware of any 'DIY-level' electrical diagnostics tests, beyond that. A good hardware hacker/EE type could do it. But not common knowledge, I think.

[ITI160]

JPLabs, thank you for the response. I will follow your suggestions. I don't have the skillset to go beyond some basics, but will proceed.

[JPLabs]

JPLabs, thank you for the response. I will follow your suggestions. I don't have the skillset to go beyond some basics, but will proceed.

It's surprising and encouraging, to me, how many faults which are diagnosed turn out to be bad wiring or a simple cracked conductor. I used to just toss failed electronics, but now if it's something special I look inside, and I've also found a few 'easy' failures, not requiring a EE background to find, but just eyeballs or a DVM.

So, good for you, diving it to see. If nothing else, you get to see how it's put together, and how well waterproofed it is by design, and maybe learn from that.

[Felix D'Kat]

Use a magnifying glass on the PCB, sometimes a dry solder joint can be the whole problem

[JPLabs]

JPLabs hit the nail on the head

My first problems were related to the switch on the DPC-14
I drowned it in heavy rain took it apart dried it no prob changed handlebars and ended up breaking a connection in the switch from tugging on the cable

It was the +red wire so I knew better then to reconnect a damaged power wire there would be resistance
so I cringed and snipped the wires and desoldered the board
40 watt iron with a steady hand and a big magnifying glass

working good as new and now I've got the cable the length I wanted

if you take it apart to inspect do a continuity test

Yes, using a big magnifying glass is key here. I'm over 50, and my eyeglasses are a bit old, so I can't see that stuff well enough right now. I've used various magnifiers, they are usable, but a pain. I treated myself to one of these for Christmas. Costs a bit more than fresh glasses, but took one click, not an appointment. I'll get the glasses later, this is fun for all kinds of things. Can't wait to get my next, tiny, splinter, or suspect counterfeit bill....and my knives are sharper than they've been in a while.

Amscope Boom Stereo Inspection Microscope, intended for circuit board inspection and similar, about $500. Weighs 70 lbs, sits on a table in the main house, at the ready, until my wife gets sick of it, then we'll see.....she is sick of pulling splinters, so at the moment we have a workable arrangement.

Kind of extravagant for a magnifying glass, and a little off-topic, but not crazy in terms of buying visual superpowers. I was surprised how cheap it is, considering the optics work pretty well, so thought I'd share.

[calfee20]

How is the depth of field and what is the magnification?

[JPLabs]

7 to 45x with the base optics. I added 0.5X and 2.0X Barlow lenses, so can go from 3.5 to 90x. Depth of field is very usable at the lower powers. As in, your whole finger can be in pretty sharp focus, it's about like having super-eyes. At higher powers it is less, and at 90x it's pretty thin. Coarse wood grain can be all focused at once, or a coin face, easily. But it's shallow.

DOF is a property of optical design, so would be the same for any brand scope with the same magnification, as I understand.

The properties I like, optically, that are not a given, are:
Focus is sharp enough all the way to the edges.
Full range of zoom is useful. It's not like a cheap pair of binoculars that zoom to high power, but don't work there. This is a practical range.
Color aberrations aren't objectionable. I haven't looked at test patterns or anything like that, but playing with it, I don't notice any color fringing.

It has one flaw, the linear rod is soft and gets rough, fast, making it feel awesome for 5 minutes, then cheap. Easy enough to replace it with case hardened ground steel for $30, which is the common mod for these. I'm doing that.

EDIT: Focus Distance is maybe 6" at low power, and perhaps 1/8" at max power, casually remembering. I don't like the LED ring light very well for 3D objects. I'm using a 12V adjustable halogen desk lamp with much better results, so save your money on the light option and do that. Color and texture are both horribly suppressed with the LED. The Halogen is FAR more beautiful, and you can move it around to optimize shadows.

Seriously, if I was shown 2 views, one with the LED and one with the Halogen, I'd think the Halogen was a 5x more expensive machine. If you've ever compared cheap and fine optics, it's that big a difference in contrast.

[calfee20]

I didn't know Barlow's went the other way. I thought they only increased magnification. The focal distance sounds very useful. I have visors with a loupe that are 5 to 10 X maybe and you have to be right on top of your subject.

[calfee20]

I have had a few of these for years.


I used to use them at work. No matter what I was repairing I needed them especially as I got older. All of my coworkers liked to make fun of me when I was using them but they all borrowed them in the end.

[Bruce Martin]

Wow, I have something similar. I just bought a new DPC-18 display, and I plugged it in, and WAM, nothing happened. I held the button down for 2 seconds, nothing. No matter what I have pushed, it wont turn on. I plug the programming cable in, everything works great. Plug display in, nothing. After getting in touch with support, they said it must be bent pins. They wanted a pic of the pins, I sent that in. Now nothing from them, its been a day and a half, and they wont answer my emails. I cant send it back yet, cause they haven't told me to yet. What to do!!?

[JPLabs]

A day and a half, going into a weekend, is frankly not giving them enough time. Give it a little while, they'll follow through. You should be able to spot bent pins, yourself, anyway.

I bent my pins when I first assembled my kit. I was able to straighten them, but it was really finicky. But, if you are in a hurry to get the bike going, and find bent pins, maybe you are going to try that. If so:

Disconnect the battery. There will still be voltage in the controller, though, so ALSO use something non-conductive like a toothpick, and don't let pins touch each other. If pins touch or get shorted, you will probably do damage, instantly.

If not in a hurry, just wait for support.

[ITI160]

I need some additional input. I put the switch under a magnifying glass and on one of the buss strips on the PCB, just off of the on/off switch, I can see a speck of color, like exposed copper. If it is exposed copper from the PCB, what is the best way to try and repair it? Thanks in advance.

[JPLabs]

I need some additional input. I put the switch under a magnifying glass and on one of the buss strips on the PCB, just off of the on/off switch, I can see a speck of color, like exposed copper. If it is exposed copper from the PCB, what is the best way to try and repair it? Thanks in advance.

Do you mean where the solder point for the switch is, you can see copper, like it is detached? As if the solder droplet is pulled up and or cracked?

If so, solder it again!

You can make a fine jumper to test further, by paralleling suspect connections like that one, if you can avoid screwing up and shorting something..

[calfee20]

He might be talking about a gap in the resist coating that was used in the manufacture of the PCB. I hope my nomenclature is correct.