Bike Computer PCB Repair

[srracer]

Hi folks,

A quick background so you know what skill level you're all dealing with - I'm a mechanical systems engineer that works in a major semiconductor Tester manufacturer.. so I know the basics of what components do and some simple DC circuits. I'm slightly better than a novice with a soldering iron, but perhaps not much. I've built some of my own circuit boards for a couple of hobby projects but all real basic (no reverse voltage protection stuff in my designs!). I do have access to some nifty tools at work, though.

I have a PCB from an SRM Powercontrol V. This board takes in signals from a power meter and incorporates them with other standard bicycle computer data (heart rate monitor, speed, etc).

The PCB will not power up. I purchased it as part of a bike purchase and the previous owner said it never worked for him (he bought it used) and believed that all that was needed was the battery needed to be replaced. When I asked him if he had the charger - he said "what charger?". The unit looked to be in *such* new condition, that I wondered if perhaps the only thing wrong was that it needed to be charged. So I bought the plug I needed and with some research of others that have used this unit, learned the wiring for the charger connection.

I connected the charger and.....nothing happened. This is when I made some seemingly bad decisions...

I thought that the unit would power up with the charger connected... after more research later, it appears that if the battery is fully gone, then even the charger connection will not power up the unit. But I made the mistake of attaching the 12v to a couple of the other connector leads to see if any voltage showed up at the battery... I got nothing up there (hopefully I didn't fry any of the sensor inputs in doing so).

Then I thought I could connect the voltage directly to the battery leads to see if I could get it to power up.. Unfortunately, my adjustable wall wart only had 3v, 4.5v and then 6v... I thought the 3v would be too low and for some reason the 4.5v setting wasn't working so I tried 6v.. and still got nothing.

So then I put it away and waited for a new battery to come.. When I soldered in the new battery I still didn't get it to turn on.. so then I measured the voltage at the battery leads while it was soldered in place and it was showing <2.5v.. and the battery was getting warm.... hmm.. so I unsoldered the battery and started poking around with the DMM.. It appeared that the battery leads were shorted together - or nearly so.

Unfortunatlely, I couldn't remove the board without desoldering (or so I thought) the Binder 719 connector in the bottom. I ended up gettting the board out, but destroyed the binder connector and one of the through hole solder rings in the process.

Once I had it off I started trying to test the diodes. Using the diode setting on the DMM (a cheap one) I did notice that the diode across the battery terminals (as an aside - what is that doing? I thought reverse voltage protection would be in series with the load?) appeared to be shorted in both directions (with the battery disconnected). I suspect that's my problem.

1st question - Is that diode my likely problem - or at least the first thing I should fix?

2nd question - If so, how do I tell what diode to buy to replace it? There don't appear to be any markings on it...

3rd question - If I get that in there, what things would I want to check next to make sure I didn't blow anything else up?

A full resolution image of the scans of the pcb can be seen, and even commented on, here:

https://docs.google.com/file/d/0BxiygIaWX7bIYjRLelN4MFRYMEk/edit?usp=sharing

Thanks for any and all help!!!
Chris

 

[srracer]

Well, I'm happy to report that I removed the diode and connected it to a power supply and it turned on! So my diagnosis in #1 was at least correct...

Anybody have any thoughts on how I would go about selecting a suitable replacement? It measures about .140" long if that's helpful.

Thanks!
Chris

 

[davenn]

look for a SMD version of a 1N4001 or similar
your circuit will work without it, It just wont have any power polarity reversal protection

some one has in the past obviously reversed the power to it

Dave

 

[dh390]

If there is space you could just use a 1N4001 or maybe a 1N914 (though my first choice would would be the 1N4001) just bend & trim the leads so they won't short on other solder joins or components & will fit back in the case. Solder it in just like the SMT diode aligning the bans. I have used through hole components several times in place of SMT parts on circuit boards. As long as they can be made to fit & not short out on other things it is no problem. If needed you could put tubing over the leads or place electrical tape over the board to prevent shorting.

 

[CDRIVE]

Just a couple of notes here. There are two types of diode protection configurations. One is the series type that you mentioned but don't have. The other, the type you have, is the shunt configuration. The Cathode (-) is connected to the positive (+) trace and the Anode (+) is connected to circuit (-) Ground. I find it very odd that shunt protection was used directly across a battery without there being a fused link between the battery and the protection diode.

Last note.. I would not use a 1N914 as it may burn open if the bat polarity is inadvertently reversed. The 1N400X series can handle much higher currents and have, in my experience, always shorted when they failed from prolonged over current. This is more preferable than burning open and exposing the entire circuit to a reverse voltage. When that happens it's transformed into a paper weight.

Chris