Restoring a dead PDB
Ugha
Posts: 543
Hello all,
Bill Chennault was very kind and sent me a Professional Developement Board that he had
accidently let the magic smoke out of by reversing polarity on the power supply.
I'm attempting a dirty-and-cheap hack to attempt to fix it with little success so far.
At this stage, the PDB has no voltage regulator and has had both filter caps removed.
I've cleaned off all the related pads (and scorch marks) to find no solder bridges
or visible fuses between traces.
I had hoped to be ready to replace the regulator and caps, but decided to stick a meter
on the board to make sure there were no fused traces I didn't notice before. Unfortunately
I found something rather disturbing...
VDD and VSS are both connected. There is a 40ohm difference between them but I can't
figure out why.
Does anyone have any idea of likely locations for shorts like this? Or methods for tracking
down a problem like this?
The only thing I can think of is to start cutting traces until I find the fused sections...
I'd really HATE to do that so any advice is more than welcome.
So my questions are:
Is there supposed to be a connection between VDD and VSS with 40ohms resistance between them?
What are the most likely components to fail on a PBD when polarity is reversed?
What is the "safest" testing method for tracking down possible shorts?
Is it possible there is a short under the PDB's breadboard? If so, what's the best method
for removing the breadboard without damaging it or the PDB?
Please note that I'm using the Rev B version of the PDB. It's the one with the USB connector.
I've attached the datasheet for the Rev. B
PS: Related question... why on earth did Parallax stop including a USB connection on the PDB?
Bill Chennault was very kind and sent me a Professional Developement Board that he had
accidently let the magic smoke out of by reversing polarity on the power supply.
I'm attempting a dirty-and-cheap hack to attempt to fix it with little success so far.
At this stage, the PDB has no voltage regulator and has had both filter caps removed.
I've cleaned off all the related pads (and scorch marks) to find no solder bridges
or visible fuses between traces.
I had hoped to be ready to replace the regulator and caps, but decided to stick a meter
on the board to make sure there were no fused traces I didn't notice before. Unfortunately
I found something rather disturbing...
VDD and VSS are both connected. There is a 40ohm difference between them but I can't
figure out why.
Does anyone have any idea of likely locations for shorts like this? Or methods for tracking
down a problem like this?
The only thing I can think of is to start cutting traces until I find the fused sections...
I'd really HATE to do that so any advice is more than welcome.
So my questions are:
Is there supposed to be a connection between VDD and VSS with 40ohms resistance between them?
What are the most likely components to fail on a PBD when polarity is reversed?
What is the "safest" testing method for tracking down possible shorts?
Is it possible there is a short under the PDB's breadboard? If so, what's the best method
for removing the breadboard without damaging it or the PDB?
Please note that I'm using the Rev B version of the PDB. It's the one with the USB connector.
I've attached the datasheet for the Rev. B
PS: Related question... why on earth did Parallax stop including a USB connection on the PDB?
pdf
619K
Comments
Add a protection diode at the input after you add fix the board to avoid smoking it a second time.··
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························ Craig Eid
··········· Triad Research and Development
Electical Engineering Design and Consulting Services
··················· www.TriadRD.com
Ring it out, what's the 'normal' resistance.
Have you checked it both ways to see if there is a difference in the polarity of the resistance?
Jim-
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When your problem seems insurmountable, read the forum for a while.
Solve someone else's problems and yours won't seem so bad.
www.edn.com/article/CA408390.html
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
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Chris Savage
Parallax Engineering
Anyone else got a PDB feel like checking their resistance between VDD and VSS to see if this is normal?
Craig Eid: I think I'll do that [noparse]:)[/noparse]
CannibalRobotics: I checked it both ways (thanks for the suggestion, I'd have never thought of that) and its the same.
Leon: That's a cool concept... I'll look into it. At the moment I don't have the parts to attempt it though.
Chris: Thanks for the suggestions... I'll try it with a regulated 5V coming from a 7805 radioshack regulator. That's about as close as I can get to a bench supply on
my budget.
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· -- Carl, nn5i@arrl.net
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· -- Carl, nn5i@arrl.net
It is great that you took some readings before installing the new parts as a sanity check. However, if you had a short the resistance would most likely be a LOT lower than that. (less than a couple ohms I expect)
With a 5V supply and a 40 ohm load you would only be drawing about 125mA of current which sounds plausible when there are some components that are probably always powered from that 5V supply whenever the board is on.
If you want to try any other resistance loads you can just throw in the values in the ohms law calculator online:
http://www.the12volt.com/ohm/page2.asp
So far it sounds ok and you may be able to just install the new parts to wrap it up....
Robert
By the way, depending on what is still connected 40 ohms may not be an unreasonably low reading.
I have absolutely no idea what they mean, but they seem a little unusual to me.
I don't have a benchtop supply so I did what I could with what I have...
I used a BOE's power supply and ran right from the BOE's VDD to the PDB's VDD (same for VSS of course).
The strange thing is, the second I connected it everything dropped down to around 2.5 volts.
The power and pulse generator LEDs light up like I think they are supposed to, and running VDD to one of the LED bank does indeed light it up.
The motor driver won't function... but I'm assuming that its due to the low voltage.
The other test I ran was on current. I placed my meter between the BOE and the PDB and found that it's using between 1 and 0.9 amps with no load
other than the two LEDs and whatever else runs off VDD full time. That sounds quite high to me. (even though the original voltage regulator did have
quite a high amp rating).
That I could tell, nothing got warm and there was no smoke or fire... so I'm kinda stumped as to what my next step should be.
Any suggestions about what to try next, or why the voltage is cut in half or the current is so high, would be greatly appreciated.
It is the PDB designed for BS1 and BS2s plus the SX28.
I'm not sure what Bill meant. I'm assuming dead short (which makes little sense to me since this is a fully functional PDB).
kwinn: Unfortunately my budget doesn't allow such a drastic expense as to replace everything on the board. I'm going to try
popping out everything socketed on the board tomorrow and hope that'll prevent the strange voltage drop I have.
Also, almost all the components are surface mount. While I can solder most surface mount, it's a serious pain as I'm sure
you know.
To all:
I had a thought... more of a half-arsed theory. Could there be a short so bad (shown by the 1amp current flow) its overloading the
voltage regulator on the BOE in some way (resulting in the voltage drop everywhere in the circuit) without tripping its internal protections?
If so... I have no clue how.
Parallax: Any chance I could get a photo of an unpopulated PDB showing the section under the breadboard just in case the short's there
somewhere?
That's a great idea. You should start a new topic, addressed to one of the Parallaxians, and pose that very question!
--Bill
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You are what you write.
I'll offer to help though, I hate to see such a fine board go to waste. On the suggestion of another thread here quite some time ago, I bought an air-desoldering tool. It's is in fact an incredibly handy thing to have, it was a great investment. It's especially gifted at blowing SM components off of the board quickly and cleanly.
If you if you really want to repopulate it, send me the board, I'll clean it off and send it back with all of the components in a baggie. You can then ring out the board and test the components individually.
Send me a PM.
Jim-
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Signature space for rent!
Send $1 to CannibalRobotics.com.
If you have an unused toaster oven you can (carefully) remove components.
I seriously doubt there are shorts anywhere hidden. Solder doesn't just jump out on the board to create shorts. You can flood solder the edges of an IC and suck it out with some braid, and never have hidden shorts. The exception would be no leg chips like bga, qfn etc.
I have had the experience of a bad propeller where it drew so much current that the chip got hot.
I have to agree with Erik Friesen's statement about solder not jumping out on the board to create shorts. The problem is almost certainly one or more of the components. Jim may also be correct about the high current draw causing the boe bot supply to drop to 2.5V. If it is a linear regulator it will shut off when it overheats and come back on when it cools a bit. I have seen this happen as quickly as 10-12 times a second, which looks like about 2.5V to a meter.
The components that are most likely to go when the polarity is reversed are electrolytic capacitors, diodes, transistors, and IC's, which is pretty much everything on the board except the resistors.
Try the following, and test by applying power after each step. Make sure you keep track of where the components came from.
Remove all the electrolytic capacitors first. It looks like most of them are through hole in the picture of the board so it should be fairly simple.
Remove any through hole diodes and transistors next. If this does not isolate the problem it's either the toaster oven or what's below.
If this does not remove the short it may be time to start cutting the power trace to isolate the problem. To minimize the damage to the board use a spade type blade so you can cut straight across the trace and curl it up slightly rather than cutting out a section. That way you can press it back against the board and run a bead of solder across the break to repair it. Much neater than soldering jumper wires on the trace. To minimize the number of cuts do a "binary" search. Try to cut where you isolate half the circuit at once. If one half is bad, then isolate half of that section again, and continue until you find the bad component.
Good luck, Kurt
I rechecked everything this morning and there is NO voltage drop, NO 1amp current used.
The voltage is just where its supposed to be.
The current is 0.2ish amps... which sounds about right.
The resistance between VDD and VSS is still about 40 ohms. But from what you guys have
said, that's not a bad thing.
Ever had a feeling that you were totally insane? That's me right about now.
Normally I'd say I had something conductive under it or something... but I was doing it on my bed! Unless bedsheets
are suddenly conductive... the only other solution is that I'm totally wacko.
By the way, in hindsight, it might not have been best to experiment with something that has a history of exploding with
flame on my bed. Fire hazard!
Is it possible that the board started acting weird because something got hot while I was doing the other experiments
and that caused the strange readings? I felt every IC and none got hot that I could tell, but that isn't exactly a
scientific process.
Bill: Thanks for the suggestion... I think I'll go directly to Parallax tech support though, if I can't figure out whats wrong.
Wouldn't unpopulated pictures or wiring diagrams of other Parallax products be nice to have though? It sure would make
duplicating small parts of boards easier! (Which is most likely why they won't do it)
CannibalRobotics: I'll most likely take you up on that if it starts messing up again and I can raise the funds to replace everything. Thanks for the offer.
Erik Friesen: I went ahead and checked the current both ways this morning and found that it reads 0.23 to 0.24 one way, and -0.23 to -0.24 the other.
I'm guessing it has some kind of smart protection to prevent conducting the wrong polarity.
Are you sure about the lack of hidden shorts on the circuit board? I could have swore that I read somewhere that boards with large ground plain (like the PDB)
have a high danger of melting a positive trace into the ground plain when a short happens.
I've tested a lot of the features of the PDB and got some good results.
First, I rigged up a makeshift power supply out of an old radioshack
voltage regulator I had laying around... it runs right from the VIN
socket, into the regulator and into the VDD and VSS sockets. I think
that its kind of cool I didn't even have to do any soldering to get
full functionality out of the power socket and on/off switch [noparse]:)[/noparse]
Unfortunately, while I was testing stuff the motor driver overheated
and got EXTREMELY hot. I'm assuming that was the cause of my previous
insane current usage and voltage drop (looks like I'm not nuts after all!).
Luckily, the driver is socketed, so I just popped it out and everything
else seems fine. I'll replace it later and hope that it was the chip
causing the problem and not something going to the chip.
I ran the following tests:
The LEDs all work.
The 7-segment displays all work.
The buttons all work.
The dip switches all work.
The two 10k pots both work.
The pulse generator seems to work (It can blink LEDs at different speeds
so I'm assuming it works. I don't exactly know what it is for otherwise)
The on-board battery has a full charge and can light up an LED.
Reset button works.
And best of all...
SX programming/running works perfectly!
BS2 serial programming/running works perfectly!
I've yet to test the following:
BS2 Programming via USB
RS-232 socket
Audio Amplifier/Speaker
BS2p40 usage (Don't have one)
BS1 usage (Don't have one)
RTC setting/usage
My question is:
The PDB Rev B I have specifies 9volt input. Unfortunately I don't have a
wall-wart that produces 9 volts.
The Rev D specifies 6-12 volts.
I have a 12 volt wall-wart that I used for testing all the features and I
was wondering if there would be any problems using 12 instead of 9 volts?
I put a HUGE heat sink on my voltage regulator which is rated for much
higher voltage than I can produce. So that section is ok... just need to
know if there are any other reasons 12v wouldn't be ok.
My favor is testing methods for the rest of the board:
If someone has some free time and feels up to helping me test this thing...
Can you write a simple SX or BS2 program that'll help me test one of the
following features of the PDB?
RS-232 socket
Audio Amplifier/Speaker
RTC
Unfortunately, I have no experience with any of those and I'd like to
ensure that everything else is working as quickly as possible before I
start to experiment and learn new things.
Thanks in advance for your help and advice... and for all the help you
guys have already provided. It looks like I'll get a PDB after all!
Thanks for the info [noparse]:)[/noparse]
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Chris Savage
Parallax Engineering
Do you think the passives will most likely be alright? I tested all the resistors I could on the board and they seem to be the correct values... not sure how to test anything else.
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Chris Savage
Parallax Engineering
Update:
I've replaced the voltage regulator and two caps. Everything seems to work fine.
The following components are bad in some way:
The L293D chip.
The RTC seems to have something wrong with it.
The Audio Amplifier seems to have a problem, but the IC works fine so its in a passive somewhere most likely.
Untested:
The RS-232 DCE
I'm currently working on the audio amp... trying to figure out where the problem could be.
Unfortunately I don't know much about surface mount packages so I was wondering if anyone knows what the large black object between
the audio IC and the external speaker screw terminal is?
Also... Does anyone have any idea what could have failed in the audio amp besides the IC without leaving any visible sign there's a problem?
I put in a 386D I had lying around (much older version with lower tolerances but same pinout) and everything works.
I'm guessing the LM386n-1 had something wrong with its "gain" circuitry because when I use just the IC with no additional
gain, it worked fine.
Either way, that's one more thing I can cross off my "to-fix" list on the PDB!