Declan, you can hardly call that a technique at all, or do you ? Smile, but still, I had a lot of fun reading, you vocabulary and expressions always make me laugh, it's quite entertaining. Greetings Friedel
At 08:44 a.m. 13/01/03 -0800, you wrote:
Well, I have the board sorted. I left it lie over the pagan holidays, and approached it again in the new year. A number of things didn't survive the hibernation, and I gave it overvoltage which took out a number more. Bulk replacements followed, and I was left with these sort of problems again. This board had 40 class B output stages from a floating 1.0V supply with fets, and was running off batteries. It transpired that all faulty cells could be eliminated with these two methods. 1. Monitoring the 1.0V supply & the supply current on no load, and switching every stage on one at a time. Either the voltage or supply current would move on a dud fet pair 2. Checking the voltage on the drivers. Some would go high perfectly and not go low; Others might go low perfectly and not high; A few did neither. I ended up swapping a lot of fets, and a couple of driver latches. One driver latch took 10 mA extra when one particular stage was switched high, although the output stage was perfect. I imagine that was the original complaint, but can't prove it. After I replaced the fets twice or three times, I finally went for the chip. -- Regards, Declan Moriarty. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ On Friday 27 December 2002 18:54, somebody wrote > The fact that "some work and some don't" would support an oscillation > or RFI problem. > > Your comments reminded me a problem we had years ago with a video > crosspoint switch. Small DMOS FETs were used as switches. They were > packaged in small metal (TO-18) cans. The boards built in engineering > worked fine but the units done by the assembly department were > completely dead. The FETs were shipped with small conductive rubber > "O" rings around the leads to protect the FETs from static. The > assembler thought they were mounting pads and left them attached. > When the FET was pushed down against the board, the "O" rings were > not apparent. I'd hate to tell you how many engineering hours it took > to find this very simple problem. > > The lesson here is when you hear hoof beats, check for horses before > you go looking for zebras. > > Fred Townsend > > Declan Moriarty wrote: > > On Wednesday 25 December 2002 06:08, somebody wrote > > > > > Two thoughts come to mind. > > > > > > 1) Might the circuit somehow be oscillating? I have seen > > > circuits of this kind that would sing. The problem went > > > unnoticed because applying a meter or scope probe added enough > > > capacitance to kill the oscillation so everything looked normal. > > > The fix was to apply a nf or so of capacitance from the gates to > > > ground. > > > > These fets are directly driven by latches. There is smoothing in > > abundance > > > > > 2) Could this be a RFI issue? Are there strong fields around, > > > such as a broadcast station in the neighborhood, that the circuit > > > is picking up and causing the FETs to turn on? > > > > I hardly think so. 2 of these work, this one works but loses 10mA. > > > > > Fred Townsend > > > > > > Declan Moriarty wrote: > > > > Here's one to tease yourselves over. I have a board to > > > > fix and for once, I do not know how to go about it. > > > > > > > > I designed the thing. It's battery powered, and is consuming > > > > batteries. There is a 1.0V line with nearly 40 class B fet > > > > stages, using logic level fets (Irlml6401 p-channel on top and > > > > irlml 2502 n-channel underneath) The gates are tied; > > > > > > > > That stage is pulling 10 mA on no load. It should draw > > > > basically nothing. It draws 10 mA even with the load > > > > disconnected. The 'Off' condition has -1.5V on the gates; the p > > > > channel fet sees the voltage from +1.0 to -1.5V and is fully > > > > on, and the n channel reverse biased. 'On' it is the reverse - > > > > the p channel is reverse biased, and the n channel sees +2.5V. > > > > > > > > Now the tracks are too thick to use a millivolt meter and trace > > > > where the 10mA is going. 10 mA doesn't show - I had difficulty > > > > tracing 100mA. How do I find the $*�"! current leak?? > > > > > > > > I had one thing - a fet test program which applies logic 0 to > > > > all gates and will drive one at a time high, as I press > > > > buttons. I couldn't find any gate leakage. The 5V will usually > > > > leak onto the 1.0V if a gate is gone, and I can find the fet > > > > pair that way. I will go around the fet pairs with a voltmeter > > > > and check the values, but expect to find nothing. -- Author: Declan Moriarty INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
-- Author: Friedel Bruening INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
