> > I have to ask why you want to do that. Classically, those circuits > > were designed so that the low gain devices work just fine. Excessive > > gain can cause it's own problems. ?? > > I have this obsolete board from the Electricity Supply Board's Turlogh > (pronounced Turlock) Hill plant which has an intermittent fault. It has > diodes, resistors, and SST117s. I can eliminate the diodes, the > resistors, and so am left looking at the transistors.
And connections. Many old boards have been fixed by reflowing solder connections. This is especially true if they have been subjected to a lot of vibration. Any sign that the board has soaked up contaminants that might lower the resistance of the board material? > When these boards classically went intermittent, what was the issue? No > capacitors here, beyond a few pF. There's 100pF across one resistor. > The caps don't leak. I can check and exercise them with Analogue > Signature Analysis. Older semiconductors have suffered from contamination problems. I believe at "end of life" this shows up as decreasing resistance and increasing leakage across the junctions. I suppose if some of those transistor were Really low in gain, this might explain it. The oldest metal can transistors in the Motorola book on my shelf list an HFE min of 20 to 40 so I guess any one that was lower than that might be bad. -=-=-=-=-=- "http://www.dibsplace.com/design/"; -=-=-=-=-=- DIBs Electronic Design | Dave Baldwin Adapt and overcome. | Buy a book and read it. -=-=--=-=--=-=--=-=--=-=-=-=--=-=--=-=--=-=--=-=--=-=-=-=- -- Author: Dave Baldwin INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- 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).
