Bill, Yes, this is the 845A, not the AB I have. Well, I first thought that about the resistor, but sure enough, the color bands are brown, green, and brown, or 150 ohms, as on the schematic. I wonder if the resistor was mis-painted at the factory with the wrong color bands? I just couldn't see a carbon comp resistor moving from that value to almost a short, but this one sure shows it, and is why, I figure, it put off enough heat to leave a bright spot on the board above it. The zener would have to sink a bunch more current over it too, and on the side of the 400 uF cap beside it, was a slight white discolored area that was given by the zener being warm. Unhooking the zener showed it still good, and even though the 400 uF caps end has a small bulge, it doesn't show a short. I didn't disconnect the resistor from the circuit while reading it, as one side goes to the power switch, and takes it out of circuit from the transformer.
Another thing I noticed was the GA2817 switching transistors show conduction one way across the collector and emitter on both, but it isn't a short. I figure they have built in snubber diodes in them, but I can't cross the number to anything, as I think they were a custom or OEM for Fluke from TI. I unplugged them so I could test the filter cap and bridge rectifiers correctly. Does anyone know anything about these transistors? Thanks, Will *********** REPLY SEPARATOR *********** On 6/24/2011 at 2:44 PM WB6BNQ wrote: >Hi Will, > >Is it possible that someone replaced the original resistor with the wrong one ? >I am suggesting that they were either color blind or were not paying attention to >what they were doing. > >You must be talking about the version that does not have the battery option. You >will notice that the battery version does not have a zener diode. This is >because the battery is slightly trickle charged via the 150 Ohm resistor and >loads the transformer output enough that the voltage to the switching circuit >never gets high enough to be a problem (assuming no failures). > >With the battery load missing in the AC only model, there is a need to help limit >the voltage out of the bridge rectifier. The transformer and bridge output are >such that the voltage is barely above the zener value. Very little current flows >through the zener. The transformer winding resistance also comes into play as it >can barely deliver the required current and just maintain the needed voltage. In >other words the winding resistance is such, along with the switching circuit >load, as to limit the amount of current the zener sees. > >Bill....WB6BNQ > > >Will Matney wrote: > >> All, >> >> Speaking of replacing electrolytic capacitors, which I am now doing on a >> Fluke 845A, I found a carbon comp resistor way out of spec. It is a 150 >> ohm, 1/4 watt current limiter, for a 10 Vdc zener. I noticed that it had >> looked to have been hot (or gave off excessive heat), and had made a shiny >> place on the PC board above it, though the resistor doesn't show any burnt >> color. When I measured it, it was reading around 1 to 2 ohms. The zener was >> still good, luckily, as was the bridge rectifiers, and filter cap (though >> bulged), and this is the supply voltage for the switching transistors, >> which run the chopper transformer. >> >> My question is, have any of you seen a 150 ohm carbon comp resistor change >> that much in value? I wouldn't think they could, but evidently, this one >> did, and if so, I wonder about the others in the meter. >> >> Thanks, >> >> Will >> >> *********** REPLY SEPARATOR *********** >> >> On 6/24/2011 at 11:22 AM Brooke Clarke wrote: >> >> >Hi Chad: >> > >> >Google found it: >> >http://techdoc.kvindesland.no/radio/passivecomp/20061223155312558.pdf >> >But it's not that informative. >> > >> >The best info I've seen on measuring components is the HP (Agilent) >> >Impedance Handbook. >> >http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdf >> > >> >I measured dozens of caps using a number of different methods to see how >> >well the combined ESR and Capacitance meter I sell works. >> >http://www.prc68.com/I/ESRmicro.shtml >> >http://www.prc68.com/I/Capacitors.shtml >> > >> >Does anyone know of a modern cap leakage tester? >> > >> >Have Fun, >> > >> >Brooke Clarke >> >http://www.PRC68.com >> > >> > >> >GMail / AnalogAficionado wrote: >> >> Cyril Bateman called Understanding Capacitors >> > >> >_______________________________________________ >> >time-nuts mailing list -- [email protected] >> >To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> >and follow the instructions there. >> > >> >__________ Information from ESET Smart Security, version of virus >> signature database 5851 (20110206) __________ >> > >> >The message was checked by ESET Smart Security. >> > >> >http://www.eset.com >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > >__________ Information from ESET Smart Security, version of virus signature database 5851 (20110206) __________ > >The message was checked by ESET Smart Security. > >http://www.eset.com _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
