I use a HP4350a and they can be found at hamfests. Maybe epay. On Fri, Jun 24, 2011 at 6:04 PM, Will Matney <xformer@citynet.net> wrote: > 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 -- time-nuts@febo.com > >> >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 -- time-nuts@febo.com > >> 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 -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. >