[time-nuts] Replacing electrolytics - any disadvantages of hightemp ones?

[paul swed]

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 at 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
> >> >
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