[time-nuts] Replacing electrolytics - any disadvantages of hightemp ones?
wb6bnq at cox.net
Fri Jun 24 21:44:22 UTC 2011
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.
Will Matney wrote:
> 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.
> *********** REPLY SEPARATOR ***********
> On 6/24/2011 at 11:22 AM Brooke Clarke wrote:
> >Hi Chad:
> >Google found it:
> >But it's not that informative.
> >The best info I've seen on measuring components is the HP (Agilent)
> >Impedance Handbook.
> >I measured dozens of caps using a number of different methods to see how
> >well the combined ESR and Capacitance meter I sell works.
> >Does anyone know of a modern cap leakage tester?
> >Have Fun,
> >Brooke Clarke
> >GMail / AnalogAficionado wrote:
> >> Cyril Bateman called Understanding Capacitors
> >time-nuts mailing list -- time-nuts at febo.com
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