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

Will Matney xformer at citynet.net
Fri Jun 24 22:04:20 UTC 2011


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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