[Test-Equipment] need some help

[Brian Harris]

Howard,

Thanks for the tip.  After turning it back on it seems to work fine so I'll have to check those capacitors.  

Brian Harris, WA5UEK
cell 214-763-5977
email cosmophone at yahoo.com

--------------------------------------------
On Mon, 10/28/13, Howard Weeks <weeksh at att.net> wrote:

 Subject: Re: [Test-Equipment] need some help
 To: "Discussion of Electronic Test Equipment" <test-equipment at mailman.qth.net>
 Date: Monday, October 28, 2013, 2:34 PM
 
 I have one that blew smoke when I
 first plugged it in.
 
 Investigation revealed that it was one or both of the AC
 bypass caps 
 across the AC line.  Plugged it back in and turned it
 on.  Worked fine
 and has continued to do so for years.  Didn't replace
 the caps.
 
 Howard K5JCP
 
 On 10/19/2013 8:05 PM, Brian Harris wrote:
 > I have an HP4261A LCR Meter however I have no manual
 for it.
 >
 > It seems to work correctly in that, when I measured a
 precision 1000 Ohm resistor, the display agreed exactly with
 the resistor value and, when I measured a 0.047uF silver
 mica capacitor, it read 0.0484uF.  The problem came
 when I tried to measure a 2.5mH RF choke.  While it
 read 2.4mH, it continually showed 'out of range'.  I
 manipulated the controls trying to get the out of range
 indication to disappear but it didn't.  After about
 five minutes the 1/2 Amp slow blow fuse blew and there was
 more than a wisp of smoke that came out near the front of
 the panel.  I pulled the cover off quickly in an
 attempt to see where the smoke was coming from but by the
 time I got it open I couldn't tell.  I pulled all the
 boards out of it including the display board and switch
 panel board but could find nothing that looked like it was
 hot except a white ceramic integrated circuit on the first
 board behind the board that drives the display board. 
 The metal lid which
 >   seals the ceramic packaged chip looked
 like it had gotten hot but then it could just be
 oxidizaton.  After spending 30 years in the
 semiconductor industry I would think if this chip got hot
 enough to discolor the metal it probably would be toast
 however, when I replaced the fuse and turned the meter on
 again (without having the RF choke attached), there was no
 more smoke and the new fuse didn't blow although I didn't
 leave the meter on more than a minute or so. 
 Note:  I didn't try to measure any other
 components.  When the fuse blew the controls were in
 the positions below:
 >
 > 1V drive
 > L
 > Auto
 > 1 KHz
 > DC Bias 0V
 >
 > Also, the DC bias jumpers were connected to the
 measurement terminals.  I don't know if these are
 supposed to be connected for measuring an inductor but since
 I had the DC bias at 0V I didn't figure it would make any
 difference.  If anyone can shed some light on my
 problem please do.  I need to try to find an electronic
 copy of the user and service manuals.
 >
 > Thank you,
 >
 > Brian Harris, WA5UEK
 > cell 214-763-5977
 > email cosmophone at yahoo.com
 >
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