[time-nuts] Anyone familiar with SR-620 repair?

[David]

I just tracked down a shorted tantalum in a Tektronix DM501
multimeter.  It was on the output of the floating -12 volt supply
bridge rectifier before the regulator.  The current level was so low
that it never heated up although I burned two fingers on the push-pull
output transistors for the floating supply.  The regulator is on a
separate module but the supply was still shorted when I pulled it and
the bad tantalum was the only part left.

I have not seen a shorted tantalum before where it could not be surge
current related until now.

On Sat, 24 Mar 2012 00:08:12 -0400, Peter Gottlieb <nerd at verizon.net>
wrote:

>I had a HP 3326 which had a power supply in foldback. All the modules are inaccessible unless you have a rather rare set of extenders anyway. The voltmeter method quickly led me to the board and a bench supply and meter again to the shorted cap. Very easy. Other times I've borrowed the FLIR camera from work, also taught the new EEs that trick as well.  It is a true lifesaver on dense surface mount boards. I haven't tried the liquid crystal sheet but it seems like an interesting idea so long as everything is about the same height. 
>
>
>Peter
>
>On Mar 23, 2012, at 11:53 PM, lists at lazygranch.com wrote:
>
>> Prior to emission or IR microscope technology, liquid crystals was how you found hotspots on ICs. I've done this with a goop that you dispense with a syringe. 
>> 
>> One trick to make this more sensitive is you bring a soldering iron close to the  liquid crystals. Not so close as to cause a change, but you get them closer to the phase change point. 
>> 
>> 
>> -----Original Message-----
>> From: Skip Withrow <skip.withrow at gmail.com>
>> Sender: time-nuts-bounces at febo.com
>> Date: Fri, 23 Mar 2012 21:07:45 
>> To: <time-nuts at febo.com>
>> Reply-To: swithrow at alum.mit.edu,
>>    Discussion of precise time and frequency measurement
>>    <time-nuts at febo.com>
>> Subject: [time-nuts] Anyone familiar with SR-620 repair?
>> 
>> You don't need expensive test equipment to find this kind of problem.  What
>> I use is a sheet of liquid crystal film with a transition temperature just
>> slightly above your room temperature.  Just lay it on the circuit board and
>> you can find where the power is being dissipated (even if pretty small) by
>> watching the colors change.
>> 
>> I think Omega Engineering sells a 8.5" x 11" sheet for about $18 if memory
>> serves me.  I have used this trick many times and it works great to find
>> shorted (bypass) caps.  No disconnecting anything, no milliohm meters, no 4
>> or 5 digit voltmeters.