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

J. Forster jfor at quikus.com
Sat Mar 24 13:45:12 UTC 2012


Nope.

The cap was cool because the thing was shorted and had no voltage across
it. Power is V * I.

As I said before, either an open or a short circuited component dissipates
no power.

The defective component is NOT always the hot one. A hot component is only
a pointer to the fault, not necessarily the problem itself.

This is especially true of fuses. Always ask "Why did the fuse blow??"

-John

=================


> 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.
>
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