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

[David]

The capacitor was cool in this case because the floating supply was
current limited do to the design so the power dissipation was limited.
The transistors driving the isolation transformer were beta limited
and the output current after the transformer was on the order of 200
milliamps.  Had the capacitor been on the input side, the short
circuit current would have been an order of magnitude or two higher
and I expect it would have been hot or even caught on fire.  The only
thing that made it easy to find was that removing the post regulator
as a module left only that one capacitor in the circuit.

I actually have a far IR camera but did not get to breaking it out for
this project.  I did manage to burn two Mark I Fingers though on the
metal transistor cans.  The DM501 I fixed supports a simple delta Vbe
temperature probe so at some point I think I will build one.  All I
need is the LEMO connector.

I would really like to know though why it failed in a non-surge
related way at such a low current level.  Previously I have only seen
them fail where it could have been surge current related.

On Sat, 24 Mar 2012 06:45:12 -0700 (PDT), "J. Forster"
<jfor at quikus.com> wrote:

>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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>
>
>
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