Help Fix an Invitrogen Blue Light Transilluminator

[Cory Tobin]

Hi everyone,

A non-functional blue light transilluminator was donated to my lab.  It's an Invitrogen Safe Imager Blue Light Transilluminator 2.0.  I cracked it open and found it was missing 3 surface mount resistors.  Otherwise the device seems to be in great shape.

See the images here:
http://imgur.com/a/kfkmU

If anyone has one of these transilluminators and wouldn't mind opening it up, I could use the values of the 3 missing resistors shown in the first picture.  It's really easy to open.  There's 9 screws on the back.  Once those are out it comes apart in two pieces and the top of the board is visible.

Thanks in advance if you can help me out.

-Cory

[Alexey Zaytsev]

Those resistors have never been populated in the first place. It's not a mistake, but a common practice to have "optional" components on the PCB, and decide if you actually want to put the components at the time of PCB assembly.

Sorry to say, but this is not the source of the problem.

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[Nathan McCorkle]

It doesn't appear to me that those resistors had ever been
placed/soldered in. This is quite common, for selling variable
products with the same circuit board... or for test/calibration
points, or for backup in case there is a hiccup in parts supply and an
alternative circuit needs to be enabled so products can keep shipping
regardless of supply fluctuations. These are usually called
'BOM_IGNORE' or 'DNI' (do not inventory). We do this a lot with
capacitors, because the parts we test are so new the exact
requirements aren't know (though simulations/modelling gets us close).

I'd check all the solder joints with a jewelers loupe or a microscope
if you can fit the board under it... or a magnifying glass. Definitely
check that IC near the resistors you mentioned... and the transistor
with the heatsink. Mechanically stressed components (metal leads were
not in a relaxed state when soldered) can lift off of solder pads or
cause cracks, heat gets things close to the melting point, so one bad
hot day with heavy use could have lifted a single pin or cracked some
joint because of thermal cycling and dissimilar temperature
coefficiencts.

Second easy thing to check would be the response of the capacitors...
a simple signal generator (just a square wave of half a volt will
often do for in-circuit testing) and oscilloscope can diagnose bad
caps in a second or two.
https://en.wikipedia.org/wiki/ESR_meter#Methods_of_ESR_measurement

pic of electrical setup
http://meettechniek.info/passive/cap-images/circuit-c-squarewave.gif

write-up
http://fullnet.com/~tomg/esrscope.htm

some folks get away using a multimeter instead of an oscilloscope...
but I like 'seeing' the 'truth'.




I picked this up for testing bad lab equipment power supplies,
specifically because it was battery-powered and thus would minimize
risk of damaging through some weird short-circuit with
high-voltage/high-power:
https://www.amazon.com/Seeedstudio-DSO-Nano-v3/dp/B00UJOA10G/

it did need some modification to work well (desoldered the internal
flash chip, soldered in a micro SD card, then re-programmed with the
BenF firmware which upgrades the signal trigger SIGNIFICANTLY), pics
here of hardware mods:
http://seeedstudio.com/forum/viewtopic.php?f=22&t=4678&sid=49b9dc37e79d270e93051a041406c271

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-Nathan

[Cory Tobin]

Ah rats! I was hoping it would be a quick fix.  Thanks for your help, guys.  Much appreciated.

I will go through each component and make sure all the solder joints are good.  I'll check back in later this week with any progress I've made - good or bad.

And thanks for the tip on the DSO Nano.  I may end up getting one if this transilluminator fix gets any more complicated :)

-Cory

[Cory Tobin]

I have identified the problem :)

I poked around and didn't find any components that were improperly soldered.  All the connections looked good.  So I pulled out my multimeter and found that there was no continuity across the fuse.  When I bypass the fuse with a wire the device works great.

Unfortunately it's a surface mount fuse with nothing identifying the current rating.
http://i.imgur.com/DczdSKH.jpg

It connects directly to the +24V DC input.  Is there any way to identify the current rating for the fuse without talking to the manufacturer?

-Cory

[Simon Quellen Field]

Use a multimeter to bridge the gap, and read the current.

As a rule of thumb, you can generally double the normal operating current to get a safe fuse rating. The circuitry can handle double the normal current for as long as it takes the fuse to blow.

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[Patrik D'haeseleer]

There should be a label at the back of the machine, usually near the power cord, that lists what voltage it expects, what wattage and/or amperage it draws, etc. Sometimes that label also lists the amperage of the fuse. If not, as Simon recommends, doubling the normal amperage is usually a reasonable starting point. 

Patrik 

[Jonathan Cline]

Marking might be on the bottom of the component. Remove it since it's no good anyway and flip it over.
Fuses dont blow by their own. Maybe someone in the past connected up a wrong power supply to it. Otherwise maybe it was liquid spill caused it to blow so double check that ie might be good to wash the board with iso.   Check any nearby cap's to see they dont have any visible damage.  Check nearby diode, there will be a reverse polarity protection diode in there somewhere near the connector, if someone connected a wrong supply with backwards polarity, that diode might have shorted and then caused the fuse to blow, so, replace it too (very common 1n4001 will do but make sure to connect it in as reversed in circuit).

I agree it is easy to rate the fuse based on the power supply connector or power supply rating.  Take that number convert it to amps and round up to the nearest fuse and/or add 30% to round up.  It is not super critical because it is only lighting LED's with 24v probably 2A switching supply limited so probably an automotive fuse holder + common automotive fuse would do.   You could also probably replace it with 24v lamp bulb if you wanted but that wouldnt fit in the case and it would get warm (it would be a nice hack, very retro looking).

To avoid the "oops connected wrong power supply" since you have it apart already, consider removing the power supply connector, putting on your own water resistant molex connector, and putting the matching connector on the power supply, so, some random power supply that only fits the jack while being the wrong rating or wrong polarity, isnt accidentally connected.   If that seems like too much trouble then at least get happy with the label maker, label the connector and the plug on the power supply and the supply itself etc, it is pretty much "all incoming equip gets the label maker treatment no matter what" in my labs.


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[Cory Tobin]

Thanks for your help everyone!  I measured the current across the fuse while running at full brightness as 210mA.  I also noticed that next to the fuse was a spot to solder in a cartridge fuse holder and bypass the surface mount fuse.  I got a cartridge holder from Digikey (part# F6094-ND) as well as a 400mA fuse.  Soldered it in and removed the old surface mount fuse.
http://i.imgur.com/DAFgL3y.jpg

"F1" is where the old fuse was.

Everything works great now.
http://i.imgur.com/JkbM4Uh.jpg

Time to go stain some gels with SYBR Safe :)

-cory