Wednesday, December 24, 2014

Noisy fan on my Owon DS7102V Digital Oscilloscope

A bit over two years ago I got one of those inexpensive, Chinese-made digital oscilloscopes to supplement my old, trusty (and still working!) Tektronix 465B analog scope.  While the old analog scope is still my "Go To" scope since it is much easier to use in most situations, there are quite a few instances where just being able to throw some cursors and lines on the screen to quickly measure and monitor various parameters - and being able to save the results to a USB memory stick is just "nice"!

I chose the Owon over the Rigol, the more popular scope at the time with similar ratings and for about the same price because:
  • The Owon has a 10 Meg sample memory depth, meaning that even though you might be operating at audio rates, you can still be sampling at many MHz!
  • It had a much larger screen than the then-comparable Rigol with much better resolution.
  • The other folks that reviewed it said that although there were fewer firmware updates available, they weren't really needed since there weren't any known, serious bugs.
  • They guy at Saelig offered it to me for the same price as the Rigol, including some accessories that would have cost extra with the Rigol!
So, I've been happily using this 'scope for a few years now, preferring its lower mass and bulk when taking it portable and occasionally taking advantage of its ability to store the scope image (and even the memory buffer) to a USB memory stick - or even via an ethernet connection.  Another interesting feature that has occasionally come in handy its its VGA output connector allowing the screen to be directed to a much larger monitor at the same time as the built-in monitor is active - something that I've occasionally used at Ham Club meetings to demonstrate something using an overhead video projector.
Figure 1:
Two of the four black screws to be removed.
There are two more on the other size, one deeply recessed
as well as the large, silver ground screw.
Click on the image for a larger version.

About 6 months ago I noticed something odd:  Occasionally, when I would turn it on, I would hear the fan screech.  I soon realized that this happened only when it was cold and (apparently) the lubrication in the fan was likely a bit thick.  If I let it screech for a minute, turned it off and then back on, everything would be fine - but it was getting to be increasingly annoying.

I finally decided to do something about it, knowing that it would require disassembling the unit.

First, some warnings:
  • You may be voiding any warranties that the unit may have by doing this!
  • This unit contains potentially dangerous/fatal high voltages!  You must disconnect it from the mains before servicing.
  • If you do not have a modicum of experience in working on electronic devices, do not work on one of these!  (If you own an oscilloscope in the first place I would guess that you'd know something, though!)
  • Take necessary precautions to minimize static buildup as there are static-sensitive components that could be destroyed if excess voltage is applied!
  • You risk breaking something and/or damaging yourself:  If you undertake this task you agree to accept all risks in doing so!
  • Work in a clean, well-lit area with a soft cloth on which to place the oscilloscope face-down and a container to store screws, noting the different sizes and types (and their locations!) as you remove them.
  • Figure 2:
    Location of the two tabs on the bottom of the unit.
    One must press on the white plastic to release these tabs.
    Click on the image for a larger version.
  • Your mileage may vary!

Having gotten that out of the way, let's get on with it.

Getting it apart:

Getting the back off is slightly tricky, but not bad if you know the secrets:
  • Remove the four black screws, two on each side.  These have small washers with them that you should not lose!  One of these screws is deeply recessed, so a fairly long, thin screwdriver is needed to remove it.
  • Remove the large, silver ground screw from the back of the unit.
Figure 3:
Location of the single tab on the top of the unit.
Press on the white plastic in the location of the tab
to release it.
Click on the image for a larger version.

Once you have removed the five screws from the back, the case will still not come apart so you must now press in just the right places to release the tabs as the pictures indicate.

Figure 4:
The power supply and the attaching wires.
The ground wire unplugs from the socket on the rear
cover while the mains wires unplug from the power supply.
Notice the yellow ground wire attached under the screw.
Click on the image for a larger version.
On the bottom, there are TWO tabs that must be released (figure 2) in order to separate the front from the rear of the unit.  To do this, press firmly on the white plastic front while prying apart the side closest to that tab and the two halves should come apart.  This is done again for the other tab on the bottom and for the single tab (figure 3) in the middle, on the top.

Once you have the tabs released, carefully work the rear cover past the power switch, noting that it is tethered to the power supply board with several wires.  Carefully unplug the yellow/green ground wire from the plug on the rear cover and then gently but firmly pull the AC mains connector from the main board, rocking it back and forth, prying slightly on connector's tab if necessary.

Once you have disconnected these wires, set the rear cover aside in a safe place.

Now it will be necessary to remove the power supply.
Figure 5:
The screw location for the power transistor.
This screw, along with the four in the corner, hold
the power supply board into place.
Click on the image for a larger version.
  • It is possible that this power supply has capacitors on it that are charged to dangerous/lethal voltages, even when powered off.
  • Do not touch anything on the back side of the power supply board or set it on a conductive surface!
  • Note from Figure 4 which screw has the ground wire connected to it! 
First, disconnect the other power cables that connect from the power supply board to the other boards.  Now, locate the four screws in the corners of the power supply and carefully remove them, noting that they are coarse-threaded screws that go directly into the plastic.

Once the four corner screws are removed locate the hole in the power supply board and note that there is a screw that fastens a power transistor to the metal structure for heat-sinking.  Using a (preferably) magnetic screwdriver, remove this screw, keeping track of any washer with it.  At this point you should be able to lift the power supply board free and set it aside on a non-conductive surface such as a piece of paper or cardboard.

Figure 6:
The display interface board being removed.
Note that the other two wires have been removed - plus
the orange/brown cable from the front panel display.
Click on the image for a larger version.
Now, the display/driver board must be removed - this being the blue board that was partially under the power supply board and the reason that we needed to remove the power supply in the first place.   First locate the brown/orange cable at the end of the board that goes to the main display panel and note how much of the gold-plated conductor from the cable is visible while it is plugged in to the connector!  Remember this as it will be helpful to know when you plug the cable back in!

This connector is white with a black or brown release tab.  To release this cable, this tab slides away from the board (along the cable) evenly by only a millimeter or two while still being connected to the socket, preferably using one's fingernails rather than metal tools:  You should gently work each side of the black/brown release catch until it is at the end of its travel, noting that when you push on one side, the other side will move back slightly.  Once this tab is released the cable should be easy to pull out with very slight force:  If you have to pull on it very hard at all, the tab is not properly released!
  • Be very careful with this release tab and cable as both are easily broken if excessive force is applied!  This release tab does not separate from the connector.
Once the cable for the display is disconnected, carefully unplug the two other cables connecting to the board noting that some of them have release tabs.
Figure 7:
Shield cover with BNC connector.  The large plastic
shield must be removed from the connector along with
machine screws that hold this cover into place.
Click on the image for a larger version.

At this point the four small screws holding the board in place may be removed and once this is done the board may be unplugged from the side-mounted connector that connects it to the rest of the circuitry.  This board should be set aside in a safe location, observing anti-static precautions.

Now, locate the shield portion with fan and the "Trigger" BNC connector and remove the black plastic shroud around it.  On this shield, locate the other screws that hold it in place to the stand-offs to the board below and remove those screws, noting that they are machine screws!

At this point you should now be able to remove the large metal shield with the fan attached, noting how the cable from the front panel is routed through the hole.  As you lift it, you will be able to remove the power connector for the fan from the main board by firmly pulling on it and rocking.  Be very careful for other impediments for removing this cover - just in case I forgot something in these instructions!
Figure 8:
The original fan.  I chose to re-lubricate this fan with
a PTFE-based oil, but now that I have a picture of it I
should be able to cross-reference it and order a new
fan ahead of time should is start to make noise again!
Click on the image for a new version.

You should now have the fan sitting in front of you.  As it turns out, I didn't a replacement for the particular fan that was used, but having had experience with these sleeve bearing fans before I knew that if the bearings were in reasonable shape (e.g. not "cooked") then it was perfectly reasonable to replenish the original lubricant and get more life from it.

To inspect the sleeve bearings slip a small, clean (de-greased) screwdriver under the label where the wires emerge from the fan and pry up, carefully pulling off the label.  Under this, in the center, you will see the end of the sleeve bearing and if you see "wet" oil rather than what looks like bits of tar, the sleeve bearings are likely to be in good shape:  Note that the oil may be dark and somewhat thick, but it seems that this is par for the course for what is used on these fans for some reason!

At this point I soaked up as much of the original oil as I could with a piece of paper towel and then dripped one drop of PTFE ("Teflon" tm) lubricant as a replacement, the brand being visible in figure 9.

Figure 9:
The fan and the supplementing lubricant, a PTFE-based oil.
Click on the image for a larger version.
Over the past several years I have had very good luck the above technique:  Usually, it is done to "resurrect" a fan for long enough time to get a brand new one ordered and on its way, but I have, on several occasions, forgotten that I have done this to a completely seized-up fan after cleaning out the gooey remnants of the original oil and realized, after a year or two, that it was still running fine:  Not the ideal thing to do, but good to know if you are in a pinch and don't have a new fan on-hand - or are really cheap!

Once the single drop of PTFE oil was added - be sure to completely clean up any that was spilled on the surrounding plastic using alcohol - re-attach the label if it came of "cleanly" - that is, if you were able to remove it without leaving any of its adhesive behind.  If parts of the label and/or its adhesive did come off when you removed it, use some "invisible" tape to cover the hole, but be sure to tape the original label of the fan to the outside of the metal shield where you can see it when you take off the cover so that you could, if need be, cross-reference it and get a new one!

Once the fan has been relubricated it is time to reassemble the oscilloscope, and as they say "Reassembly is the reverse of disassembly:  Please refer to the steps above - and your notes - when putting it back together.

A few points to remember:
Figure 10:
The ground stud:  Note that it must be aligned so that it is
flush with the back panel or else it will not go on!
Click on the image for a larger version.
  • The machine screws that hold the metal cover in place.
  • The plastic piece around the BNC connector.
  • When plugging in the display interface board, make sure that the connector properly aligns with the main board:  If it doesn't seem to fit, it isn't plugged in correctly!
  • The small screws that hold the display board into place.
  • The two cables that plug into the display board - one for the back light and the other from the front-panel buttons.
  • The LCD panel cable:  Be sure that you release the black/brown part of the connector before trying to insert the cable.  Once the cable is pushed in all of the way, re-seat the black/brown lock.
  • Re-install the power supply:  Put in the four corner screws first, then the screw that holds down the power transistor, remembering to install the ground wire.  Refer the the picture, above, if you need to be reminded as to where the ground wire attaches to the power supply.
  • Make sure that all cables are reconnected!
  • Reconnect the power and ground wires to/from the rear cover.
  • Making sure that no wires are pinched, place the rear cover into place, carefully working your way around the power button.
  • Note that you need to carefully align the BNC connector and ground screw stud to the back panel.  Note that the ground stud actually protrudes into the back panel and is flush with it and that the rear panel cannot be properly installed until this is done!
  • Carefully engage all three snaps.
  • Reinstall the four long, black screws with their small washers.
  • Reinstall the large, silver ground screw.
  • You are done with reassembly!

If all has gone well the fan will now be quiet.

One final recommendation:  Turn the oscilloscope on and let it run face up (so the new lubricant will work its way down into the fan) for an hour or so so that it gets warm and then shut it off, leaving it face up while it cools down.  This will allow the new PTFE-based lubricant to permeate the sleeve bearings within the fan, particularly as it cools and the lubricant withdraws back into the sintered bronze sleeve bearings themselves.

If the fan becomes noisy again (or is still noisy) you will now be able to cross-reference the original fan with a new one and know how to install it.  If you do get a new fan, it would be best to get one that is specifically designated as having ball bearings and/or from a well-known brand so that you don't have to replace it again!

For your convenience, the general specifications of the original fan - which appeared to use sleeve bearings - are as follows:
  • Size:  60x60x10mm
  • 12 Volt, 0.15 Amp
  • Noise:  27-29dBA
  • Speed:  3600-4700RPM
  • Air Flow:  11.4-16.5 CFM
Some suitable replacements using ball bearings are:

Digikey:  603-1407-ND  (Delta model number EFB0612MA)
Mouser:  664-AD0612MX-G76T-LF  (ADDA model number AD0612MX-G76(T)-LF)

Other fans may be found with sleeve bearings, etc. and any of them should work in a pinch.  Since this 'scope does not run particularly warm, picking one with somewhat less air flow would probably not be a "show stopper", particularly if it isn't used in very warm environments.

Best of luck!


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