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re problems with Ohmeda-Biox 3100 pulse oximeter

Discussion in 'Electronic Repair' started by W. Curtiss Priest, Feb 10, 2007.

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  1. ***
    W. Curtiss Priest, Director, CITS
    Center for Information, Technology & Society
    466 Pleasant St., Melrose, MA 02176
    Voice: 781-662-4044

    Ohmeda Biox 3700 Pulse Oximeter Technical Note
    June 9, 2006
    Revised February 10, 2007

    Failure Modes:

    I. Probes fail
    II. "Low Signal" problems
    III.Calibration trim pot goes out of range
    IV. Power/Standby off/on failure
    V. Lead-acid battery life

    Keyworks: repairing oximeter, fixing oximeter, oximeter probes,
    oximeter batteries


    One reason "hospital quality" medical instruments are expensive
    is because such instruments must meet quality standards regarding
    safety, and be FDA approved for use. The sale/purchase of medical
    equipment requires a physician's authorization.

    In recent years there has been a gray zone about various
    medically-related devices. As a result, there are lower
    cost pulse rate devices and oximeter devices. These devices
    typically lack the "alarm features" of a pulse oximeter intended
    for medical use and lack, say, abilities to record data internally
    or, via RS-232 to an external computer.

    In general, repairs to hospital equipment should only be made
    by qualified facilities. This ensures the integrity of the
    device. However, such devices are now regularly sold on the
    used market, both on eBay and various Internet-based sellers.

    Also, in general, no hospital or treatment center would
    purchase such second-hand devices. Should a patient be
    harmed via the failure of such a device, the care center would
    likely be found negligent.

    So. The information provided here is intended only for repairs
    for the 3700 only for non critical care or use. In our belief
    that perspective patients benefit from gaining knowledge about
    their bodies, we would rather see a valuable pulse oximeter
    repaired and used, rather than discarded.

    Thus, this article contains no medical advice and the details
    are strictly informational. We believe the information to be
    reasonably accurate, but we do not assume any risks that
    the misuse of this information may entail. In summary, do
    not either purchase an unknown instrument nor modify it if
    the intended use might be injurious or life threatening. We
    describe what we have done which is appropriate for our
    environment and situation.


    It is typical for two or three problems with an electronic
    device applies to the majority of the units built and sold.

    When it was actually cost effective to repair electronics,
    the repair service depends on this because while the
    experienced repair service knows these faults and can often
    fix a unit in a few minutes. This compensates for a unit
    with an unusual failure which might occupy a full day of
    diagnosing, and which, after that, might just be returned
    to the customer as "not repairable."

    I have a DIY approach to repairs. As I do not do repairs
    for a living, I typically hold on to a piece of equipment
    for many years, and obtain the service manuals. So, if
    there is a failure and the "groups" have not discussed it
    (especially, see, and
    I must spend extra time to sleuth the problem, and, then as
    the groups have given me useful leads on some problems, I
    publish my results, a technical note, as a gift.

    Further, some people do crossword puzzles, but I like
    the "natural" puzzles of fixing something. When I had
    spare time (I wonder where that has gone to), I might
    get something at a flea market, to add to a museum collection,
    and repair the item even if I might never use it. The
    challenge of the hunt and the "solve" is what I enjoy.

    My nonprofit Center conducts sleep apnea research. We plan
    to offer a DIY "self-test" kit for people, concerned about the cost
    of a $2000 sleep study, wanting to know if it is wise to get such
    a study. So, we have monitored patients in our research and one
    valuable instrument has been the pulse oximeter. We received ours
    as a gift, in 1992.

    The overall quality of the device has been fine, considering
    that at $3000, sales are limited to the thousands. And while
    today there are other units that sell for $100-200, the Model
    3700 stores an 8 hour night of data in memory, and can be asked
    to dump the data to a PC via its RS-232 interface. As one seldom
    finds one of these instruments with its manual, for those
    interested the output "MODE" is 1200,7,O,1. To retrieve
    the internal data:

    1. Hold the SaO2 Trend 20/60 on Power ON
    2. Let the machine self test
    3. Hold the SaO2 Trend 20/60 for 3 seconds
    4. Press SaO2 a second time to start the output

    The data are the readings for every 12 seconds. To exit during
    output, press the SaO2 key again. The output looks like:

    SaO2=XXX PR=XXX YY (YY -- is an error code)

    An alternative to internal recording is to record directly
    from the serial port while in use. We have a B&W laptop PC
    that we dedicate for this. We use the shareware DOS Procomm
    program, recording RS-232 in in ASCII to a named file. In
    the morning we write the file to a 1.44 floppy.

    To chart SaO2 on 8 1/2 x 11 paper in landscape mode, we post-process
    the output file with a program we wrote. We then ask "Cricket Graph" to
    chart the data. And, we send the chart to the Laser printer.

    As there are too many data points for 11" paper, our program
    takes 9 data points and pulls out the lowest SaO2 reading during
    that period. As oxygen lows are a critical number, this proceedure
    provides easily observed charts. Also, as REM periods critically
    affect sleep, we prefer recording to the PC as the patient may
    sleep longer than 8 hours. Should anyone wish a copy of our
    program (written in C), please write the author. And, as
    I believe Cricket Graph is "oldware," you may inquire about that too.

    Difficulty of repair:

    As we have seen five failure modes, and some related failures,
    this is difficult to describe.

    For someone with little electronics construction skills, the
    information about the probe may help the user know whether to
    purchase another.

    For battery life and substitution this is fairly easy. It
    does require some carving of plastic to accomodate the
    different "form factor" of a single 6 VDC battery.

    For, the off/on repair, purchasing the relay and replacing it
    is of moderate skill. For repairing the existing one, this
    is of greater difficulty, and requires steady hands.

    For hunting down a thermal problem related to the calibration,
    this is difficult. And, while we truly believe that if "one
    does it, they all do it," we are uncertain about whether TI-
    made LM358's, with a manufacturering date of around 1992 (the
    date number "92," say, is above the chip number all develop
    a thermal failure. As NTE does not supply a substitute, finding
    another requires a Google search for providers of out-of-date
    ICs. If anyone finds the same fault, kindly e-mail .

    I. Pulse Oximeter Probes

    In the second month, under warranty, a probe failed. It was
    replaced, so I have no information about the failure mode.

    Recently the unit grew fussy, indicated low signal from the
    probe, and sometimes thought the probe was off the patient
    when it was still on the patient.

    A new probe is $250. A refurbished probe is $175. A
    refurbished probe, with exchange, is $125.

    Five years ago, the unit was a little fussy. Finding
    that a little more pressure helped, we fitted the probe
    with an O-ring located 3/4" from the pivot point. We
    lightly notched the edges of the probe to hold the ring
    (1/8" thick, 7/8" I.D.). We also used some silver colored
    hot melt glue to assure the ring didn't wander.

    This year the probe was fussier. In observing the
    quality of the signal it is important to know that the
    unit processes many readings from the probe and produces
    a delayed output about the signal quality. To understand
    why, it is useful to know how the probe senses both oxygen
    desaturation and pulse rate.

    I am indebted to various group discussions for an insight
    about this. I had thought, perhaps, the pulse rate was
    taken using, say, a "pressure sensitive" rubber. However,
    both readings are derived from just 2 LEDs on one side of
    the probe and one phototransistor on the other side.
    One LED lases red; the second lases infra-red.

    In an era of digital meters, there is still a very good
    purpose for a handheld analog meter. My treasure is an
    NCR Model 310-C. The probes of the meter have a nice
    bite to them; they are better pointed than standard probes.

    Now, in testing LEDs and phototransistors it is important
    to know that the "resistance" on an analog meter with
    a semiconductor is not simply a measure of resistance, but
    the flow of current in a circuit which applies a known
    voltage. These meters apply 1.5 VDC in the X1, X10, X100,
    X1K, position and 9 VDC (or 15 VDC) in the X10K position. On
    the NCR, the 15 VDC is applied in the X1K position.

    This voltage is important because a simple "PN" silcon
    junction does not conduct until at least .6 VDC is applied,
    as that is the "avalanche" voltage for a typical silicon
    diode. And, the actual observed resistance also depends
    on the resistance of the meter coil. So, when you "test"
    diodes such as LEDs it is good to use the same analog meter
    and, when in doubt, grab a known good device for comparison.

    So what I see with my NCR on X1 (with a 20,000 ohms per
    volt coil) is somewhat relative. However, do understand that if
    I grab a random red laser diode, I get exactly the same
    "resistance" as I do with the "red" side of the LED pair
    in the oximeter probe.

    Like all diodes, we expect no conduction when the leads
    are placed in one polarity, and we expect conduction based
    on the above (and the character of the LED junction) in
    the other polarity. So:

    1. Red LED (mounted to the left on a tiny circuit board
    in the probe):

    NCR @ x1 ohms
    4.5 K
    NCR @ x10
    22 K

    2. Any Red LED in my parts drawer

    NCR @ x1 ohms
    4.7 K
    NCR @ x10 ohms
    22 K

    3. The IR LED (mounted to the right on a tiny circuit
    board in the probe):

    NCR @ x1 ohms
    500 ohms
    NCR @ x10 ohms
    3500 ohms

    4. An IR LED in my parts drawer:
    NCR @ x1 ohms
    500 ohms
    NCR @ x10 ohms
    3300 ohms

    5. The Photo-transistor
    NCR @ x 1 ohms
    150 ohms
    NCR @ x10 ohms
    900 ohms

    6. Any typical NP junction for a silicon transistor
    NCR @ x 1 ohms
    100 ohms
    NCR @ x 10 ohms
    700 ohms

    These measurements can be taken at two places. If you carefully
    pry up the rubber on the inside of the probe with the LED --
    note -- the rubber has a steel shield that is 3/32" wide, that
    travels the perimeter of the rubber rectangle -- keep the
    shield with the rubber, you might see a lot of green. This
    green is cement used to tack things down. You can wash the
    area with alcohol and a Q-tip. You will see the tiny circuit
    board, and you will see 3 gold pads. Wires Green, Red, and
    Orange will connect to those pads. So, when the gold pads
    are cleaned, you can place the meter probes there. You can
    also do this test from the probe plug -- but -- you are
    now testing two things: the LED/sensor and the wires. As a common
    failure mode for a probe worn during sleep is a broken
    wire (wiggling, etc.), consider testing the wires separately
    from testing the components.

    The remaining component is the phototransistor which has
    a flat cable polymer connector (DuPont trade name Parlux),
    with 3 leads to the other side. One lead is not used and
    appears to be directly connected to one of the other leads.
    As the Black and uncovered wire are openly soldered to this
    cable, it is easy to measure. With my NCR, I get 60 ohms
    with leads in one polarity, and no conduction (again X1)
    in the other polarity.

    In summary, both the LEDs and the photo-transistor are
    "semiconductors." As a rule of thumb, if one observes
    conduction with meter probes in one polarity, and no
    (or little) conduction in the other direction, and when
    these differences are pronounced, the semiconductor is
    typically good and working. If all 3 components in the
    oximeter probe show good semiconductor behavior, these
    components are unlikely to be a problem. If the probe
    misbehaves, look for proper pressure against the finger,
    cloudiness on the 2 plastic windows facing the components,
    and connections/wiring. (For a few minutes you are welcome
    carefully operate the probe, at the least, with the rubber piece
    over the LEDs, removed. As the label on the probe reminds
    us, the other side is put towards the palm. So without
    the rubber cover/small O-ring/window, one's fingernail
    is pressed directly against the 2 LEDs. And while we
    expect the light levels to be higher because a somewhat
    cloudy window is now removed, the unit automatically
    recalibrates for the change in intensity.

    Note: The copper traces deposited onto polymer are
    difficult to connect to, with wire. So, one possible
    failure point is the connection from the polymer trace
    to the wire. And, the trace side of the cable is down, so
    the trace is not exposed. To test such a situation, take
    a new Exacto knife blade and, with magnifying goggles, scrape
    the top, but to the side, the trace area. Understand, the
    polymer is tough, it will be hard to penetrate. However, the
    deposited copper trace is fragile and thin. You need to scrape
    enough to suddenly see bright copper, and scrape no more. An
    alternative is to take a shirt pin and punch a hole to the
    copper -- but -- unlike using a shirt pin into insulation on
    a wire, the copper will be pierced, and the connection will
    be unsure. Another possibility is to disconnect both
    black and bare, and to rotate the parlux so that the copper
    plated side is exposed. Then we expect only a very thin
    protective cover on the trace, more easily removed.

    And, should you find the connection from the polymer trace
    to the black or shield wires to be open, you need to very
    cautiously take a very fine strand of insulated wire, around
    # 32 guage, and with a very low power, very pointed soldering
    pen, get some solder to take on the exposed trace (you may
    use an acid (HCL) flux provided you apply it with a tooth pick
    and wash the excess when done with water). Once you see, even
    a tiny patch of solder has taken, you can tack the 32 guage
    wire on, make a "service loop" (to remove strain), and then
    solder that back at the black or bare wire. As the
    photo-transistor side is a "high impedance" circuit, do cover
    and seal any intrusions. A quick dab with a colored nail
    polish (aka lacquer), will seal the area. And the lacquer
    will bond to the polymer as the lacquer solvent (acetone and
    toluene) will etch plastic. Allow the lacquer 1/2 hour to dry.

    I would never suspect a break in the polymer trace, itself.
    However, if you expose both traces, and don't see resistance to
    the photo-transistor, you must assume the connection from
    the traces on the other side of the probe has failed. I
    have looked at that side. As I understand, there are two
    styles of rubber probes -- soft and hard -- and the rubber
    cushion on my photo-transistor side is hollow. I pealed
    away some of the cushion, noted that one needs a true rubber
    cement to reattach the bottom edges of the cushion (not contact

    There is an alternative to soldering. This is also employed
    in repair of the trim pot (below). For the repair of auto
    rear windshield defoggers, NAPA, for example, sells the
    Permatex product called "Quick Grid Rear Window Defogger
    Repair Kit." It is Permatex Item# 765-1460. In a small
    bottle is .05 fl. oz. (1.4 ml) of a repair compound. This
    compound contains metallic copper in a quick drying cement.
    As defogger traces cannot be soldered to, this compound will
    reliably bridge breaks in such traces. (when traces under
    high current are thin, they become excessively hot, and
    this is a "run-away" condition -- i.e., the trace becomes
    thinner, it becomes hotter, until it open circuits)

    The same compound can be used where no flexibility is
    required. I.e., if you have a break in the probe and can
    (using, say, cement or hot melt glue) assure that both
    sides of the break will not wiggle, this compound can be
    applied for a lasting connection. Remember, both sides of
    "the bridge" must be fully conducting. I.e., if this is
    to repair a trace with any coating on it, the coating must
    be carefully scraped away, say, with an X-Acto X611 blade.
    And, always use magnifying goggles. Harbor Freight (and
    others) sell a magnificent "Magnifer head strap w/lights"
    for as little as $5. There are multiple magnifiers, allowing
    for magnification from 1.8 x's to 4.8 x's. While the
    two lights on each side of the band sound useful, they
    are totally useless and I suggest unscrewing them and throwing
    them away. They will "never" point light where you need it.

    Instead, place a 120 VAC, 50 watt, "Par 20" miniature spot
    in a gooseneck lamp. The light is particularly well focussed,
    compared with, say, a miniature flood light or a standard bulb.


    When done inspecting on the LED side I noticed a small copper
    shield over the outside of the incoming cable. However, I
    did not find that that copper nor the steel perimetry was
    ever connected to a probe wire. I presume this shielding
    is done to thwart static electricity. As I was unsure of the
    connection of the left and right sides of this, I carefully
    soldered a small piece of copper solder braid (used for wicking
    solder) to assure conduction to this aspect. As the rubber
    is still connected (mostly) and I thought leaving it that way,
    convenient, I used a solder clip on the steel to make sure heat from
    soldering didn't melt the rubber. (such clips have a claw
    area and a spring to hold them shut) Should one bother? Hey!
    Some engineer decided this should be here. Let it be. (That
    this depends on the pressure of 2 metal ends onto the copper
    cover -- heck -- I didn't design such a lame way of connecting
    them. If the original design was good, then joining the two
    halves is better.)

    As the rubber cover (notice, it includes a small O-ring to
    assure finger pressure is not applied directly to the 2
    exposed LEDs), with the steel surrounds, presses back into
    place, the only thing I see to do is to prevent intrusion
    of oils and salt from the finger, and I do that with a light
    coating of vaseline petroleum jelly at the edges where the
    steel/rubber edges meet the probe arm. If we, instead, glue it,
    we don't have easy access for next time.

    Now, all this, and I find my 3 components are healthy. But,
    the probe is no longer a mystery. As for how it measures
    oxygen and blood pressure --

    1. Blood changes color with its degree of oxygen
    saturation. The haemoglobin cells which
    transport oxygen will change the amount of
    light received by LEDs to the receiving sensor

    2. We presume the use of two LEDs, and some sophisticated
    circuitry, allows the oximeter to calibrate to
    the actual percentage of desaturation. Exactly
    how is probably a guarded secret by instrument

    3. Meanwhile, the pulse rate causes dimensional changes
    in the finger which are translated into periodic
    changes in the change in light. So we presume
    that there are circuits that look for this periodic
    change, and convert those changes into the display
    for pulse rate

    Now, it is not necessary to open the probe if all three
    components are fine and if the wires are fine. We can
    take measurements from the probe connector.

    This is a nine pin, gold pin connector, with a flat spot,
    and one labeled pin (8).

    The typical numbering, looking into the male end -- the
    end from the probe, would be as follows:

    1 9 7
    2 8 6
    3 4 5

    Official numbering may differ.

    The connections are:

    7 - green - red LED
    5 - red -- LED common (red and IR)
    9 - orange -- IR LED
    1 - black -- photo-transistor
    6 - bare -- photo-transistor

    2 - one end - 51K ohm resistor in the probe plug
    4 - other end - 51K ohm resistor in the probe plug

    So, 2 & 4 "officially" tells the unit if there is a probe
    plugged in.

    II. Low Signal Problems

    So far, as our probe appears fault-free, the only remedy
    has been the addition of some added pressure via the O-ring.

    As rubber stiffens as it ages, this compensates for 15
    years of exposure to skin oils. (Note, skin oils are the
    primary cause for vinyls and other plastics to age.)

    However, our unit got quite irratic. Instead of a steady
    heart beat, there was the sound of 2 or 3 beats. The
    unit would either say "check the probe site" or might
    say "probe off patient."

    We noted a "daughter board" (inside the unit) between the
    probe connector and one of the 3 circuit boards. Such
    boards are inserted when a design is touchy and a redesign
    is "added" to the unit. Now, all connectors from the front
    panel to the probe were gold plated. However, even gold
    contacts can become dirty. So, we decided to contact clean:

    1. the probe connector to the panel connector
    2. the probe connector inside to the daughter board
    3. the daughter board connector to the circuit board
    4. the panel connector J3 to the top board

    We used Caig's Deoxit D5 spray. (If we are dealing with
    non-gold contacts in harsh environments we use Caig's
    Deoxit to clean contacts and then Caig's Preservit to
    prevent further decay.) We spray the cleaner onto the
    female receptacles, we then slide the cleaner onto the
    male pins and shuffle the connector half a dozen times to
    assure cleaner transfer and mechanical cleaning. (Would
    the Radio Shack "contact cleaner" spray work as well?

    The results from this were quite good. Typically
    the signal strength was barely hovering at the first
    level, and we would get a fault within 30 seconds. Now.
    The signal strength gradually headed towards the top, and
    provided the probe site was kept still, will mostly
    stay at the highest level.

    This was unexpected. We saw gold contacts on all except
    for J3, and we, say in PCs, have gold to gold contacts
    on boards that last for years. But, we will not argue
    with success. (In retrospect, we note that, 1.) connectors
    to the boards are difficult to get fully seated, and those
    with "clamps" are not always easily seated where the clamp
    comes over the connector. As for probe signal stability,
    we note that the most likely cause for poor signal is when
    the probe is attached to an unsteady hand. So, in making
    tests, be extremely careful to clip the probe to a hand
    that you do not move, and, be sure to squeeze the probe
    as you place it, as it comes to rest better.)

    III. Calibration / trim pot goes out of range

    This was a problem we encountered in the first few years
    of the device's use. One presses power-on. The unit
    goes through a lengthy checkout process.

    However, there is exactly one hole underneath the unit
    for one adjustment. This is usually a situation where
    a design engineer finds no way to electronically compensate
    for a problem. As this trim pot is in an area with analog
    JFET amplifiers, we know that some analog comparator
    needs adjustment due to changes in components over time.

    The need for this is announced on the unit's LCD display
    during startup. It indicates that the adjustment is
    out of range, and asks that it be set to +/- .1

    When this happened, the value displayed was about +1.2
    Further, the trim pot was not able to bring the value
    within range. We examined the local circuit and found:

    a TL082CP (U21) NTE 858M
    IC-Linear, Dual Low Noise JFET Input OP Amp

    We suspected it, but, adding an IC socket, and substituting
    did not change the measurement. See Appendix A for how
    we remove ICs and replace with sockets, for "through the
    board" ICs.

    In this area of the board we find:

    a 10K resistor
    (next to R26, in line with screw on pot)

    and the trim pot:

    trim pot 1K (R25)

    We noted that by reducing the 10K resistor a bit, the
    trim pot brought the calibration in line.

    After another two years, the calibration drifted further,
    and, we recognized it had a thermal component to it. And,
    the drift would sometimes be to +2 or +3 and based on that
    we could tell that there is a rough correlation between
    this number and how far off the SAO2 reading is on the

    The ICs that are most important are:

    U14 -- an LF398 Monolithic Sample and Hold Circuit
    U15 -- an LM358 low power, dual op-amp
    U16 -- another LF398 Monolithic Sample and Hold Circuit
    U21 -- the TL082CP IC-Linear, Dual Low Noise JFET Input OP Amp

    Details about U14-U16 are given in the 1982 National Semiconductor
    Corporation Linear Databook. U21 is by Texas Instruments and
    is also shown in the NTE databook and web site (NTE 858M).

    A typical design that employs two "hold circuits" is for
    a slowly varying signal that has a high and and a low over,
    say, seconds. Then, the difference is taken by a comparator
    (here, U15) and that signal is sent to U21.

    What is critical to consider is that the change in light
    level at the probe's phototransistor is delayed by five-
    ten seconds, either in this circuit, or in the microprocessor.
    This means that the calibration number displayed is the
    result of measurements in this area, a number of seconds
    ago. And, in that the problem appeared thermal, we now
    need to selectively heat and cool parts of the board with
    these ICs, knowing that the sensitive area will be where
    the heat gun or arctic freeze changed the component
    temperature "a while ago." The board did respond to this
    inspection, and we finally reduced the problem to U14-U16.

    As these are ceramic packaged ICs we can put some heat
    sink compound on each, and use the soldering pencil to
    heat them and carefully drizzle arctic freeze on them
    to cool them. The IC that was thermal was U15. Indeed,
    if we cooled it enough, the op amp on leads 1, 2 and 3
    would go from drifting to full failure, and the calibration
    setting would jump to 5.4 We presume 5.4 is the upper

    Fortunately we had a spare LM358, we pulled the
    defective IC, we socketed the location, and inserted the
    other LM358. Suddenly the calibration was rock solid,
    and, we even had to remove the 10K resistor (above) to
    get calibration back to +/- .1

    Now, in constantly readjusting the trim pot (via the small
    hole near the center of the oximeter's underside) we managed
    to hurt it. And, this is a remarkably small, 10 turn
    potentiometer, so we had no spare. Above we mention one way to
    repair without soldering. This could be used here. In the case of
    this trim pot, the screw goes to a worm gear inside. That worm gear
    moves a slider on a carbon film layer deposited on a ceramic square,
    about 1/4" by 1/4." The center lead (the varying resistance
    lead) had become disconnected from a silver film that
    connects to the slider. Carefully using very pointed meter
    leads, we found the break to occur just where the silver
    film goes around the edge of the ceramic, to connect to the
    tinned copper lead that solders to the circuit board.

    It is fairly easy to expose this area, normally encased in
    the black plastic box that contains the pot. You take a
    1/16" chisle bit in a battery Dremel, on low speed, and you
    create an opening to the side of the brass screw, in the
    center, and on the side where the center lead comes up.
    Do this with magnification -- it is difficult to put back
    pieces of worm gear, or slider, or ceramic.

    We first coated the carbon film/slider with a contact cleaner,
    and then carefully washed the area where the silver film came
    to the edge of the ceramic. We carefully washing this
    location with alcohol on a toothpick, and then put a small
    drop of the copper-based cement. After drying 1/2 hour, we
    found continuity to be excellent. As this area points downward,
    we simply left the "access door" open. In any further adjusting
    of the pot, be careful not to let the jeweler screwdriver
    slide over and break this repair.

    IV. Power/Standby off/on failure

    Last year the unit simply refused to power on. We traced
    the problem to the failure for the front panel membrane
    switch to actuate a "power relay" located on the top board.

    There are two such relays. Off/on is the relay closer
    to the front. Carefully removing the top of the relay
    with a rotary cutoff wheel (parts extend all the way to
    the thin top cover, so, place the wheel in the same
    plane as the top cover), we found that a toothpick inserted,
    pressing a white plastic contact holder allowed the
    unit to power on. So, for a while, it was simply used
    that way.

    Then we diagnosed the problem in the last few days. Again,
    with no circuit diagram, we followed the circuit from the
    off/on relay (K2) to the front membrane switch. This is
    actually quite difficult to do, as there are many safety-
    related IC gates, and following the true path of the signal
    was often thwarted. However, we now know:

    1. The membrane power switch on the front panel
    connects to the center board of the stack of three
    circuit boards via a fifty pin ribbon connector

    2. Off/on is triggered by a voltage on pin 3 of this
    connector where the voltage is regularly 8.2 VDC
    and goes to zero when the membrane switch is pushed

    3. Pin 5 is also part of the power on circuit as well
    as ground provided via the three wire cables from
    the front cover to the top, power board

    4. The off/on signal trace wanders around the middle board
    and then is connected to pin 12 of J2 (the shorter
    ribbon cable that connects the middle board to the
    top board)

    5. That signal goes to pin 2 of U18 (a CD4093, quad
    NAND gate). Capacitor C35 connects here, providing
    a "de-bounce"

    6. The output of this NAND, pin 3, goes to pins 5 and
    6 of another NAND in the same IC. I.e., this
    NAND is simply used as an inverter

    7. The output on pin 4 goes to the "Set" (pin 8) on
    the second flip/flop contained in U9 (CD4013B)

    8. The output, Q2 on pin 13, goes into the Clk (pin 3)
    of the first flip/flop contained in U9

    9. The output of that flip/flop, Q1 on pin 1 goes
    to the base of transistor Q1

    10. The emitter of the transistor goes to ground, and
    the collector goes to the "low side" of the coil
    in relay K1

    11. The "high side" of the coil in K1 is provided 9 VDC
    via relay K2. K2's purpose is to sense that the
    unit is properly connected via the power cord to
    120 VAC

    The fault was "mechanical." These relays K1 and K2 are
    Omron part number G6A-234P-ST-US-DC9 And these relays are
    still widely sold by Omron. This particular design, the
    "G6A-234P" uses a remarkable white plastic slide piece that
    contains a magnet and 2 pole pieces. I.e., the magnetic
    flux of the coil is applied by a rectangular bar that
    comes out of the coil and sits inside the 2 pole pieces
    of the moving magnet. This design provides a very high
    force, and the berillium copper moving contacts can
    achieve up to 3 amps of current on the switched leads
    of this relay.

    The input power of this relay is 200 mW or 22.2 ma.
    at 9 VDC. The coil resistance is 405 ohms, and there
    is a "snubber" diode across the coil to prevent inductive
    surges from hurting the Q1 transistor. "ST" stands for
    "standard sensitivity" and the "DC9" identifies this as
    a 9 volt coil.

    The manufacture date of the relay, 1992, corresponds to
    the manufacture date of our oximeter. Now. When Omron
    terminated the varnished coil wire (an astounding 48
    gauge wire, think tiny), they used an unreliable technique
    of wrapping the end of each coil wire to the square edged
    lead wire, with just 2 turns. I.e., the wrap must be
    vigorous enough for the square edge of the lead wire to
    break through the varnish and then create a "cold weld" from
    the copper wire to the lead.

    One of these connections failed. As the relay is a production
    part, it can be purchased through various distributors. Note:
    only a few of the distributors stock the 9 VDC version as the
    5 VDC version is much more popular.

    However, it takes less time to repair this relay than to
    get another, remove the old relay, and install a new one.

    As we had already carefully removed the top of the relay,
    we only had to cut "access windows" to where the magnetic
    wire is wrapped onto the leads. Clipping the lead on
    the large electrolytic next to the relay and using a razor
    blade to separate the silicone rubber holding the cap. to
    the board, it was now easy to take a small carbide burr
    and cut two vertical slits on the sides of the plastic
    case of the relay -- finishing the cut using an Xacto
    blade -- being extremely careful not to nick the coil.

    With magnification, put a tiny amount of paint/varnish
    remover onto the two turns of coil. Do not drip the
    remover/solvent onto the coil! Now, take some flux,
    either rosin or NoKorode paste or even an acid-based Dunton's
    NoKorode, and, again with a toothpick, dab these turns.
    Now, take a very pointed solder pencil, and solder the
    leads to the pin. We do this on both sides as we don't
    know which side has failed. Now, spray the area with
    a light solvent (e.g., rosin flux cleaner) to remove anything

    Note: we inspected an G6A-234P manufactured in 2006.
    As we suspected, this fault must have been common, as
    the newer device has six turns, and, is properly soldered
    to the lead pins.

    So, we consider all such relays manufactured the "old way"
    to be truly "accidents waiting to happen."

    Otherwise, in examining this "top board" we did look at the 3
    +9 VDC regulators -- TL497AN's in a column, to the
    right (facing machine) -- each next to a toroid

    As these ICs are not listed in the NTE replacement series,
    we provide the pinout:

    1 Compensation Input
    2 Inhibit
    3 Freq. Control
    4 Substrate
    5 Ground
    6 Cathode of interior diode
    7 Anode of interior diode
    8 Emitter out
    9 No Connection
    10 Collector out
    11 Base (connects to the base of the output transistor)
    The output transistor is called the "Power Switch."
    The maximum "switch current" is 750 mA
    12 Base Drive (connects to output of interior Oscillator,
    and is the Base of the output transistor, before an
    internal resistor
    13 Current Limit Sense
    14 Vcc -- 9 VDC

    This is an early "switching voltage regulator." Curiously
    I find it in the 1st Ed. Texas Instruments "The Linear Control
    Circuits Data Book for Design Engineers (1976)," but
    by the 2nd Ed. (1980) it no longer appears. This explains
    the absence of an NTE replacement. On page 238 of the
    1st Ed. (yes, we have one) we are shown 4 sample circuits. The
    switcher could be used for "positive" step up, "positive" step down,
    "positive" to "negative," and as a positive regulator
    with "buffered output" (i.e., an external transistor to
    handle the regulator's current output). The "M" version
    operates over the 2nd highest temperature range -55 C to 125 C.
    The "C" is -65 C to 150. No letter, the operating range is
    -25 C to 85 C.

    As the circuits are non-obvious, and the actual output
    can be taken off from the diode or the emitter, the common
    element is the voltage into pin 1 (the comparator, aka,
    the "error amp." In all cases that voltage is compared to
    an internal fixed reference of 1.2 VDC.

    So for the regulators, back to front, pin 1 reads:

    #1 -- - 3.28
    #2 -- -13.85
    #3 -- + 1.21

    and these voltages are typically the output between
    two series resistors, where one resistor runs to the
    actual output voltage, and the other resistor runs to ground.

    So, following the 3 "high legs" of the dividers, we
    find the actual supply voltages:

    #1 -- - 5.0 (R36)
    #2 -- -15.15 (R40)
    #3 -- +15.24 (R42)

    We measure other supply voltages:

    Pin 14 - LM339 - +5 VDC
    Pin 14 - these regulators - 9 VDC
    Caps C1/C2 - +14.50 VDC

    Those familiar with instrumentation know that many
    circuits were designed to run with +15 V and -15 V.
    So outputs #2 and #3 are expected. Further, some
    circuits require a negative voltage with respect to
    ground, and -5 (#1) is a commonly use value, for example,
    it is still included in PC computer power supplies.

    Of course almost all TTL ICs and many CMOS ICs run
    from +5 VDC. The +9 VDC was probably chosen as
    a source for these 3 switching supplies.

    As the switching voltage outputs are all relatively
    low, we do not expect failures in the switching
    transformers as we see when switchers are used to
    create higher voltages, such as in CRTs. If
    the designer used proper "inductive surge" protection,
    and 15 years of life suggests he did, we might only
    expect failures if something shorts their outputs,
    but only if the "current limit sense" does not
    properly "crowbar" the output, protecting the IC.

    So, if one of these devices is not at the proper voltage,
    the first step would be to remove the load by cutting
    a board trace, disconnecting it. If the voltage then
    measures OK, then find the short. Also while the
    Internet is an excellent source for obsolete ICs one
    could just as easily purchase a +15/-15 supply module
    for around $10 and simply take the offending supply
    out of the circuit. And, of course, any small supply
    does 5 VDC, and if isolated, one is free to turn that
    around and make it a -5 VDC supply.

    However, do note that these supplies should be "instrument
    quality." If the supply is inserted just where the switching
    supply provides their voltages, then the further filtering
    for those voltages would remove unwanted ripple. (Do
    not use a cheap AC adapter as their voltages are unregulated
    and they would have a 60 Hz component to the output which
    the filter circuits are not designed for.)

    The capacitors to pin 3 of TL497 can range from 5 to 1000 pF
    which produces from 385 down to 10 kHz switching

    V. Lead-acid batteries

    Because the unit is built for hospital use and high
    reliability, the unit can run on internal batteries.
    And as small lead-acid batteries are constructed using,
    not plates, but screens, these deteriorate over time
    whether the unit is used, or not.

    The unit requires 4 x 1.5 VDC cells rated for 2.5 AH
    (ampere hours). When we first replaced these 4 cells
    we used originals, called "D" cells, from
    The total with shipping was around $30.

    However, as high powered hand lamps have become available in high
    volume, and these use a single sealed battered with 4 cells,
    and at a post Christmas Target sale we bought several at $5
    each, it was much cheaper to buy one of these. And, the
    rating is 4 AH. The plus and minus battery clips were
    compatible. We used a piece of galvanized steel wire to
    go from one screw-down to another and used a rotary cutoff
    wheel to remove some plastic in the cover. Total retrofit
    time, about 15 minutes.

    In closing, I notice one 3700 (with no probe) was just sold
    on eBay for $111. I see that online resellers want
    $500-$1000 for a good, working instrument. As we
    encourage people with sleep problems to have access to
    useful instruments, we hope this (detailed) note is useful.
    And, the extra security of a unit built for hospital use
    does provide a level of safety greater than consumer devices.


    Chief Engineer

    Appendix A:

    Finally, for removing ICs without damaging traces or the
    IC, I have this down (for pins through the board).

    1. One makes a rectangular loop out of #12
    guage copper wire (now a tool one keeps)

    2. One flattens one side, inside and out so
    that flattened sides go up against
    the 2 x 7 IC leads on the solder side

    3. One makes sure that the leads are straight --
    i.e., the oximeter boards has all 14 leads
    bent over -- individually take a soldering
    pencil and straighten them

    4. Now, take an IC puller adapted with a screw and
    nut to tighten the puller to the IC.

    5. Hang a 3 pound vise connected to the puller, where
    the puller is pointed towards the ground (or
    better inside a drawer)

    6. Place excess solder so that all 7 x 2 leads
    are connected by solder

    7. Take the copper wire rectangle (using its
    original insulation on a leg, as a handle)
    and work the flat sides against the
    14 leads

    8. Within 4-6 seconds, the IC and vise drop into
    the drawer with a resounding thunk

    9. Now desolder each hole, and wack the board
    onto your thigh, to remove solder

    10. Finally, clear the holes for a socket by drilling
    with a # 69 number drill

    Total time, about 10 minutes to socket an IC.
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