Hand-held dopplers are delicate instruments in a high-paced environment, and it is a commonplace to have these devices in the repair department due to some form of damage or failure.

The key to troubleshooting a broken doppler is to isolate the problem to the components affected, and this is more simple than you might think.  A doppler is made up of only a few components, many of which are replaceable as a complete assembly.

MAIN UNIT: In recent years, most manufacturers have been restricted to only supplying parts that do not involve exposing the PC board, which means that aside from the batteries, the main unit of the doppler can be considered an assembly.  Inside the main unit, there is a PC board (usually a single PC board with all components printed/soldered together), a speaker, casings and batteries.

POWER SOURCE: Most dopplers will run off of alkaline batties and get approximately 500 assessments out of one battery; however, some models use AC adaptors and/or charging stands with a NiMH battery in the main unit.  Batteries are a source of many problems with dopplers, and rechargeable models generally run better with OEM batteries than aftermarket ones.

COILED CABLE: The cable between the main unit and the Probe will usually have a connector on both ends; however, some dopplers are hardwired to the main unit and/or the Probe.  It is a lot easier to isolate a problem with a removable coiled cable.

PROBE: A doppler probe is made up of a small PC Board, a Crystal assembly and a casing that will include the connection of the Probe to the PC board.  Crystals are the fragile part of the doppler that will break when the doppler is dropped or struck.  Like the Main Unit, the Probes are mostly sold as complete assemblies because the PC board would be exposed with a repair. It is infrequent to replace a probe PC board unless a non-waterproof probe has been submerged in fluids, in which case, you are likely going to be replacing the complete Probe. 
Now that we have identified the main components of the doppler, we can isolate problems quickly to one of these components and begin the troubleshooting.

If there is no indication of any power or sound, you can isolate the issue to either the Power Source or the PC board of the Main Unit.  The Cable and Probe will not affect the main unit while powering up.  Dopplers tend to be sensitive to voltage and amperage, and some models will shut down or not start up due to the voltage being below the threshold.  Even if your battery is brand new, do a quick voltage and amperage test on the battery to make sure that the output is as required.  It is important to note that some rechargeable 9V batteries run at 7.2V, and this is often close to the threshold where the doppler will power off or not power on.
If your battery has the specified power by your doppler, it will likely be that your main unit PC board (includes power on/off switch) has been damaged or has failed.  Most manufacturers no longer sell PC boards, so replacing the main unit or sending it to the factory for service will be your best option.
If a battery has exploded in the battery compartment, the terminals should be cleaned with isopropyl alcohol to ensure you are getting a solid contact.  It is risky to put alkaline batties in any doppler with an open charging port as the alkaline battery has a high chance of leaking or overheating.

If the doppler turns on, the Power Source is good.  At this stage, it could be one of three components causing this issue to occur, and it needs to be isolated.  If you have an additional probe that you can exchange it with, this will eliminate the Probe.  The most common issue with the Probe is a cracked crystal, and you can test this by putting thumb pressure (similar to opening a can of pop) on the probe tip, which will separate most cracks and create a squelch as the crystal is pressed.  (This doesn't work if the crystal has been completely smashed or the wire connections inside the Probe have been compromised.)  Some models of dopplers have probes that come apart relatively easily, and if it is outside of warranty, you can open the casing to see if any wires have disconnected from the solder points on the PC.
The coiled cable occasionally will have a broken wire and need to be replaced.  A simple continuity test from either end of the cable will identify if the cable has been compromised.
The main unit will come down to the speaker, and the simplest way to test this is to bypass the speaker by plugging in a set of headphones into the 3.5 mm jack if the doppler has one.  The headphone jack is usually wired so that you will hear the sound even if the speaker is not working.  Speakers are connected to the PC board and generally not available by OEM part supply but are easily sourced for any doppler not under warranty.

The doppler turns on, and you have sound, but there is a weird noise that shouldn't be there.  The sound will either be squelching from the Probe or static from a bad connection or bad speaker.  Using headphones, plug into the main unit and see if the sound goes away.  If it does, it is likely the speaker that needs to be replaced.  If the sound continues, move on to the cable.  If you can replace the cable easily, that is the fast test but if not, check the continuity of the cable.  After that, it is down to the Probe, which is the worst offender.  A cracked crystal will usually sound more like squelch but can also create a static noise.  By unplugging the Probe, does the static go away?  Most probes are no longer serviceable and will have a repair/exchange option available for them.

Ensure that the correct Probe for the application is used.  The probes are limited to read BPM/FHR to the range that they are intended, and as Fetal Heart Rate and Beats Per Minute are in different ranges, the display may not show the reading.
If you are using the correct Probe, but it has cracked crystals, it may not send the numeric reading, even when you can hear it.
If the display is fading in and out or not operating regardless, the display may have failed.  The LCD is usually part of the PC board; these are repaired at the factory only for most makes.
This is the basics for troubleshooting any doppler, and we would love to hear your questions or comments below.  Our goal is to assist the Bio-Meds with the servicing of dopplers to the best of our ability!

Back to blog