The platform: how the 170 Series measures, what is inside and how it connects
Identify which 170 Series model is on the bench, explain how it derives fetal heart rate and uterine activity, and name the assemblies, connectors and interfaces that carry each signal from the patient to the strip chart.
Four models, one chassis
The Corometrics 170 Series is a family of four bedside fetal monitors built on the same enclosure, recorder and power concept. The model number tells you what the front panel can do, and you should read it before anything else, because a fault report like 'no FECG' means nothing on a monitor that never had FECG.
The 171 and 172 are antepartum monitors: the 171 has one ultrasound channel and external uterine activity, the 172 has two ultrasound channels for twins plus external uterine activity. The 173 and 174 are intrapartum monitors. Both add direct fetal ECG through a spiral electrode and internal uterine pressure through a catheter or strain gauge. The 173 has a dedicated FECG socket and one ultrasound channel; the 174 has a combined socket that takes either an ultrasound transducer or the FECG legplate cable, plus a second ultrasound socket, so it can monitor twins as US/US or FECG/US. Fetal movement detection is a purchased option on any model and may or may not be installed.
Two things are absent, and both matter for service. There is no screen: the monitor shows its values on seven-segment LED displays, and the strip chart is the clinical record. There are no maternal parameters on board: maternal blood pressure or fetal oxygen saturation appear on the chart only when an external monitor sends them through a serial port. If you came from multi-parameter monitors such as the BSM-6000 or IntelliVue MX courses, forget menus, modules and networks; this is a dedicated instrument whose output you judge on paper.
Fetal heart rate by pulsed Doppler ultrasound
The ultrasound channel sends short bursts of a carrier of about 1.15 MHz into the maternal abdomen at a repetition rate of 2 kHz and listens between bursts. Echoes from moving structures, mainly the fetal heart walls and valves, come back shifted in frequency. The receiver mixes the echo with the transmit frequency, so anything stationary cancels and only the Doppler difference frequency survives. That signal goes two ways: to the loudspeaker as the familiar Doppler sound, and to an envelope detector whose output the processor analyses for the repeating cardiac pattern.
The rate is not counted from single peaks. The monitor uses autocorrelation: it compares the current stretch of envelope with earlier stretches and finds the interval at which the pattern repeats best. This is why ultrasound FHR is smoother than ECG-derived FHR and why it is relatively robust against single noise spikes, but it also explains two classic clinical artefacts you will be asked about: the monitor can lock onto the maternal pulse in a large vessel, and it can double or halve the rate when the pattern is ambiguous. Neither is a hardware fault, and no repair fixes it; repositioning the transducer and confirming maternal pulse is the clinical answer.
The transducer itself contains nine piezoelectric crystals, eight around the edge and one in the centre, giving a wide beam so that the heart stays in the field when the fetus moves. On dual ultrasound models the two channels share one transmitter and receiver and take turns, switched by PIN diodes; that is why a second transducer that is merely plugged in can still affect the first channel if its plug or cable is faulty.
Direct FECG, toco and intrauterine pressure
On the 173 and 174 the fetal ECG is picked up by a spiral scalp electrode attached to the presenting part, with a reference contact against the mother's thigh through the legplate. The monitor detects each R wave and computes the rate from every R-to-R interval, so the trace is beat-to-beat and shows true short-term variability. The amplifier must handle signals from a few tens of microvolts to a few millivolts; an optional artefact elimination setting rejects beats that jump by more than a set amount from the previous ones.
External uterine activity is measured with a tocotransducer: a small pressure-sensing button on a strain gauge bridge, strapped over the uterine fundus. The bridge is excited with a fixed DC voltage from the monitor and returns a millivolt-level differential signal. The reading is in relative units from 0 to 100, because the force on the button depends on belt tension and maternal tissue as much as on uterine tone; the UA reference button simply sets the present reading to a chosen baseline. Internal uterine pressure uses a strain gauge or an intrauterine pressure catheter with the same bridge principle and a known sensitivity, so it reads in mmHg and is zeroed to 0 instead of a relative baseline.
On intrapartum models the FECG and IUP front ends sit on a separate FECG/IUP board mounted on the main board. It has its own floating power supplies and passes signals across the isolation barrier optically and through transformers. That board is what keeps the electrode inside the uterus isolated from mains, and it is the board whose integrity your leakage test proves.
Inside the enclosure
Lift the top cover and the layout is simple. One main board carries almost everything: the processor with its program flash and separate boot flash, working RAM, and a battery-backed RAM with real-time clock that holds the configuration, the time and the error log. On the same board are the ultrasound transmitter and receiver, the uterine activity amplifier and its excitation reference, the audio amplifier, the serial ports and the recorder drivers. The main board comes in a different version for each model, because the number of ultrasound channels and the FECG/IUP mounting differ, so a board from a 172 is not a spare for a 174.
The power section on the main board takes the 12 V DC input and makes the logic rails, the clean analogue rails for the ultrasound front end and the higher rail that drives the thermal printhead. A comparator watches the input and stops recorder printing and memory writes when the supply sags, which protects the stored settings during brown-outs.
The top cover is not just plastic. It carries the display board with the LED digits and the membrane switch panel, both connected to the main board by ribbon cables, so the cover stays tethered when you lift it. The membrane panel is bonded to the cover and is replaced together with a new top cover, not peeled off. The recorder module combines the printhead, the paper drawer with its roller, the stepper motor and a small sensor board with the paper-out and paper-misload optical sensors and the drawer switch.
Connectors and automatic mode selection
The front sockets are round, colour-coded and mechanically keyed: blue for ultrasound, dark grey for FECG, blue with a grey centre for the 174 combined socket, white for uterine activity. The monitor selects the mode from the plug, not from a menu. Inside each transducer plug, enable pins are wired to ground, and the monitor reads which enable line is pulled to decide whether it sees an ultrasound transducer, an FECG cable, a toco or an IUP device. Some third-party catheters have no enable link, so the FECG/IUP board also recognises an IUP device by the current it draws from the excitation supply.
This design has a direct service consequence. A plug with a broken enable jumper, a bent pin or a cracked shell looks to the monitor like nothing is connected: no dashes, no mode message on the chart. A plug rebuilt with the wrong jumper wiring selects the wrong mode. Learn to read 'no mode at all' as a plug or socket problem before you think about boards.
Rear panel: the DC input from the external adapter, two remote mark jacks (one for an acoustic stimulator, one for a hand-held event marker), a 15-pin telemetry connector for a matching telemetry receiver, a nurse call jack reserved for future use and two RS-232 serial ports on RJ-45 connectors. When a telemetry mode is active, the equivalent front-panel inputs are ignored, so a monitor left connected to a telemetry receiver can look dead on its front sockets.
Recorder, paper and the serial interfaces
The recorder is a thermal array printer on Z-fold paper with two heart-rate scales: 30 to 240 bpm and 50 to 210 bpm. The two papers have different grid spacing, and the monitor must be configured for the paper actually loaded; otherwise every trace is drawn at the wrong height against the grid, even though the digital display is right. Chart speed is configurable at 1, 2 or 3 cm/min, typically 3 in North America and 1 elsewhere. The paper has black marks that the misload sensor reads; paper loaded upside down produces a misload alarm and no printing. Use only the paper specified for this recorder: the manual warns that other paper can damage the printhead.
The serial ports connect to central perinatal systems using a Hewlett-Packard compatible protocol, to an external maternal NIBP monitor, to a fetal pulse oximeter and, in later software, to older Corometrics-compatible host protocols. Each port has a mode and a baud rate set in the service setup mode, and a mismatch is the commonest reason a central station sees nothing. The software version and a full configuration summary can be printed on the chart from the service setup mode; do it at every visit and keep the strip with the record.
Power comes from an external universal AC-to-DC adapter that accepts mains from 100 to 230 V and delivers 12 V DC. Only the adapter model named on the rear label is approved. There is no battery for monitoring: pull the adapter and the monitor stops. The only cell inside is the backup for the clock and settings memory.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Main board (model-specific) | Processor, memories, US and UA front ends, power rails, recorder and serial drivers | Dead after liquid ingress, failed self-test, one US channel or UA channel lost |
| 02 | FECG/IUP board (173/174 only) | Isolated FECG and IUP front ends with floating supplies | No FECG or IUP, noisy FECG, raised patient leakage after damage |
| 03 | Top cover with display board and membrane panel | LED displays, keys and indicators | Dead segments, keys that stop responding, worn or cracked membrane |
| 04 | Recorder printhead | Thermal array that prints traces, grid annotations and messages | Missing dots, faint print on one side, damage from wrong paper or dirt |
| 05 | Recorder motor and roller | Moves the paper at the set chart speed | Wrong or uneven speed, noisy gears, paper not advancing |
| 06 | Recorder sensor board and drawer switch | Detects paper out, paper misloaded and drawer open | False misload or paper-out alarms, dust on sensors |
| 07 | Nautilus ultrasound transducer | Nine-crystal pulsed Doppler probe | Cracked face, broken cable at strain relief, damaged plug pins |
| 08 | Nautilus tocotransducer | Strain gauge button for relative uterine activity | Drifting baseline, no response, seal damaged by cleaning, calibration lost |
| 09 | FECG legplate cable | Connects the spiral electrode and reference plate to the FECG input | Broken wires, contaminated plate, poor seal, intermittent FECG |
| 10 | External AC-to-DC adapter | Converts mains to the 12 V DC the monitor runs on | No output, intermittent connector, damaged cord |
Typical scenario: a 'faulty' twin channel
Typical scenario, not a recorded case. A labour ward reported that the second heart rate on a Corometrics 174 'never works'. The engineer found the monitor configured correctly and the second ultrasound socket healthy with a known-good transducer. The ward had been plugging the FECG legplate into the secondary blue socket, expecting FECG/US twins, and an ultrasound transducer into the combined socket. On the 174 the FECG cable fits only the combined socket; the secondary socket accepts ultrasound only. Once the staff were shown which plug goes where, both channels worked, and the engineer labelled the sockets on the unit.
Know the socket map of each model before suspecting a channel. On the 170 Series the plug selects the mode, so a plug in the wrong socket looks like a hardware failure.
Cheaper electrodes, lost signal (Monica fetal monitor)
An engineer looked for cheaper alternatives to the original single-use electrodes of a Monica fetal monitor, a different design that picks up fetal ECG through electrodes on the maternal abdomen. A colleague reported that with several alternative electrode brands the signal dropped out whenever the patient moved, even with correct skin preparation and placement. After trying a few, the clinic kept one alternative that mostly worked, accepting occasional signal loss. The case is not about a Corometrics, but it shows the same rule that applies to the 170 Series spiral electrodes and legplates.
Electrodes and legplates are part of the measuring chain. On a 173 or 174, intermittent FECG with substitute consumables is a consumables question first; test with the specified electrode and legplate before you suspect the FECG/IUP board.
On-site checklist
- Model number (171, 172, 173 or 174) and serial recorded
- Fetal movement option noted as installed or not
- Software version and configuration summary printed from the service setup mode
- Paper scale setting matches the paper in the drawer
- Correct external adapter model in use, cord and connector intact
- Serial port modes and baud rates noted for any connected central system or external monitor
- Telemetry receiver connection noted