The HT70 platform: micro-piston pump, valves, sensors, oxygen, power and the patient circuit
Identify which HT70 variant is on the bench, trace the gas from the intake filter to the exhalation valve and back, name the boards and assemblies that do each job, and know which parts are field-replaceable before you open the case.
Which HT70 is in front of you
The HT70 family is a compact electrically powered ventilator built for transport, emergency, sub-acute and home use. In Level 1 you met ventilators that take wall air and oxygen and mix them; this one is different at the root. It needs no compressed gas at all. It draws room air through a filter and generates flow with its own twin micro-piston pump. Oxygen, when the patient needs it, is added at the intake by an accessory, not by an internal blender. Keep that picture in mind: almost every model-specific fault in this course follows from it.
The service manual lists five configurations across three models. The HT70S and HT70SM are the Basic versions without pressure support and trends; the HT70 and HT70M are the Classic versions with pressure support; the HT70PM, the HT70 Plus, adds an on-airway flow sensor with waveforms, exhaled volume monitoring and flow triggering. The letter M means the built-in oxygen sensor is fitted. Some parts are ordered differently for the Plus and for the later HT70-2 build, notably the single board computer, so record the full model designation and the serial number from the label before you order anything.
Why it matters on site: a Basic unit with no oxygen sensor cannot show FiO2 and will never raise an oxygen alarm, whatever the clinician expects; a Classic unit has no exhaled volume, so a complaint about 'wrong exhaled volume' on it is a misunderstanding, not a fault. Half of the first call is establishing what the machine is supposed to do.
The gas path, read from intake to patient
Follow the gas in one direction, as you learned in Level 1. Room air enters on the right-hand side of the case through a semi-transparent cover and the air intake filter behind it. The cover lets the user see the filter without tools, which is why a grey filter is the first thing you look at on any call. From the filter the air passes a muffler, which damps the pump's suction pulses, and reaches the inlet of the pump. On units with the oxygen sensor, the sensor is mounted on the muffler (the service manual shows them as one assembly), in the intake stream, so it reads the oxygen fraction of the gas the pump is about to deliver.
The pump is a pair of small pistons driven by a motor. The software sets the flow it wants for the breath and the servo loop sets the motor speed to achieve it; there is no flow control valve in the delivery path. Check valves on the pump keep the pistons pushing gas forward. The output goes into a manifold, which carries the valves described below, and leaves the ventilator at the gas output port, where a standard bacteria filter is fitted before the patient circuit.
The circuit is a single-limb circuit with three connections at the front of the ventilator: the gas output, the proximal pressure line and the exhalation valve drive line. The exhalation valve sits near the patient. The proximal line measures pressure at the patient end and runs through an inline filter that protects the internal transducers. On the HT70 Plus the on-airway flow sensor at the patient end adds two more small tubes that go to a differential pressure transducer inside.
The valves: exhalation control and safety
The manual divides the pneumatics into three subsystems: gas delivery, exhalation control and safety. Exhalation control has two valves on the manifold. An on/off solenoid valve pressurises the exhalation valve drive line during inspiration, closing the exhalation valve diaphragm so that the breath goes into the lungs. In expiration it releases the drive line. When PEEP is set, a proportional solenoid valve holds a controlled pressure on the drive line, so the diaphragm resists expiratory flow and the airway pressure settles at the set level. A wet, kinked or disconnected drive line therefore shows up as wrong PEEP or a breath that leaks away, not as a pump fault.
The safety subsystem has three parts. A mechanical high-pressure relief valve opens on its own at about 100 cmH2O. An electrically driven safety valve gives a second level of relief, opening above about 120 cmH2O, and it also opens to relieve the circuit when an occlusion persists for longer than about 10 seconds. An emergency air intake valve lets the patient draw room air into the circuit if the pump fails, and exhale through the exhalation valve. The emergency intake on the bottom panel and the fresh gas intake on the side must never be blocked; when you set the unit down on a soft surface in a workshop, think about that.
A small group of auto-zero solenoid valves switches the pressure transducers to atmosphere at intervals so the software can correct their zero. They click during operation and during the service screen tests; a dead auto-zero valve shows up as a zero error, not as a delivery problem.
Electronics: who does what
Behind the rear panel are the two main boards. The Main Control Board (MCB) drives the motor and pump, carries the airway and internal pressure transducers with their colour-coded tubes, runs power management and battery charging, and holds the hardware part of the safety system. The Single Board Computer (SBC) plugs onto the MCB and runs the user interface: the touch screen, the membrane keys and the indicator LEDs. In the top case, a display board connects the LCD, the backlight inverter, the touch panel and the membrane switches to the SBC through the LCD data cable. On the Plus, a flow sensor add-on board with its own transducer handles the on-airway sensor.
The alarm system is deliberately split in two. Clinical alarms use two speakers controlled by the SBC, and the software checks that the speaker is actually being driven. Hardware faults use a separate piezo buzzer for the Device Alert alarm, which can force the ventilator into a safe state independently of the processor. That buzzer is powered by a super-capacitor so that it can sound for at least two minutes after power is lost. The dual LEDs in the handle flash red for high priority, red and yellow for medium and yellow for low.
Two more details save time. The nurse call relay is held energised while there is no alarm and drops out on alarm, so a loss of power also signals the nurse call system. And the power switch is a momentary switch: a press asks the software to shut down, and the user must confirm on the screen. A ventilator that 'will not switch off' with one press is behaving as designed.
Power and batteries
The HT70 can run from external AC through its own external power supply, from external DC in the 12 to 24 V range, for example a vehicle adapter, and from two internal lithium-ion batteries. The Power Pac is the large pack that latches onto the back; it is hot-swappable and can be charged on or off the ventilator. The Backup Battery is inside the case and is not user-replaceable. External power reaches the ventilator through the Power Pac, whose electronics decide which source is in use and report to the MCB. The order of precedence is external power, then Power Pac, then backup battery. Both batteries charge whenever external power is connected, whether the ventilator is ventilating or not.
The manual gives the Power Pac as 14.4 V, 6.5 Ah and the backup battery as 14.4 V, 2 Ah. With new, fully charged batteries the two together can run the ventilator for up to about 10 hours at the manufacturer's standard settings, and the backup battery alone is rated for at least 30 minutes. Treat those as best-case figures for a new system, not as what a three-year-old pack in an ambulance will do. The batteries carry their own gas gauge electronics, and the alarms for low charge, high battery temperature and backup battery failure come from that information.
Two small consumers are easy to forget: the coin cell that keeps the clock, and the super-capacitor behind the shutdown buzzer. Both appear later in the maintenance and verification lessons. Also note that a wrong external supply is a real risk: the manual allows only the manufacturer's own AC power supply and Power Pac with this ventilator.
Before you open it
The service policy is assembly replacement. The manual describes removal of the rear panel, the SBC, the MCB, the power switch, the top case, filters, fan, backup battery, display board, LCD and touch screen, handle, membrane switches, speakers, the solenoid valve group, the on/off valve, the safety valve, the pump and the manifold with its proportional solenoid. For several internal faults, such as a leaking check valve, the troubleshooting table calls for the pump assembly rather than repair of its parts. It also points at the pump assembly for a safety valve that will not close, although the safety valve is listed as a separate spare; confirm the part with the manufacturer before ordering.
Before any work: disconnect external power, remove the Power Pac, disconnect oxygen, and work on an ESD-safe bench with a wrist strap. The backup battery stays connected inside until you disconnect it at the MCB, so the unit is never fully dead just because the mains lead is out. The manufacturer restricts service to factory-trained, authorised personnel, and after any service the ventilator must pass the full Operational Verification Procedure before patient use. Lessons 2 and 5 cover that procedure.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Air intake filter, cover and gasket | Filters room air before the muffler and pump; oxygen accessories attach to the cover | Clogs grey and starves the pump; gasket loose and leaks |
| 02 | Muffler (with oxygen sensor on M models) | Damps pump suction noise; carries the FiO2 sensor in the intake stream | Comes loose and the pump gets loud; sensor ages and drifts low |
| 03 | Twin micro-piston pump assembly | Generates all inspiratory flow, set by servo-controlled motor speed | Motor fault, internal leak, worn diaphragm or check valve; replaced as an assembly |
| 04 | Manifold with proportional solenoid | Carries delivery gas to the outlet and sets PEEP pressure on the exhalation drive | Leaking o-rings, malfunctioning proportional solenoid: unstable or wrong PEEP |
| 05 | On/off valve, safety valve, relief valve, emergency intake valve | Exhalation drive switching; second-level and mechanical over-pressure relief; ambient air if the pump fails | Leak causing low pressure with normal flow; valve stuck open or not clicking |
| 06 | Solenoid valve group (auto-zero valves) | Switches transducers to atmosphere for zeroing | Auto zero errors, unstable baseline |
| 07 | Main Control Board (MCB) with Paw and Pint transducers | Motor drive, pressure sensing, power management, battery charging, hardware safety | No AC operation, no charging, motor fault, transducer calibration errors |
| 08 | Single Board Computer (SBC) | User interface, membrane keys and LEDs, software | No display or unresponsive touch with good cables and display board |
| 09 | Display board, LCD/touch screen assembly, membranes, handle LEDs | Screen, backlight, touch and keys; visual alarm indicators | Dark screen, dead buttons or LEDs; loose LCD cable |
| 10 | Power Pac and internal Backup Battery (Li-ion) | Hot-swappable main battery and internal reserve battery | Will not charge, short run time, switching to backup battery alarms |
A portable ventilator that blinked and would not start: the battery chain first
On a Pulmonetic LTV 1200, another electrically driven portable ventilator, the unit would not start normally from its external supply and only blinked a test indicator. The engineer wanted to know how to test the power supply on its own. A colleague suggested taking the optional external battery out of the chain, connecting the supply straight to the ventilator, and measuring the internal battery. Across a fleet of six such ventilators, he added, it was the external batteries that failed, not the supplies. The case was discussed rather than closed, but the method is the point.
On a portable ventilator power passes through a chain of sources and packs before it reaches the boards. On the HT70 external power enters through the Power Pac, so a failing Power Pac can look like a failed supply or a failed MCB. Isolate the chain one link at a time before you replace a board.
Concentrator oxygen into a low-pressure input
In a discussion about ventilators supplied from an oxygen concentrator instead of a cylinder or pipeline, engineers described two ways of connecting it: through a regulator set to a very low pressure, or into a dedicated low-pressure oxygen input where the gas bypasses the machine's own mixing valve and the oxygen fraction is set by the flow on the concentrator's flowmeter and watched by the oxygen cell. They added a typical complaint on one fleet: oxygen 'not regulating', which turned out to be a low-pressure oxygen mode left switched on in the settings.
The HT70 low-flow reservoir works on the same principle: FiO2 depends on the oxygen flow and on the patient's minute volume, not on a dial. Always confirm FiO2 with the sensor or an analyser when oxygen comes from a flowmeter.
Clock failure after a long storage period
A Hamilton C3 that had stood unused for a long period showed a real-time clock failure on start-up. The cause was the small battery that keeps the clock running while the ventilator is off; it had run down during storage. The engineer replaced it and planned a software update recommended for that model at the same visit.
The HT70 has its own coin battery for the clock, and the manufacturer lists it among the parts replaced at the 24-month service. Units stored for emergency preparedness are exactly the ones whose small batteries run flat unseen.
On-site checklist
- Model designation (HT70S, HT70SM, HT70, HT70M, HT70PM) and serial number recorded from the label
- Software version and total hours read from the screen and recorded
- Oxygen accessory in use noted: none, entrainment mixer or low-flow reservoir
- Intake filter colour and fan filter condition seen before you start
- External power supply is the manufacturer's own unit, cord and connectors intact
- Power Pac charge indicator and backup battery level noted
- Circuit type, exhalation valve and proximal inline filter identified; flow sensor present on a Plus
- External power and Power Pac removed, oxygen disconnected, ESD strap on before opening