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METCAL MX-500P-11 TECHNICAL DOCUMENTATION

This documentation was carefully reverse engineered from several
actual MX-500P units, and although it has been meticulously triple
checked, it may contain errors and omissions so use it at your own
risk. It is provided solely for the purpose of helping you satisfy
your personal curiosity about how a Metcal MX-500P works, and you must
never use it for any other purpose, especially not for any commercial
or business purpose, and certainly not as an aide to experimenting
with or performing work on MX-500P units, as it is inadequate for such
unintended use. Reproduction is strictly forbidden.

MECHANICAL DESCRIPTION:

The small plastic cover at the two RF output connectors is held in
place by it's two plastic hooked clips which descend into the aluminum
housing at the top and bottom. Beneath this plastic cover are two hex
nuts that bolt the RF connectors to the aluminum housing. The RF
connectors are soldered directly into the circuit board. On the rear
of the MX-500P are four deeply recessed T15 tamper proof Torx screws
which hold the two halves of the aluminum casting together. Loosening
the single screw in the upper-middle back of the MX-500P by a few
turns releases the internal heat sink which is attached to the circuit
board. The internal heat sink couples heat into the MX-500P aluminum
case from where it can dissipate into the ambient air. Good thermal
coupling between the internal heat sink and the case is aided by a
thin coating of white thermal compound. The circuit board inside the
MX-500P is fastened down by six internal screws, four of which are
rather large because they also hold down the line transformer. The
tiny grub screw on the upper right hand side of the MX500 controls the
Auto Sleep feature and should not be tightened past the point where it
gently activates the switch. Note that it can be dangerous to power up
an MX-500P unit that has been taken apart or that has been reassembled
by anyone other than an trained Metcal service technician.

DESCRIPTION OF FUNCTION:

The MX-500P Power Unit provides RF energy at 13.560MHz to the
Soldering Tip Cartridge, which contains an induction heater consisting
of an 18 turn AWG33 wire coil wound around a 0.11" diameter by 0.5"
long slug. The slug is composed of a copper core, clad in a thin
magnetic alloy having a curie point equal to the desired soldering tip
temperature. The magnetic alloy absorbs RF energy from the coil,
causing the slug to heat up until the curie temperature is reached. At
this point  absorption stops and heating ceases, because the RF energy
is now reflected back to the power unit by the copper core.

The On/Off switch atop the power unit controls the 18V power supply
U8, which runs all the supervisory circuits. When the 18V supply is
off, Q6 turns off thereby causing Q7 to turn on and disable the RF
generator.

Q5 and Q8 control a small DC bias voltage out to the soldering hand
piece, so that U2a can sense an intermittent or disconnected hand
piece cable, in which case the yellow LED DS2 will light and U2b will
latch Q7 on, thereby disabling the RF generator until the On/Off
switch is cycled.

U5a senses small changes in RF generator output power to the
soldering hand piece. If no changes are detected for half an hour then
sleep mode timer U6 times out causing U7 to latch Q11 on, thereby
disabling the RF generator until the On/Off switch is cycled. This
functionality can be disabled by backing out the tiny grub screw in
the upper right side of the unit.

If thermal switch TS1 detects an over temperature condition inside
the power unit then Q9 will turn on and disable the RF generator until
the temperature drops back down to normal.

If  Forward Power at T3 and C33 exceeds reasonable limits due to a
fault in the power unit circuitry, then Q19 will turn on and disable
the RF generator until Forward Power returns to acceptable levels.

U5b monitors the supervisory circuits and lights green LED DS1 if
everything is OK, in which case Q12 will be on, enabling U4 to power
up the RF generator.

U1 provides a 13.560MHz square wave out to class C driver stage Q3,
which in turn drives the class C final output stage Q4, providing RF
power to the soldering hand piece. Note that Q3 is unusual in that it
has an input capacitance of only 55pF and a gate threshold voltage of
only 1.6V.

Diodes D8 and D9 sense the RF voltage level coming out of the RF
generator, providing negative feedback to switching power supply U4 Q1
Q2, which powers the final RF output stage Q4 of the RF generator.

J1 provides a DC voltage which is proportional to the power being
delivered to the hand piece. It can be connected to an analog meter
movement or other measuring instrument.

TRIMPOT DESCRIPTIONS:

RV2 adjusts the RF-Output-Power delivered to the hand piece; if this
adjustment is incorrect then the voltage at C8 will likely not
correspond to the values given elsewhere in this document.

RV1 sets the Forward-Power-Fault safety shutdown circuit trip point;
if this adjustment is incorrect then the voltage at C16 will likely
not correspond to the values given elsewhere in this document.

RV3 calibrates the signal out to any Meter connected at J4; the
signal at J4 is not normally used so it is hard to imagine how this
adjustment could have any impact upon the operation of the unit.

Calibration is well beyond the scope of this document and must not be
attempted by anyone other than a qualified Metcal service technician.

MEASUREMENTS FROM SOME GOOD WORKING UNITS:

Whenever unit is plugged into the AC line:
      Voltage at C2 will measure approximately 26 VDC
      Voltage at C6 will measure approximately 53 VDC
Whenever the power switch is on and unit is plugged in:
      U8 pin 3 will measure 18 VDC
      U7 pin 14 will measure 12 VDC
Whenever a hand piece is connected and the green LED is lit:
      U4 pin 4 will measure 1.3 VDC
      U4 pin 5 will measure 0.0 VDC
If the green LED is extinguished but the unit is plugged in:
      U4 pin 5 will measure 26 VDC
Whenever the hand piece is idling hot in the stand:
      Voltage at C8 will measure between 17 and 18 VDC
      Voltage at C3 will measure between 14 and 15 VDC
      Voltage at C16 will measure approximately minus 1.2 VDC
      U1 pin 14 will measure approximately 4.8 VDC
      U1 pin 4 will have a 13.56 Mhz waveform approximately 2.8 Vpp
When the soldering hand piece is heating up from a cold start:
      Voltage at C8 may temporarily rise as high as 21 VDC
      Voltage at C16 may temporarily rise as high as minus 0.22 VDC
When the hot hand piece is touched to something cold:
      U5 Pin 1 will pulse high momentarily
Characteristics of a cold MX Soldering Tip Cartridge:
      Inductance at 1kHz is somewhere around 2.8 uH
      DC resistance is fairly close to 0.21 ohms