Market Forge ST-24M36G Service Manual

ST-24M SERIES

12 PAN PRESSURELESS STEAM COOKERS

PARTS AND SERVICE MANUAL

EFFECTIVE FEBRUARY 22, 2016

The Company reserves the right to make substitution in the event that items specied are not available.

ERRORS: Descriptive and/or typographic errors are subject to correction.

Superseding All Previous Parts Lists.

MARKET FORGE

Telephone: [phone] Fax: [phone]

www.marketforge.com

P/N 14-0290 Rev B (2/16)

TABLE OF CONTENTS

PRINCIPLES OF OPERATION ..........................................................3

TROUBLESHOOTING ..................................................................5

WIRING DIAGRAMS ...................................................................9

REPAIR AND REPLACEMENT ........................................................ 12

ILLUSTRATED PART LIST

CONTROL PANEL ASSEMBLY ........................................................ 13

CABINET ASSEMBLY ................................................................ 14

DOOR ASSEMBLY ................................................................... 16

DOOR LATCH AND HANDLE ......................................................... 17

CONDENSER ASSEMBLY ............................................................ 18

FEBRUARY 22, 2016 2 ST-24M SERIES CONVECTION STEAMERS

PRINCIPLES OF OPERATION

GENERAL

The ST-24 Pressureless Steam Cooker consists of two
identical cooking compartments, one above the other, in

a single cabinet assembly. Each compartment is tted

with independent electrically controlled steam circuits and
spring-loaded, self-sealing doors with slam action latches.
Compartments can be used separately or simultaneously
for either constant steam or 60-minute timing. The prin­ciples of operation in this section include an explanation
of steam, steam condensing, and electrical circuits and
their functioning.

PLUMBING CIRCUITS

The plumbing circuits consist of the piping, steam sole­noid valves, orice, drain, and cold water condenser re­quired to provide controlled steam application to the cook-

ing compartments. A simplied diagram of these circuits

is shown in below

NOTE: Figure 1 is strictly a pictorial schematic diagram

and is not intended to show the actual congura­tion of the plumbing. All components are shown
in correct relationship with each other. However,
the diagram does not show their actual locations
or position within the cooker.

As shown in the diagram, steam inlet and exhaust con­nections are connected at the factory directly into a steam
boiler or direct-connected steam plumbing enclosed
within the base cabinet on which the cooker is mounted.
The boiler (or direct-connected steam control system) is
equipped to supply constant, regulated steam at 14–15
PSI. Steam exhaust, having been reduced to water by the
cold water condenser, is directed into the boiler (or direct­connected steam control) drain system.

Steam inlet lines for compartments are equipped with
normally closed solenoid valves operated by the electri­cal control circuits. The inlet valves are opened whenever
the compartment control circuit is activated by use of the
60-minute timers.

STEAM INLET LINE

A steam supply line is plumbed from the boiler output (or

direct-connected steam control) to a 1/2-inch barb tting

connected to the input sides of both steam inlet solenoid
valves. When a cooking compartment is not in use, the
valve for the compartment remains closed to prevent
steam from entering. During operation, the appropriate
inlet solenoid valve is opened by activation of the con­trol circuit. Steam is projected onto the surface of pans

of food loaded into the compartment by an orice located
inside the compartment. Steam continues to ow through

the compartment in this manner until the control circuit
closes the solenoid valve.

Figure 1

FEBRUARY 22, 2016 3 ST-24M SERIES CONVECTION STEAMERS

PRINCIPLES OF OPERATION

STEAM EXHAUST AND DRAIN LINES

Perforated strainers at the drain line openings inside each
compartment allow only steam, condensation, and liquid
cooking drainage to enter. Prior to discharge into the boil­er drain system, steam is converted to water by the cold
water condensing systems for each compartment.

STEAM EXHAUST CONDENSING SYSTEM

The steam condensing system consists of the identical,
two-position, normally closed cold water solenoid valves,
with outlet sides connected into the exhaust plumbing for
each cooking compartment. A spray nozzle directs cold
water about the inside of the drain lines to increase cold
water contact with exhausted steam. Valve inlet sides
are connected remote from the supply line of the steam
boiler (or direct-connected steam plumbing). The valves
respond to a thermostatic switch located inside the com­partment. When the timer starts the cold water solenoids
will energize.

ELECTRICAL CIRCUITS

The electrical circuits of the cooker control the power to
activate timer motors and energize solenoid-operated
valves and circuits, which in turn control application of
steam to the cooking compartment and condensation
of steam from the exhaust line. The cooker operates on
120V, 2 amp, 60Hz electrical service connected to all cir­cuits from the circuits of the steam boiler (or direct-con­nected steam controls) contained within the cabinet on
which the cooker is mounted. Power is supplied to the
control circuit at all times when the shut-off device (Circuit

Breaker) for the unit (installed by a certied Electrician) is

in the ON position.

Control Circuit Components

A brief description of the electrical circuit elements fol­lows.

60 Minute Timer/Constant Steam

The timer contains a 120-volt AC synchronous motor that
drives a timing dial through a gear reduction and clutch
mechanism. The timer dial is manually set for any interval
of operation from 0 to 60 minutes or constant steam as
read on the calibrated dial face. The manual rotation of
the dial moves the common element (1) of the timer switch
from the neutral (OFF) position to contact (3), which con­nects with the steam inlet solenoid valve operating circuit.

The cooker is placed into automatic operation with the
setting of the timer dial. Its timing cycle, however, is auto­matically delayed by a thermostatic switch, which assures
operating temperature is achieved before the timer motor
begins to “time out.” When the timer motor has operated
for the preset duration, the common element is trans­ferred to contact (4), returning the inlet solenoid valve to

the closed position and energizing the buzzer. Contact to
the buzzer circuit remains closed until the dial is manu­ally turned to the OFF position, returning the common ele­ment (1) of the timer switch to the neutral position.

Indicator Lights

An indicator light is included for both compartments. The
light remains on (red) at all times when the coinciding tim­er dial is set and the door interlock switch is closed. The
light turns off at the end of the timed cooking duration or
when the set time has elapsed.

Buzzer

The buzzer is an alarm device that operates by oscillation
of a striker against the core of an electromagnet. When
the 60-minute timer dials reach the “0-Minute” position,
the buzzer coil is energized to sound the buzzer. Move­ment of the timer dial to the OFF position opens the con­tact to the buzzer coil to shut it off.

Door Interlock Switch

The interlock switch is a single-pole proximity switch with
normally open contacts. The switch is operated by the
proximity of a magnet within the door. When the door is
open, the switch contacts remain in the open position.
When the door is closed and securely latched in place,
the magnet is near the switch to close the contacts.

Connected between the operating contact (3) of the tim­ers and the steam inlet solenoid valve, the door switch
acts as a protective device to interrupt valve operation
unless the door is closed.

Thermostatic Switch

The thermostatically operated switch is a two-position,
normally open switch mounted on the cooking compart­ment. The switch functions to activate the cold water sole­noid valves of the steam condensing system and to delay
timer motor operation until the compartment temperature
reaches 195°F, thus assuring that cooking temperature
exists throughout the timed duration.

Digital Timers (Optional)

The timer controls the steam to the cavity by either a con-

stant steam setting or a used dened timer that will be

programmed by the user into the timer

FEBRUARY 22, 2016 4 ST-24M SERIES CONVECTION STEAMERS

TROUBLESHOOTING

TROUBLE-SHOOTING GUIDE

A trouble-shooting guide for use by service personnel is
given in the table below.

ELECTRICAL FAULT ISOLATION

Correction of an electrical failure rst requires isolation of

the fault to a single circuit or component. In most cases,
the nature of the failure and its effect upon the operation

of the cooker will be sufcient to narrow it down to one or

more circuit elements. The Electrical Fault Isolation table
is provided as a guide for isolating electrical faults.

ELECTRICAL TROUBLE-SHOOTING PROCEDURES

Before performing the trouble-shooting procedures in this
section, the serviceman must be familiar with the function
of all controls as described in the Operation section as
well as with the Principles of Operation.

ELECTRICAL FAULT ISOLATION GUIDE

FAILURE FAULT LOCATION

1. Will not operate in either CONSTANT STEAM or 60-MINUTE TIMER po­sitions.

2. Operating in CONSTANT STEAM position but not in 60-MINUTE TIMER
position.

3. Operates in 60-MINUTE TIMER position but not in CONSTANT STEAM
position.

4. Steam solenoid valve fails to open with indicator light on. a. Solenoid valve coil

5. Indicator light off when steam solenoid valve open. a. Indicator light

6. With indicator light on and steam solenoid valve open, timer dial fails to
turn.

7. Buzzer fails to sound at end of 60-MINUTE TIMER mode. a. 60-Minute timer contacts

8. Steam ows continuously from boiler drain line. a. Thermostatic switch

The electrical trouble-shooting procedures that follow re­quire access to components and terminals of the electri­cal control panel. Electrical controls are reached by re­moving screws that fasten the control panel to the frame.
The panel may be pulled forward for testing while inter­connected to the cooker circuits or disconnected at the
pin connection for complete removal and repair.

INCOMING POWER

Before trouble-shooting any of the electrical parts or as­semblies, verify that power is being supplied to the cook­er. Incoming power is connected at the boiler (or direct­connected steam) control box located in the base cabinet.
With power connected to the cooker, an AC voltmeter is
used to measure 120 volts across L1 and L2. If 120 volts
is present, and the cooker will not operate, the fault lies
within the electrical circuits of the cooker.

a. Incoming power
b. Timer
c. Door interlock switch
d. Wiring
a. 60-minute timer
b. Wiring
a. Timer
b. Wiring

b. Wiring

b. Wiring
a. Compartment thermostatic switch
b. Constant steam position
c. Timer motor
d. Wiring

b. Buzzer
c. Wiring

b. Cold water solenoid valve
c. Wiring

FEBRUARY 22, 2016 5 ST-24M SERIES CONVECTION STEAMERS

TROUBLESHOOTING

ELECTRICAL INSPECTION

The rst step in any electrical trouble-shooting procedure

is a thorough physical inspection of all wiring connections.
To access electrical components, remove the control pan­el as explained in Exterior Panel Removal.

WARNING

Before removing control panel or checking
connections and wiring, be sure that the cir­cuit breaker for incoming power is OFF. When
power is supplied, all exposed terminals of the
control panel carry 120 volts.

Check all wiring connections by hand to assure that both
ends of all connection points are tightly secured. Use a
screwdriver to tighten connection points. If necessary, vi­sually inspect all quick-disconnect terminals for evidence
of corrosion. Terminals in this condition should be sepa­rated, cleaned with emery cloth until shiny, and tightly re­connected.

60 MINUTE TIMER

Timer Contacts

Defective timer contacts will result in failure of either cook­er compartment to operate. When this occurs, remove the
control panel (see Exterior Panel Removal), and proceed
as follows:

1. Turn off power to the cooker at external circuit break­er.

2. Disconnect all ve wires from timer terminals

3. Connect an ohmmeter between terminals 1 and 3.

To conrm timer motor condition, proceed as follows:

1. Carefully check motor wire leads and tighten loose
connections.

WARNING

Use care while working with control panel.
Terminals carry 120 volts.

2. Turn on power to the cooker.

3. Set timer dial (any setting beyond “0-Minute”). If op­eration is correct, the motor will turn the dial toward
“0-Minute.” If the motor fails to operate, it is defective
and the entire timer must be replaced.

4. Shut off power to the cooker.

DOOR INTERLOCK SWITCH

Malfunction of the cooker door interlock switch prevents
timer indicator lights from turning on and steam solenoid
from opening when the timer dial is set. If steam does not
enter the compartment and the indicator light fails to turn
on with the door latch securely engaged, the fault may be
in the door interlock switch. Proceed as follows:

1. Turn off power to the cooker.

2. Disconnect wires to the door switch terminals.

3. Connect an ohmmeter between the terminals of the
switch.

4. Actuate the switch by closing the cooking compart­ment door. If a zero reading cannot be obtained, the
switch is defective and must be replaced.

4. Rotate timer dial beyond the “0-Minute” point (any
setting) to obtain a reading of zero ohms on the
ohmmeter. If zero ohm reading cannot be obtained,
timer contacts are defective and the timer must be
replaced.

5. Move ohmmeter leads to terminals 1 and 4.

6. Rotate timer dial to “0-Minute” position (an audible
click indicates correct position). If zero ohm reading
cannot be obtained, the timer is defective and must
be replaced.

7. Remove ohmmeter and replace all ve leads on timer
terminals.

Timer Motor

A defective timer motor will cause continuous operation in
the TIME mode, with the timer dial failing to return to the
“0-Minute” position. Since thermostatic switch failure can

cause the same symptom, fault must rst be isolated to

the timer by testing the thermostat.

FEBRUARY 22, 2016 6 ST-24M SERIES CONVECTION STEAMERS

5. Remove the ohmmeter and replace the leads on
switch terminals.

STEAM SOLENOID VALVES

When either inlet solenoid valve fails to operate, the fault
may be a defective coil. A defective coil is found using an
AC volt-meter to check the voltage at the coil wire ter­minals, with the cooker compartment operating in either
CONSTANT STEAM or 60-MINUTE TIMER mode. If volt­age of 120 volts is present and the coil fails to open the
valves, the fault is in the valve coil. Defective valve coils
are not separately replaceable, requiring complete valve
replacement.