Middleby Marshall PS200 series - Manual (Page 98)

97
SECTION 3 - SERVICING COMPONENTS
Digital Temperature
Controller
MUST BE SET TO PID
OPERATING MODE
Amplifier Board
Modulating Valve
To thermocouple
white +
red -
purple
To transformer
24VAC input
purple/white
blue
blue/white
orange orange/white
bypass adjustment
screw (under cap)
MUST BE FULLY
CLOSED
Figure 71 - Modulating Gas System
K. Modulating Gas Valve (if so equipped)
Ovens that use a PID (modulated) gas regulation system
are equipped with a modulating gas valve instead of a sole-
noid valve. The valve is located between the combination
gas control valve and the burner.
In response to the millivolt signal from the thermocouple,
the Digital Temperature Controller sends a 4-20mA signal
to an amplifer board. The board, in turn, passes on a 0-
20VDC signal to the modulating valve itself. The valve opens
or closes proportionally to the signal from the amplifier board,
allowing any setting from 0-100% gas flow.
When the oven is switched on, gas demand is at its peak
and the valve is fully open, permitting 100% gas flow. As
the oven’s temperature approaches the set point, the valve
slowly begins to close, restricting gas flow to the 35-60%
range. After the oven is fully preheated, the gas flow usu-
ally stabilizes at about 30%. The actual oven temperature
can remain constant, with NO temperature swing, because
the valve is adjusted constantly according to the demand.
Modulating valves are used on all PS360EWB, PS555, and
PS570G gas ovens. They are also used on PS200 ovens
produced 4/02 or later and PS570S ovens produced 9/00 or
later.
The Model PS360EWB oven uses a Maxitrol M520 modu-
lating valve with a 3/4” gas line, P/N 32570. All other
Middleby oven models equipped with a modulating valve
use the Maxitrol M420 valve with a 1/2” gas line, P/N 41647.
The modulating valve features a bypass adjustment screw,
shown in Figure 71. For all ovens except the PS536, the
screw should ALWAYS remain fully closed; no adjustment
is necessary. The screw is pre-set to the fully-closed posi-
tion from the factory. If the screw is adjusted to any setting
other than fully-closed, the oven may not be able to main-
tain the set point temperature.
IMPORTANT: For proper functioning of the modu-
lating valve, the temperature controller MUST be
set to the PID operating mode. Refer to Tempera-
ture Controller on Page 31.
L. Amplifier Board (ovens with a modulating gas
valve only)
Ovens equipped with a modulating gas valve have an ampli-
fier board mounted inside the machinery compartment.
In response to the millivolt signal from the thermocouple,
the Digital Temperature Controller sends a 4-20mA signal
to the amplifer board. The board, in turn, passes on a 0-
20VDC signal to the modulating valve. The valve opens or
closes proportionally to the signal from the amplifier board,
allowing any setting from 0-100% gas flow.
All Middleby oven models equipped with a modulating valve
use amplifier board P/N 31651.
PS360EWB
with Controller 32571
only
purple/white
purple
To amplifier board terminal 6
To amplifier board terminal 5