Summary of major steps:

1) Application of External Power. Immediate OFF switch open.
2) Immediate OFF switch closed. K1 engages. (We are here.)
 2A) Optional step to verify Power ON lockout prior to Thermal Reset.
3) Thermal Reset. Also if Step 2A was performed verify Power ON lockout   
until Power OFF.
4) All Interlock loops OK. "Normal" Relay 105 engages.
5) Power turn ON. K2 engages and power supply turn on sequencing.   
(Interruptible if any of four "main" supplies does not turn on.)
6) Memory warms up.
7) Ready.
8) Power turn OFF.

I) Interlock shutdown. Fans stay running.

IBM 1620 Power Sequencing

1)      Circuit Breaker PCB applies power to Transformer T1 through fuses
F15 & F16 [01.90.10.1]

1.1)    Transformer T1 applies 48VAC to Thermal Lamp through Relay 104-3
contact [01.90.10.1, 01.05.10.1]

1.2)    Transformer T1 applies 24VAC to Contacter K1 through Immediate
Off switch [01.90.10.1, 01.05.03.1]

<with Immediate Off switch pulled verify Thermal Lamp is only lamp lit and
that no voltages are present on Meter. remove power and reset Immediate Off
switch. restore power.>

2)      Contacter K1 applies power to Transformer T2 through fuses
F1 & F2 [01.90.10.1] and to PSTB5-6 & PSTB5-7 through fuses F6 & F5
[01.90.32.1]

2.1)    Transformer T2 applies 110VAC to Convenience Outlet Gate B and
PSTB5-1 through fuse F4 [01.90.10.1, 01.90.32.1, 01.90.35.1]

2.2)    Transformer T2 applies 24VAC to PSTB4-5 & PSTB4-7 through fuse
F13 [01.90.10.1, 01.90.32.1]

2.3)    Transformer T2 applies 24VAC to 24V rectifier bridge and to Thermal
String through fuse F9 [01.90.10.1, 01.90.40.1, 01.90.42.1]

2.4)    24V rectifier bridge applies 24VDC to Power Switch 14-2 and Meter
[01.05.03.1, 01.90.40.1, 01.90.45.1]

<with Power Switch OFF, verify 24VDC on Meter is only voltage present and
that Thermal Lamp is lit.>

2A      If Power Switch was turned ON then, Power Switch 14-2 N/O contact
applies 24VDC to "Fans" Relay 101 and PSTB4-1 [01.05.03.1, 01.90.32.1,
01.90.40.1]

2A.1)   Relay 101-1 contact closes, connecting 48VAC to Power On Lamp
[01.90.10.1, 01.05.10.1]

2A.2)   Relay 101-2 contact closes, connecting 110VAC to Gate B Fans and
Power Supply Fans through fuse F11 [01.90.10.1]

2A.2)   Relay 101-3 contact closes, connecting 110VAC to Gate A Fans and
Gate D Fan through fuse F11 [01.90.10.1]

<turn Power Switch ON, verify Power On Lamp lights and all fans run.
verify NOTHING else changes. if ANYTHING else changes pull Immediate Off
switch!>

3)      Pressing Reset Switch causes Reset Switch 15-1 contact to close
Thermal String, applying 24VAC to Relay 104 [01.05.03.1, 01.90.40.1]

3.1)    Relay 104-2 contact closes peripheral Reset Out & Thermal loop
302a & PSTB4-11, applying 24VAC to Relay 104 holding it after Reset Switch
is released if both the 1620 and peripheral Thermal Strings are OK.
[01.90.32.1, 01.90.40.1]

3.2)    Relay 104-3 contact opens, removing 48VAC from Thermal Lamp
[01.90.10.1, 01.05.10.1]

3.3)    Relay 104-1 contact closes, IF Power Switch 14-2 is Off then the
DC-Monitor loop applies 24VDC to "Normal" Relay 105 and Power Switch 14-1
[01.90.40.1, 01.90.21.1, 01.90.20.1]

<with Power Switch still ON, press Reset Switch. verify that Thermal Lamp
goes out and stays out. verify that NOTHING else changes. if ANYTHING else
changes pull Immediate Off switch! If Thermal Lamp lights again after Reset
Switch is released then the Thermal String is open someplace. Troubleshoot
as needed>

4)      When Power Switch is OFF then, Power Switch 14-2 N/C contact applies
24VDC through DC-Monitor loop to "Normal" Relay 105 and Power Switch 14-1
[01.90.40.1, 01.90.21.1, 01.09.20.1]

4.1)    Relay 105-2 contact closes, bypassing Power Switch 14-2 N/C contact,
applying 24VDC to "Normal" Relay 105 holding it after Power Switch is ON
[01.90.40.1, 01.90.21.1, 01.90.20.1]

4.2)    Relay 105-1 contact closes, preparing the 1620 for Power On
[01.90.40.1]

<turn Power Switch OFF. verify that "Normal" Relay 105 actuates. verify
that Power On Lamp goes out and fans stop. verify that NOTHING else changes.
if ANYTHING else changes pull Immediate Off switch!>

5)      When Power Switch is turned ON, Power Switch 14-2 N/O contact
closes and Power Switch 14-1 contact closes [01.05.03.1, 01.90.40.1]

5.1)    Power Switch 14-2 N/O contact connects 24VDC to "Fans" Relay 101
and through already closed Relay 105-1 contact to "Mem Heater" Relay 102
and Contacter K2 [01.90.40.1]

5.2)    Power Switch 14-1 contact connects 24VDC to the power supply
voltage sense logic relay chain and ready logic relay chain [01.90.40.1]

5.3)    Relay 101-1 contact applies 48VAC to Power On Lamp [01.05.10.1,
01.90.10.1]

5.4)    Relay 101-2 contact applies 110VAC to Gate B and Power Supply Fans
and Relay 101-3 contact applies 110VAC to Gate A and Gate D Fans [01.90.10.1]

5.5)    Relay 102-2 contact opens, isolating "Ready" Relay 106 [01.90.40.1]

5.6)    Relay 102-3 contact applies 110VAC from Fuse F10 to Memory Heater
and Neon Lamp [01.90.10.1]

5.7)    Contacter K2-5 contact opens [01.90.40.1]

5.8)    Contacter K2-2 contact closes [01.90.30.1]]

5.9)    Contacter K2 applies power to the 1250W Regulator Transformer and
415W Regulator Transformer [01.90.10.1, 01.90.20.1] and C.B. Motor through
Fuses F18 & F22 and Typewriter Motor through Fuses F17 & F21 and PSTB5-5 &
PSTB5-4 through Fuses F7 & F8 [01.90.10.1, 01.90.32.1]

5.10)   The 1250W Regulator Transformer applies Regulated 133VAC to 12V 20A
Supply A, 12V 20A Supply B, 12V 16A Supply, and 3V 5A MC Supply [01.90.20.1,
01.90.21.1]

5.10.1} The 12V 20A Supply A applies regulated -12VDC to Relay 110, -12VDC
Gate A Bus, PSTB1-1, and Meter [01.90.20.1, 01.00.15.1, 01.90.30.1,
01.90.45.1]

5.10.2} The 12V 20A Supply B applies regulated -12VDC to Relay 111, -12VDC
Gate B Bus, PSTB1-3, and Meter [01.90.20.1, 01.00.17.1, 01.90.30.1,
01.90.45.1]

5.10.3} The 12V 16A Supply applies regulated +12VDC to Relay 112, +12VDC
Gate A Bus, +12VDC Gate B Bus, PSTB2-1, and Meter [01.90.20.1, 01.00.15.1,
01.00.17.1, 01.90.30.1, 01.90.45.1]

5.10.4} The 3V 5A Supply applies regulated +/-3VDC to +12VDC MC Switch
and Meter [01.90.21.1, 01.90.30.1, 01.90.45.1]

5.11)   The 415W Regulator Transformer applies Regulated 133VAC to 36V 2A
Supply [01.90.20.1]

5.11.1} The 36V 2A Supply applies regulated -36VDC to Relay 109, -36VDC
Gate A Bus, -36VDC Gate B Bus, PSTB1-5, and Meter [01.90.20.1, 01.00.15.1,
01.00.17.1, 01.90.30.1, 01.90.45.1]

<if any of the four supplies: 12V 20A Supply A, 12V 20A Supply B, 12V 16A
Supply, or 36V 2A Supply does not turn on, the Sequence will stop here>

5.12)   The power supply voltage sense logic relay chain 112-2, 111-2,
110-2, and 109-2 applies 24VDC to "Mem Voltage" Relay 103 [01.90.40.1]

5.13)   Relay 103-1 contact opens, disconnecting 2ohm 10W Bleeder resistor
from +30VDC Bus [01.90.45.1]

5.14)   Relay 103-2 contact applies Regulated 133VAC from 1250W Regulator
Transformer to 30V 4A Supply [01.90.20.1, 01.90.21.1]

5.14.1} The 30V 4A Supply applies regulated +30VDC to Relay 117, +30VDC
Gate A Bus and Meter [01.90.21.1, 01.00.15.1, 01.90.45.1]

5.15)   Relay 103-3 contact applies Regulated 133VAC from 415W Regulator
Transformer to 48V 6A Supply [01.90.20.1, 01.90.21.1]

5.15.1} The 48V 6A Supply applies regulated +48VDC through Contacter K2-2
contact to PSTB1-7 and Meter [01.90.21.1, 01.90.30.1]

5.16)   Relay 117-1 contact closes, bypassing Power On switch N/O contact,
holding Contacter K2 [01.90.40.1]

5.17)   Relay 117-2 contact applies regulated +30VDC to Relay 115
[01.90.45.1]

5.18)   Relay 115-1 contact opens, placing 150ohm 10W resistor in series
with Relay 117 [01.90.45.1]

5.19)   Relay 115-2 contact applies 24VDC to ready logic relay chain
[01.90.45.1]

<verify that Power On Lamp is lit, all Fans are running, and ALL supplies
on Meter. If ANYTHING fails turn off the Power Switch. If Contactor K2
does not drop within a few seconds pull Immediate Off switch.>

6)      Memory Undertemp Thermostat contact in ready logic relay chain
closes when Memory Core heats to 85F [01.90.40.1]

6.1)    Memory Heater Control Thermostat contact opens when Memory Core heats
to 105F, causing "Mem Heater" Relay 102 to drop [01.90.40.1]

6.2)    Relay 102-2 contact closes, completing ready logic relay chain,
applying 24VDC to "Ready" Relay 106 [01.90.40.1]

6.3)    Relay 102-3 contact opens, removing 110VAC from Memory Heater
and Neon Lamp [01.90.10.1]

6.4)    Relay 106-1 contact closes, bypassing Relay 102-2 contact, "Ready"
Relay 106 holds [01.90.40.1]

6.5)    Relay 106-3 contact closes, connecting 48VDC from Ready In loop to
Power Ready Lamp and Reset Switch 15-2 [01.90.40.1, 01.05.10.1]

6.6)    Memory Heater Control Thermostat contact opens when Memory Core cools
to 95F, applying 24VDC to "Mem Heater" Relay 102 [01.90.40.1]

6.6.1)  Memory Core temperature cycles from 95F to 105F as memory heats,
with Neon Lamp lit [01.90.40.1, 01.90.10.1]

6.6.2)  Memory Core temperature cycles from 105F to 95F as memory cools,
with Neon Lamp dark [01.90.40.1, 01.90.10.1]

<verify that Power Ready Lamp is lit and that Memory Core heater is cycling.
If this fails, troubleshoot as necessary.>

7)      The 1620 Computer is now ready for operation.

7.1)    Load a program and start the computer according the applicable
procedure.

8)      When Power Switch is turned OFF, Power Switch 14-2 N/O contact
opens and Power Switch 14-1 contact opens [01.05.03.1, 01.90.40.1]

8.1)    Power Switch 14-2 N/O contact opens and 14-2 N/C contact closes
[01.90.40.1]

8.2)    Power Switch 14-1 contact opens disconnecting 24VDC from the power
supply voltage sense logic relay chain and ready logic relay chain
[01.90.40.1]

8.2.1)  The Power supply voltage sense logic relay chain cause "Mem Voltage"
Relay 103 to drop [01.90.40.1]

8.2.2)  The ready logic relay chain causes "Ready" Relay 106 to drop
[01.90.40.1]

8.3)    Relay 106-3 contact opens, disconnecting 48VDC from Power Ready Lamp
and Reset Switch 15-2 [01.90.40.1, 01.05.10.1]

8.4)    Relay 103-1 contact connects 2ohm 10W Bleeder resistor to +30VDC
Bus [01.90.45.1]

8.5)    Relay 103-2 contact opens, disconnecting Regulated 133VAC from
1250W Regulator Transformer to 30V 4A Supply [01.90.20.1, 01.90.21.1]

8.5.1}  The 30V 4A Supply discharges causing Relay 115 to drop [01.90.21.1,
01.00.15.1, 01.90.45.1]

8.5.2)  Relay 115-1 contact closes, reconnecting Relay 117 directly to 30V 4A
Supply [01.90.45.1]

8.5.3)  Relay 115-2 in ready logic relay chain opens [01.90.40.1]

8.6)    Relay 103-3 contact opens, disconnecting Regulated 133VAC from 415W
Regulator Transformer to 48V 6A Supply [01.90.20.1, 01.90.21.1]

8.7)    The 30V 4A Supply discharges causing Relay 117 to drop [01.90.45.1]

8.7.1)  Relay 117-2 contact opens, disconnecting Relay 115 [01.90.45.1]

8.7.2)  Relay 117-1 contact opens, removing 24VDC from Contactor K2,
"Mem Heater" Relay 102, and "Fans" Relay 101 [01.90.40.1]

8.8)    Contactor K2-5 contact closes [01.90.40.1]

8.9)    Contactor K2-2 contact opens, removing 48VDC from PSTB1-7
[01.90.30.1]

8.10)   Contactor K2 contacts open, removing 110VAC from 1250W Regulator
Transformer and 415W Regulator Transformer [01.90.10.1, 01.90.20.1]

8.11)   The 1620 Computer is now powered down.

<verify that only 24VDC supply is present on Meter and that Fans, Neon Lamp,
and Power On Lamp are off>


I)      Any Interlock trip breaks DC-Monitor chain (Thermal through Relay
104-1) removing 24VDC from Power Switch 14-1 and causing "Normal" Relay
105 to drop [01.90.40.1, 01.90.21.1, 01.90.20.1, 01.05.03.1]

I.1)    Removal of 24VDC from Power Switch 14-1 disconnects 24VDC from the
power supply voltage sense logic relay chain and ready logic relay chain
[01.90.40.1]

I.1.1)  The Power supply voltage sense logic relay chain cause "Mem Voltage"
Relay 103 to drop [01.90.40.1]

I.1.2)  The ready logic relay chain causes "Ready" Relay 106 to drop
[01.90.40.1]

I.2)    Relay 106-3 contact opens, disconnecting 48VDC from Power Ready Lamp
and Reset Switch 15-2 [01.90.40.1, 01.05.10.1]

I.3)    Relay 103-1 contact connects 2ohm 10W Bleeder resistor to +30VDC
Bus [01.90.45.1]

I.4)    Relay 103-2 contact opens, disconnecting Regulated 133VAC from
1250W Regulator Transformer to 30V 4A Supply [01.90.20.1, 01.90.21.1]

I.4.1}  The 30V 4A Supply discharges causing Relay 115 to drop [01.90.21.1,
01.00.15.1, 01.90.45.1]

I.4.2)  Relay 115-1 contact closes, reconnecting Relay 117 directly to 30V 4A
Supply [01.90.45.1]

I.4.3)  Relay 115-2 in ready logic relay chain opens [01.90.40.1]

I.5)    Relay 103-3 contact opens, disconnecting Regulated 133VAC from 415W
Regulator Transformer to 48V 6A Supply [01.90.20.1, 01.90.21.1]

I.6)    The "Normal" Relay drops [01.90.40.1]

I.6.1)  Relay 105-1 contact opens, isolating "Mem Heater" Relay 102 and
Contactor K2 circuuits from the "Fans" Relay 101 circuit [01.90.40.1]

I.6.2)  Relay 105-2 contact opens, disconnecting the DC-Monitor chain from
24VDC until the Power Switch 14-2 is turned OFF [01.90.40.1]

I.7)    The 30V 4A Supply discharges causing Relay 117 to drop [01.90.45.1]

I.7.1)  Relay 117-2 contact opens, disconnecting Relay 115 [01.90.45.1]

I.7.2)  Relay 117-1 contact opens, removing 24VDC from Contactor K2 and
"Mem Heater" Relay 102 [01.90.40.1]

I.8)    Contactor K2-5 contact closes [01.90.40.1]

I.9)    Contactor K2-2 contact opens, removing 48VDC from PSTB1-7
[01.90.30.1]

I.10)   Contactor K2 contacts open, removing 110VAC from 1250W Regulator
Transformer and 415W Regulator Transformer [01.90.10.1, 01.90.20.1]

I.11)   The 1620 Computer is now shutdown with the fans running.

<verify that only 24VDC supply is present on Meter and that Neon Lamp,
and Power On Lamp are off. verify that the Fans are still running>

I.12)   Turn the Power Switch OFF, after the machine cools to reset the
interlocks through Power Switch 14-2 N/C contact [01.90.40.1]

I.13)   The 1620 Computer is now powered down.

<verify that only 24VDC supply is present on Meter and that Fans, Neon Lamp,
and Power On Lamp are off>