Undervoltage warning.
The supply gives a warning indication when the mains voltage has fallen so low that regulation is about to be lost. The Headroom Indicator subsystem illuminates all the decimal points on the DPM, giving a clear signal that something is amiss.
The voltage across the pass transistors is monitored by TR3; when this voltage falls too low, TR3 turns off, turning off TR1. TR1 collector is then pulled up to +40V by R26, charging C7 through D5, and so turning on TR2. This connects the DPM decimal point connections to the local 0V (actually +26 TO +29V above supply 0V) via current-limiting resistors R32-34. The peak-hold action of D5/C7 is required as the voltage across the pass transistors includes a large ripple content that would leave the decimal points rather dim if they were being strobed at 100 Hz.
An identical circuit (TR9, TR10) monitors headroom in the -17V supply. The voltage across the pass transistors is monitored by TR10. When this voltage falls too low, TR10 turns off, turning off TR9. TR9 collector is connected to the LH PCB via connectors CN8-CN7, and the cessation of current drawn indicates low headroom. On the LH PCB, when this current ceases D12 anode is pulled high by R78, and TR2 turns on, illuminating the decimal points on the meter.
The -17V Supply. (RH PCB)
The actual 17V section of this is identical to that in the +17V section.
The +12V subrail circuitry is much simpler as it only provides power for the driver TR11, the 723, and the fan. There is no mains meter circuitry here.
The Temperature Protection System.
This is designed to protect the +/-17V rails only. The +48V and +8V regulators incorporate internal thermal protection, and in any case, their heat dissipation is very small compared with the heatsink size. The +/-17V rails, however are vulnerable to blocked cooling vents or fan failure, so protection is provided that shuts down the 17V rails if the TO3 device cans exceed 100 degC.
The thermal sensor on the RH heatsink is LM35DZ TR31. It outputs 10mV per degreeC above freezing point (0 degC) and applies it to IC5-B non-inverting input. IC5-B is used as a comparator, with R114,106 giving a small amount of hysteresis to prevent dither or oscillation.
IC reference TR29 produces a stable 1.237V which is reduced to 1.00V by R104,112. This is applied to the inverting inputs of IC5-A,B as the shutdown temperature reference, and represents 90 degC at the TO3 can top*. TR27 senses the temperature of the LH heatsink.
* Temperature shutdown takes place at 100 degC on some earlier issue PCB’s.
If either heat sensor exceeds 100 degC the output of the associated op-amp goes high, applying voltage to the top of R109, and enabling the relaxation LED-flashing oscillator IC6-B. LED7 flashes at approx 2 Hz.
If JMP2 is fitted in the "ON" position, shutdown of the +/-17V rails is also implemented. TR30 is turned on via R117, which turns on TR28, and shuts down the -17V supply. The mutual shutdown system then also shuts down +17V. If JMP2 is in the "OFF" position, only the LED flashing occurs.
