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Tornado TV TAB DU: A4 PCB (digital interface board)

introduction permalink:
Board A4 is marked with black paint as a marker. The board has part number 229-012652-02 and is manufactured by Marconi Avionics for project K0656. The board name is 'digital interface'. Therefore the boards function is to handle the digital data. The design is rather clear. It's remarkable that heat all components are placed on a aluminium frame that acts as a heatsink. The board is coated with a resin like substance. The resin is likely applied to protect the electronics from moist and acts as a glue to reduce mechanical impact on the components.

All the logic chips are surface mount devices. It's likely the package style of the chips is known as a FMQB package. Despite some research, the information about the FMQB package is rather minor... I found in a datasheet that DMQB, FMQB and LMQB are(/were?) used for military grade devices with environmental and burn-in processing. The meaning of the letters of the package style designation is unfortunately unknown.
The board consists of mainly logic chips (26 pieces). The three white Beckman 'chips' at the bottom right are 4K7 resistor networks. Alls the chips are ceramic versions. Usually SN is the plastic consumer indication and SNC has the 'c' for ceramic. The (SN)7xxx is the type number of consumer logic chips, the 'same' chip in military grade starts with a 5 instead of a 7. So the SN7400 is functional the same as the military ceramic SNC5400 chip. So the 'exotic' looking chips are 'just' high quality versions of the well known logic family chips. The big white chip marked [F 7420 54150 FMQB] contains 54150 which is basically the same as the SN54150. This is therefore a 16 input multiplexer to one output. By configuring four bits (0000...1111) the desired input is selected and 'connected' to the output (pin 10). FMQB is the package designation. 7420 is the production code. The first two digits is the year of production: (19)74 and the production month is the 20th week. The chip is manufactured in week 20 of 1974. The production years of the chips are week 20 of 1974 and week 44 of 1981. The board is therefore assembled after week 44 of 1981.

no EMC capacitors
(Very) remarkable is that there are no EMC suppression capacitors at each chip! By the logic switching, the power draw at the input pin generates a (high frequency) pulse which can result in interference. It's a good engineering practice to add a capacitor at the power input pin for each chip and close as possible placed to the chip pin. Maybe there's a built in capacitor in the chip or the chips are rather immune to interference...

locking pins
The connector at the bottom side is connected to the wire harness of the device. Each of the six board is color coded to prevent mix up of the boards. There are also locking pins. For this A4 board the locking pins are 8 and 56. For each board the locking pins are different so the board will only fit in the correct position. The locking pin is a plastic bushing placed on a pin of the connector. The connector pin is missing and drilled out to fit the plastic bushing.

diagnose connector
There's a connector at the top of the board. This connector is not used during normal operation. It's very likely that the connector is used for diagnose purposes. The pins are connected to strategic pins of the board. This makes is much easier for diagnosing purposes when the unit is is in use. Due to the plug in boards it's easy and fast to replace a board when there's a problem. The board can be swapped and the broken faulty board can be repaired off-site. Due to the heavy duty design ans quality components, problems would be very rare I guess...

A4 printed circuit board permalink:
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