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Old 25th Nov 2020, 11:29 pm   #23
trh01uk
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Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
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Default Re: Racal Dana 9916 Frequency Counter.

The fact that the decimal points are all working is useful, because it proves that the driver of that part of the display is working. So Q28 (and circuits feeding that) plus the multiplexer transistors Q18 to Q25 are working.

IC1 pin 4 is the Blanking Input. It forces all display segments drivers (which are open collectors outputs) in IC1 to OFF when it is LOW. However you are measuring the odd voltage of 3.314V on pin 4, with 12mV of ripple on it. Its fed by 3 NAND gates type 74LS03 - these have open collector outputs, so to get a HIGH, its depending on R111 (2k2) to pull it up to 5V. So why are you measuring 3.314V? Something is pulling that voltage down slightly - perhaps a fault with the 74247, but you are replacing that anyway. And that voltage is high enough that it should not activate the Blanking function on IC1.

The TI datasheet I've looked at says the 74247 is electrically and functionally equivalent to the 7447A (whether that 'A' suffix is important I don't know).

The obvious test to do is to activate the Lamp test - when IC1 pin 3 is LOW, it forces all 7-segment drivers to ON, so you should be seeing all the displays with all segments on. The fact that you don't suggests that IC1 is dead - since we have already said that Q18 to Q25 are apparently OK. A scope on IC1 pins 9 to 15 would check this point - just in case you somehow have open circuit tracks - or open circuit resistors in 1R4.

If IC1 has failed, then the next question is likely to be why? That may not matter if the fault is cleared with a new 74247 in circuit.

Richard
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