Amber Beacon Repair and Modification

[supergalley]

Hi everyone. I could just replace this beacon which is meant to be rated for 12v and 24v DC operation but which burnt out when connected to a HGV 24v supply, but I thought it would be an educational experience to go through the motions of diagnosing the fault and shortcomings of the circuit itself.

I don't know if the power transistor is NPN or PNP, I guess it is evident from wether there is a positive or a negative pulse from the IC.

Is there anyone clever enough to work out what the Integrated Circuit actually is and also which model of power transistor would be a suitable replacement? I would imagine that someone with a little electrical experience would be able to work out wether the power transistor was NPN or PNP due to pin one being grounded, pin two taking power from the LEDs and pin three being the Base. (I would assume the first two pins were either collecter or emitter and this would give it away as PNPs take their positive from the Emmiter and vice versa)

The only ICs I know of are the 741 and the 555 but I can't quite confirm these as the wrong pins have been used on the IC for ground, +V, the RC's and the outputs as far as I can tell from pin-outs I have found on Google images.

R1 & R2 are Brown, Black, Red
R3 Brown, Black, Orange
C1 is marked 104
C2 is 100 microF Electrolytic
D1 is marked C5V 5V (not sure if Zener or standard germanium)

Also can someone please point out why this circuit would blow it's power transistor at between 24-28.2v and what would be the best method of piggy-backing some protection for it without disproportionally dimming the LEDs in operation. Is the IC adequetly set up to be safe at such a wide voltage range of 12 to 28.2v that should by design have been able to cope with it?

Many thanks

 

[Heliman]

According to your schematic, the arrow points out on the transistor. That would be a NPN.

 

[supergalley]

You're totally right there. I don't know if it is or it isn't NPN though, I just drew it that way without thinking!

 

[Alec_t]

I think the 'NPN' is more likely to be a logic-level N-channel MOSFET, given the appearance of that LED array. The array looks quite current-hungry.
Whatever that IC is, you could replace it with a 555 and re-route the connections appropriately.
As for why the circuit fried, it could be that the 5V zener failed, so everything got hit with 28V. I'd be inclined to replace all the components. There aren't many, after all.

 

[supergalley]

Oh yes, didn't even think of a MOSFET.

Good call.

I think I'm going to test the timing circuit first though before I try bodging a mis-terminalled IC in there. I know for sure that the MOSFET is cooked as it is physically damaged. I just assumed that the timer circuit was damaged as well because the MOSFET was in a permanent closed state, and had a temperamental Emmiter connection on that leg but the Base end was in good condition and there was no pulse effect observable on the LED array at all so I just assumed there was no 2Hz input.

I'm just waiting for delivery on a Lab PSU off eBay so I can power up the circuit at home to use my test equipment on.

Thanks for your advice.

I think the 'NPN' is more likely to be a logic-level N-channel MOSFET, given the appearance of that LED array. The array looks quite current-hungry.
Whatever that IC is, you could replace it with a 555 and re-route the connections appropriately.
As for why the circuit fried, it could be that the 5V zener failed, so everything got hit with 28V. I'd be inclined to replace all the components. There aren't many, after all.