Electronics > Repair

Repair old Hurst tachometer

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4cx10000:
Hi!

Yesterday a guy came in with an old Hurst, two digit tacho meter from the mid 70's. No sign of light from the display he said that it smells burnt from one of the circuit boards. At a quick look I found it to be a small dc/dc converter because the display board has a two digit VFD. A tantalum electrolyte was found blown.

At first I did not see that the transistor also was blown so I only swapped out the tantalum with a radial 47uF/25v which was part of the incoming low pass filter. Hooked up the tacho meter to my bench psu and limited the current to 100 mA and the current draw was found to be no more than 60 mA. The display did not lite up and accidentally, I took this board in my hand and I can promise you that there was quite a high voltage that measured 220 volts. I do not know why the dc/dc converter delivered such a high voltage thinking of the blown transistor.

Anyone recognise this transistor? Marking is partly gone so I would say that it could be really hard to see.  :(

fzabkar:
Could we see photos of the tacho PCBs?

What are the 3 characters on the bottom row? EBC (emitter-base-collector)?

4cx10000:
fzabkar, yes of course! I am not in my workshop at the moment, but I will take some photos tomorrow morning. :)

4cx10000:
Yes fzabkar, it is EBC and a bipolar transistor.

4cx10000:
Here are som pictures of both PCB's - dc/dc converter and display board. I will make a schematic for the dc/dc converter as soon as possible; not that I have to, but just for fun.  :)

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