It’s alive! For the most part, that is. Turns out one half of one H-bridge in the interface is broken: M3 won’t turn right. Let’s check the transistors.
[#]FischertechnikComputing #fischertechnik
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Output Q6 of the 8-bit shift register IC4 only delivers around 1.6V to the base of T6 instead of 5V. And now I wonder which of the two parts are broken.
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Without T6 the voltage is back to 5V. So I guess that's it.
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Dang! Even with a replacement transistor the voltage at the gate is back to 1.6V…
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I've narrowed it somewhat down to transistor T18: if it's placed the voltage at the gate drops to 1.56V. If I remove it the voltage is 5V. But even if I replace T18, the voltage drop occurs. Maybe D11 is broken?
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Found it. D12 is shorted.
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Using a replacement diode I've found in my #Busch electronic 2070 studio center the output level at the shift register is now back to nearly 5V.
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Sigh. Now M3 won't turn in the other direction…
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Turns out one BD136 apparently got a little bit too hot. I’ve replaced it with one from the PC interface which I’m going to fix later.
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