Electronics > Repair

HP/Agilent 4284A/4285 electrolitic capacitor defect & A2/A3 board schematic

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scroky:
Hi ZhuraYuk,
thank you very much for your picture, now i can proceed to the repair of the instrument. Best Regards
Alessandro

scroky:
Hi everyone,
Thank you again for the photo and advices.
Since the resistor burned out in a few seconds because a downstream electrolytic capacitor was shorted, I don't think it's a 2.21 MOhm resistor, but more likely a 21.5 Ohm resistor.
In fact, I think the maximum voltage on those boards is 24V: the calculation p=V^2/R gives 2.7 x 10^(-4) W, less than a milliwatt.
Looking at the colors, the value 2.21 MOhm seems correct, but I have a doubt: can you please measure the resistor in question?
Attached is a photo of the same board in which I highlighted the component with the illegible name that seems to have heated up a lot. The case is that of a TO220 power transistor. Can you please read what is written on that component? Notice that there are two TO220 components placed one behind the other very close, the first (the unknown and highlighted one) hides the second.
can you help me once again, please?
Thanks
Alessandro

ZhuraYuk:
The TO220 reads 7912CT  NLFI 012. I did not desoldered resistor, in scheme it measures as 21 Ohm for DC current and 19 Ohm measuring with LCR meter.


scroky:
Hi ZhuraYuk,
from the new photo and your measurements. I would say that there are no more doubts:
RED, BROWN, GREEN, GOLD (x0.1) and BROWN (1%) so: 21.5 ohm (1%)
Thanks for all.
Best Regards
Alessandro

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