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I came across this circuit in pioneer brand vsx522 model amplifier circuit diagram. This supposed to initialize the microcontroller during AC power up.

enter image description here

But it looks like wrongly drawn. The resistor on the left is directly connected to the 3.3V linear regulator which is powered by a tiny mains 240V 50 Hz standby transformer. The output of this circuit, at least in my perception, is used to initialize the micro controller and it looks like will locked up at high state (3.3V) indefinitely. The transistor and everything else around it cannot pull the RESET to low.

Am I right? Or do I miss anything? The values of components are not fully specified.

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    \$\begingroup\$ Is this copied from the actual Pioneer schematics or did you redraw it with your own schematic editor? Sometimes very small details can make a difference. \$\endgroup\$ – owg60 Nov 18 '16 at 18:14
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    \$\begingroup\$ Can you post the actual schematic. One of the diodes may be a zener. \$\endgroup\$ – Spehro Pefhany Nov 18 '16 at 19:13
  • \$\begingroup\$ Its not redrawn by me but captured from service manual. drive.google.com/file/d/0B-6wFWf1VJ38MWt6RlA2TGtBRWs/…. Pls see at lower left of page 15. \$\endgroup\$ – soosai steven Nov 19 '16 at 2:34
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At first I thought there were 3 errors and then considered what the spec should be.

Either way, it looks wrong because there is no base DC pull down to 0V so cap. current makes it have a hair trigger on Vc spikes via diode capacitance .

If you put 30pF across each cap and make input R=100 and 1M to R to ground, you might see a 10us reset pulse. but act like a glitch detector and not a good PoR.

  • ok for logic but uC usually expect 100ms

  • if used for uC reset then reverse top diode and swap bottom R and C.

    • then it will protect Vbe and ignore input glitches with high in, low out for base RC time constant to drop to 0.5V from input high to guarantee output high after >100ms
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  • \$\begingroup\$ It could be as simple as the top diode is wrong drawn in reverse direction. But how such a thing can happen in well established company's technical document? \$\endgroup\$ – soosai steven Nov 19 '16 at 2:39
  • \$\begingroup\$ it's a crappy reset circuit with leakage dependencies. I'm surprised too. \$\endgroup\$ – Sunnyskyguy EE75 Nov 19 '16 at 13:09
  • \$\begingroup\$ I'm still working on the actual unit. Will find out how exactly its built in real design. The values of the R and C should shine some light on this curious design. \$\endgroup\$ – soosai steven Nov 19 '16 at 14:25
  • \$\begingroup\$ far better approach is RC-CR with Schmitt Trigger, the lack of values in repair manual indicates poor document quality, or intentional IP protection. even intentional errors. BTW I liked your FB video on XFMR fire. Too bad they don't monitor PD to avoid these failures. Ask them if they want better protection. \$\endgroup\$ – Sunnyskyguy EE75 Nov 19 '16 at 16:37

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