VOGONS


First post, by PcBytes

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I've been keeping it for a long time (it was upgraded to a Zida 6DLX after the issue happened) but I'd like to try and fix it if possible.

Issues:

- will POST ONLY with any non-MMX Pentium chip, Cyrix 6x86 non-MMX and any CPU that does not require dual-voltage

- will NOT POST with any MMX Pentium chip, AMD K5 (strangely) and any K6-2 chip

- will also NOT POST with any AGP video card

Any ideas where the issue could be? I'm suspecting either the PSR16C40DT near the CPU socket or the CEB703AL MOSFET right below it. Board has already been recapped with newer OST capacitors (2007, old caps were marked En and were dated around November 1998)

Finally, I'll leave a few photos of the board. The CMOS holder is not my work and is exactly hoe I received the board.

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Main: i5-3470, 16GB RAM DDR3, Gigabyte B75M-D3H
G.T.2: P2 266MHz, 256MB SDR, Zida/Tomato 6DLX "LX98-AT"
Shaman: P3 650MHz, 384MB RAM, Soyo SY-6BA+ IV

Reply 1 of 5, by Roman555

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PcBytes wrote on 2020-06-29, 11:30:

- will also NOT POST with any AGP video card
Any ideas where the issue could be?

I suspect it might be a trouble with a 3.3v voltage regulator that is near dimm slots.
Did you measure voltage of the regulators?

Reply 2 of 5, by PcBytes

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You mean the big PSR16C40DT near the SDR slots?

Main: i5-3470, 16GB RAM DDR3, Gigabyte B75M-D3H
G.T.2: P2 266MHz, 256MB SDR, Zida/Tomato 6DLX "LX98-AT"
Shaman: P3 650MHz, 384MB RAM, Soyo SY-6BA+ IV

Reply 3 of 5, by Roman555

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PcBytes wrote on 2020-06-29, 20:03:

You mean the big PSR16C40DT near the SDR slots?

It's a part of the step down regulator. So measure voltage after the coil (I think it's the left pin) or on the left capacitor (see your last photo). My idea is the voltage drops when load becomes bigger. Maybe the capacitors of this VRM was soldered not properly during recap.

Reply 4 of 5, by PcBytes

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I have 3.27 on both pins of the PSR16C40DT near the SDR slots, and 2.8V on the cap next to it. The one at the corner of the board (to the right of the coil) reads 4.9V (so around 5v). This is without an AGP card.

With AGP card (Rage Pro Turbo AIW), I get 2.8V on both pins of the PSR16C40DT, and 2.79V on the capacitor next to it.

Main: i5-3470, 16GB RAM DDR3, Gigabyte B75M-D3H
G.T.2: P2 266MHz, 256MB SDR, Zida/Tomato 6DLX "LX98-AT"
Shaman: P3 650MHz, 384MB RAM, Soyo SY-6BA+ IV

Reply 5 of 5, by Roman555

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PcBytes wrote on 2020-06-30, 08:14:

I have 3.27 on both pins of the PSR16C40DT near the SDR slots, and 2.8V on the cap next to it. The one at the corner of the board (to the right of the coil) reads 4.9V (so around 5v). This is without an AGP card.

With AGP card (Rage Pro Turbo AIW), I get 2.8V on both pins of the PSR16C40DT, and 2.79V on the capacitor next to it.

Sorry, but I don't understand how you have measured voltages on SR16C40DT. I usually measure voltages relatively to GND rail.
But if you've measured voltages 2.79 between two pins of the capacitor than the voltage 2.79v is wrong - it should be 3.3v.

P.S. An article to understand better how the regulator works
Switching Regulator Basics: Step-Down Operation Principles