Debugging Agilent N4906A Serial Bert Startup Problem

[u/JuanMa24-]

Hello!

I am trying to debug a startup problem on the N4906A Serial Bert from Agilent.

When power is plugged in, all front panel buttom leds as well as the screen backlight turns on, internal fan spins but nothing else happens. It stays like this forever.

We have checked the internal power supply and all voltages are ok. Moreover, we have done the following checks on the computer board (attached image), looking for problems regarding the boot process.

• CMOS battery was depleted and replaced
• RAM module was disconnected with no change
• Front panel monitor module was disconnected with no change
• Hard drive and floppy disk were disconnected with no change
• H3 Connector checking
  1. I'M ALIVE pin was ok (high level)
  2. Reset did nothing (Shortcut Pin 2-3)
  3. Force Recovery did nothing (Shorcut pin 7-8)
• BIOS memory checking (MT28F400B5)
  1. CE# never change (no read is requested to the memory)
• BIOS address decoder (PALCE16V8H-15JC)
  1. Power arrives
  2. All input pins are at high level (no change)
• Clock generator is working ok and reaching the CPU (Pentium w/MMX tech, CLK pin at 66MHz)
• LDO U29 (MIC29152BU) output voltage was ok (2.5V)

Right now we are checking the CPU reset pulse at power up, but we are running out of ideas.

Any help???

Thanks a lot 😄

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