andy120 wrote on 2024-11-12, 16:42:

new old bare board. have a dallas chip modified for coin batter and on pcb oscillator with decouple caps. 29ee010 bios chip. with 2 ram sticks and cache installed, the boot code card just has - - - - volt on evcept 3.3v. where to start working out its issue?

If the POST card is installed in the PCI slot, missing "3.3V" is normal on 486 boards. Those boards don't supply 3.3V to PCI slots. If the card is installed in an ISA slot, the 3.3V is actually a -5V LED (assuming you have the "standard Amazon/AliExpress PCI/ISA POST card"), and it is normal to not have -5V if the board is supplied from a newer ATX supply with an adapter. In either case, that voltage should not prevent booting. Your post card should also have a RESET LED. That LED should light a short moment after powering on and while the reset button is pressed, but be off all the other time. If the RESET LED is permanently one, something is wrong with the reset generation. To get something onto the POST card, all you need is a BIOS chip with a proper image in it and a working processor. As far as I know, the HOT-433 is not auto-voltage, so a no-POST issue might be caused by powering a 5V processor with 3.3V. Powering a 3.3-3.6V processor with 5V will likely work long enough to get some POST codes, but is absolutely not recommended as well.

You do not need any memory or cache installed to get to the first POST code. You also don't need any PCI or ISA card except for the POST card. I thus recommend to remove everything except the CPU and the POST card from the board and check whether you get POST codes then. Also make sure the processor is correctly fitted into the socket. It ocurred multiple times to me that a no-POST issue was caused by a processor that was not fitted correctly into a ZIF socket. You can run any kind of 486 processor for some minutes without a fan and heatsink (but please don't do that with Athlons!), so if it helps checking whether the processor is correctly installed, removing a clip-on fan for a first test is no problem.

If you still get no POST, you might have a broken trace on the board. Please check for visual damage. Buzzing traces from the processor socket to the VL slot is also an option to check for breaks near the processor socket. Another issue except broken traces may be a short between two traces, e.g. when pins are heavily bent on the solder side, so they touch the next pin.