Vaxstation 400/90 power supply

On Mon, 23 Mar 2020, Brent Hilpert via cctalk wrote: [...] A correlation with an expired DS1287A chip seems plausible, though not fully verified. I have a 4000/90 that seemed dead having been stored alive and left unused for a prolonged period. The symptom were all LEDs steady lit after power-up, as if the CPU has failed to execute any instructions after coming out of reset. Multiple consecutive power-ups didn't help. The 4000/90 has a couple of potential reasons as to why its CPU would not start executing: - a broken CPU (obviously) or chipset, - flash ROM corruption (with no write-protection provided regrettably all too easy to happen), - bitfile ROM corruption (used with the CEAC and SQWF FPGAs in the /90 only; replaced with ASICs and the ROM removed in the /90A and the /96), - power-good signal not asserted by the PSU. NB the LED latches sit on EDAL (as does the DS1287A), so if the CEAC FPGA does not come up they won't budge even if the CPU executes from main flash ROM. If I were to believe documentation, I'd expect the very first couple of instructions executed from the reset vector to poke at the LEDs rather than the DS1287A to indicate execution has started, but I haven't actually looked at the code there (I should have!). I didn't therefore consider the DS1287A, especially as it was already dead in that machine before it was left to sit. It appears to make a difference however whether the DS1287A has been only discharged enough not to keep the contents across a power down or whether it has been indeed deeply discharged (the chip obviously continues to suck power from its lithium cell even past the point it cannot hold contents spares, but it didn't help. Frustrated I put the machine aside and left it untouched for a couple of months. I came back to it once again, disassembled, reassembled, disassembled again and verified the tantalum capacitors in-circuit (all measured fine) replaced the PSU, replaced the DS1287A a couple of times again, and then suddenly with one of the replacement DS1287A chips it started to work. I tried the chips that didn't again and oddly the machine continued to work, and kept working on many occasions for a couple of years more until I last checked it sometime last year. Hopefully it still does (and I plan to replace the DS1287A once expired with one of my reworked chips as shown here: <ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/ds1287/>; so far I only did the replacement with a couple of DECstation 5000 systems), however I cannot verify that now due to the current chaos in Europe making travel impossible and keeping me away. I need to warn you too that the H7819 PSU the 4000/90 uses also suffers so it's at risk of developing catastrophic leaks (I don't remember offhand if the capacitors are safely upside down as in the H7821, and I can't easily check right now; I highly suspect it is the case though, so try keeping the machine in the right position while in storage). Furthermore much of the contents of this PSU has been treated with hot melt, making any diagnosis or repair a real pain. I would recommend examining the PSU just in case anyway. It might be on the verge of giving the power-good signal. I realise I may have added confusion rather than helped, but at least there may be hints as to the causes for odd behaviour and a hope for full recovery. Maciej