SRAM woes, ISA bus diagnostics

[CkRtech]

scope the 5v & 12v lines on the motherboard psu connector.

I can't get to the psu connector with the probe as-is (unless I jammed a paperclip down into the plug and then clipped on to that). I did see a bit of hard drive activity (for example) inside the small waveform (~100-150mv ptp) of +12V while scoping +12V at the ISA connector. The voltage didn't dip on the whole.

Solid Tantalum caps don't loose capacitance with age.  There's little point in replacing them unless you suspect they have been overstressed, or degraded by prolonged moisture penetration at elevated temperatures, or unless one of the same type on the same rail is showing signs of running hot or has already catastrophically failed.

I was wondering about that. I've certainly had to replace vented/leaking/failed electrolytics for various projects, but I haven't done anything regarding tantalums. This computer was stored in an attic for awhile (years) and probably saw temperatures of about 90F-110F (~32C-43C) @ 100% humidity during the hot times of those years.

However if the PSU is also vintage, recapping that selectively (All small Aluminum electrolytics + any large ones that fail on capacitance below lower tolerance limit, high ESR or physical signs of distress + any others directly in parallel with a defective one) is probably worth doing.

The PSU is also vintage, however I switched to another PSU (A younger ATX + AT adapter) that I keep on hand for testing after I first saw the problem. It was one of the first things I did. Will most likely crack open and recap the original PSU once I stabilize the system with this one... assuming there are no concerns regarding this one.

Do all of the items I pointed out in the previous point cause any concern other than power?

[CkRtech]

Using a scope with the same power supply on a different motherboard produced a much more solid value for the CLK signal. No hard drive or graphics activity was represented in the waveform.

I've been probing around a bit on the resistor networks (most reading "B 330J") on the problem board. Each of the ones I have hit have a single white square marked on the PCB to one side - so they should be bussed. That being the case, how much can I really determine from an ohm check with them still in circuit?

Walking one probe across pins 2-8 with the other probe on the common pin while each resistor network is in circuit yields the following (three RN examples) -

Name	pin 2	pin 3	pin 4	pin 5	pin 6	pin 7	pin 8
RN1 -	33	650k	650k	650k	650k	650k	650k
RN2 -	33	650k	650k	650k	650k	650k	650k
RN3 -	33	650k	650k	2M	2M	2M	2M

Those that measure 2M on RN3 start near 1.8M and climb quickly with the probe attached. Moving to the next pin in the same RN starts at the same value as the previous pin.

RN3 is far from the previously corroded area.

In a similar fashion, pins B20, B28, and B30 on any given ISA connector on the problem board start at about 1.5M and quickly sail upward when being given a continuity check to ground (ground pin B1 on the ISA slot, specifically). In contrast - B20, the clock pin, held at 4.7k when measured on the working 486 motherboard. Naturally the boards didn't have the exact same values for a matched given pair of pins (i.e. Problem A1 vs Working A1) between them, but a stable 4.7k on working board's B20 vs a sailing 1.5M on the problem board's B20 seems to be a stark contrast.

Pardon my naivete, but can I draw any conclusions from this?

[Rasz]

desolder rn3 and measure again

[CkRtech]

Hmm. Guess they were isolated instead of bussed. 33 for each pair.