8573-P75 Failing Capacitors
P75 Planar Capacitor Locations
Diamond Jim (Shorney) says "25 V 105 °C radial lead can be used for all units."
Removing Original Capacitors
Diamond Jim (Shorney), computer repair for the stars wrote:
Temper Tantrum Tantalum
Diamond Jim put one of his starlets on hold long enough to type:
P75 Plasma Adapter Capacitors
Minority Report: Fixing 14902 Plasma Error on P75
Peter Wendt spoke:
I took the P75 apart and removed the videoboard. Then I replace the
5 round, silver electrolytic caps on it. These are of the same type and
style of those that cause the problems with the IBM
[Almost] forgot to note the discolored and greyed solder pads. Usually a good indication for either a bad spot or "something chemical" happening on it. I hand-tested the removed caps and two have an almost open circuit and the two 33 µF have a rather low internal resistance and heat up when applying 12 V permanently to them. Oof.
These caps are worth shit on the long term. Back then they were the smallest types you could get in that capacity range - but time takes its toll and the effects we experience today is the price to pay. I think these caps are responsible for a lot losses of gear in the past years, starting with FDDs as well as cards and planars.
After reassembling the system came up as normal with the orange plasma screen and only a single "OK" beep. As said: I don't really know if that was the original cause. That particular machine often greeted me with a plasma display error and sometimes it automagically resolved "by itself" with running the video tests from the advanced diagnostic and running Set Configuration once again. This time it didn't and that was the reason to replace the capacitors in a first step before delving deeper into the problem.
Let's see how it behaves on the longer run.
Dr. Jim Shorney examines his patient and says:
> After reassembly, the system came up with the orange plasma screen and a single "OK" beep.
After thinking about it for a while, the P75 finally decided to put video on the plasma. 162, 163, and some text gobbledygook. Pushing on the DAC32 IC caused the gobbledygook to turn into the NOT OK symbol and the IBM manual graphic. IMO, the DAC32 IC probably needs to be resoldered - a project for another night. This IC did have the worse corrosion over the CRTC32. Worst case, I may need to torch if off the board and clean the leads and solder pads. Ugh...
Conclusion: these caps do go bad. And they DO leak. Examine solder connections on ALL such capacitors in the P75, and nearby soldering. They should appear clean, smooth, and shiny; if there is and graying, roughness, or obvious corrosion, the capacitor should be replaced immediately. I notice that the planar also uses many such caps (William, take note!). It might not be a bad idea to replace them ALL as a preventative measure. Radial lead electrolytics are an acceptable substitute, and can be laid over on the PCB if clearance is a problem. I recommend using caps that are rated for 105 °C temperature for ANY replacement, regardless of ambient temperature; they are better constructed and last longer than 85 °C caps. Order from DigiKey, Mouser, or your favorite parts house.
Ed. Beware that some caps may handle 105 °C, but their working voltage is reduced. Also, pay attention to the MTBF, the smaller sizes tend to be affected more at high temps.
> As said, I don't really know if that was the original cause.
I think it's safe to say that we are onto something here: the capacitor(s) of death. The other P75 with the garbled text mode display will go under the knife on another evening. As I find time, I will try to come up with a complete capacitor list for the P75 video and planar.
MCA Mafia members, DO NOT throw away your dead P75s!
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