Ardent Tool of Capitalism

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:
   It's easier than that, actually. Whatever is happening chemically with the goop from inside the caps seems to weaken the solder joints, which are already thin to begin with. What I have experienced so far is that trying to heat the corrupted solder joint seems mostly futile, the solder does not want to melt easily. Rather, the SMD legs actually pull free of the contaminated solder before it has a chance to melt. I wouldn't try just pulling the caps off as the default method, there is always the risk of pulling the pad up. Do try to desolder, but don't be surprised if they come off without meltage.

Temper Tantrum Tantalum

Diamond Jim put one of his starlets on hold long enough to type:
   Solid Tantalum are not tolerant of surges or spikes, so are not suitable for power supply decoupling when connected directly across the main supply voltage. They can literally explode. Fosgate had some real problems with this in the early 80's car stereo amps. I don't know about organic Tantalum, I've never used them.

P75 Plasma Adapter Capacitors

Minority Report: Fixing 14902 Plasma Error on P75

Peter Wendt spoke:
   Most likely this report does not apply to all cases of a P75 (and probably a P70 as well):

1. System comes up with a black plasma and double beeps.
2. Attached external video works and shows a 14902 Plasma error.
3. In the advanced diags the plasma display is still shown but testing it ends with an error "Replace Plasma display".
4. Testing the display high voltage (smaller black connector wrapped in shrink isolator rubber, 6 positions, only 5 used) between blue and white shows +180 VDC, between blue and yellow +5 VDC, between black and red +5 V as well.

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
PS/2 floppy drives. There were 3 x 10 µF / 16 V and 2 x 33 µF / 16 V. I had the 10 µF but not the 33 µF, so I took 47 µF for them, which should not cause any trouble, since they are DC buffer capacitors only.

[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:
   I started with the grungy P75 with no plasma video. I found six caps on my video board. The solder connections appeared discolored and corroded, and one had lost electrical and mechanical integrity entirely. The pads were somewhat difficult to clean off, I had to scrape a couple of them with a knife. Additionally, the pins of DAC32 and CRTC32 closest to the nearby cap also had visible discoloration. Note that I did not remove the video board to replace the caps, they were all accessible.

> 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!