Corroded legs on IIsi CPU

[JeffC]

I'm doing some troubleshooting on my IIsi board. I put it under the microscope this morning and took a bunch of photos so I can sit down later and take a very close look for broken traces, however as I was taking the photos all the traces looked in excellent condition, even the ones around the old caps (recently replaced). I did notice the legs of the CPU closest to C34 were looking pretty grungy. I used a pick and pushed on one of the corner pins, and it broke loose from the board. Fortunately, the leg itself looks in decent shape and doesn't look like it is going to fall off the IC.

I'm looking for suggestions on how to deal with this. I am working with a nice solder station and a cheap hot air station. I have recently taught myself to solder SMD ICs, and I am pretty good with chips like the RAM and UE5 chips, but I have never done any fine-pitch work. From what I have seen on youtube videos the CPU might be a good candidate for removal with ChipQuik and re-soldering using solder paste, but I have never used either. The other option I see is to scrape the pads and legs as clean as I can with a pick, add a bunch of flux, and re-flow.

Any suggestions? Thanks!

Edit: added second photo of CPU, zoomed out

 

[nottomhanks]

Would you be open to repairing other IIsi boards?

 

[YMK]

Apply flux to the pins, reflow with hot air. Clean with acetone, repeat.

 

[JeffC]

Apply flux to the pins, reflow with hot air. Clean with acetone, repeat.

I ultimately wrote this board off and repaired another IIsi board I had. I did a bunch of work to this one and could never get rid of the death chime. I replaced the onboard RAM with known good chips, replaced all the ALS245s, beeped out all the traces going to/from the RAM and ALS245s. A bunch of other stuff I don't recall the details of as well. There is probably a broken trace somewhere but I wasn't able to find it. Since I had another board that was a much easier fix, I have relegated this one to a parts board.

@nottomhanks sorry no, I am hesitant to work on other people's machines, and definitely not a IIsi board. They are notoriously finicky. What is wrong with yours?

Edit: @YMK I ended up removing and re-soldering the CPU. I made multiple attempts to reflow the legs, there were a couple that I just could not get to take. I scraped the legs and pads, added flux, and after many attempts I was never able to get a couple of the pins to make a good connection to the pads. Just too much grunge on the pads/legs. After I removed the chip and cleaned the pads it soldered back down nicely.

 

[blturner]

I did a bunch of work to this one and could never get rid of the death chime. I replaced the onboard RAM with known good chips, replaced all the ALS245s, beeped out all the traces going to/from the RAM and ALS245s. A bunch of other stuff I don't recall the details of as well. There is probably a broken trace somewhere but I wasn't able to find it.

I am dealing with a IIsi exactly like this and have gone through the exact same steps!

 

[JeffC]

I am dealing with a IIsi exactly like this and have gone through the exact same steps!

I was able to fix my board, I'm not sure what the original problem was, probably related to the ALS245s. The final issue I was able to fix was related to poor installation of the replacement RAM chips. I did not have a lot of experience soldering PLCC-package chips at that time, and there were some legs that looked like they were correctly soldered, but were not. If you replaced any PLCC chips do a very careful inspection with a jeweler's loupe, microscope, or macro mode on your phone camera to make sure the legs did properly solder to the pads. The way the legs curl under the chip body make it difficult to inspect the solder joints.

What troubleshooting/repair steps have you taken so far?

 

[blturner]

What troubleshooting/repair steps have you taken so far?

Well, this board was previously working before I washed and re-capped it. Afterwards, the issue was that it would boot, but only without RAM SIMMs installed which left it with only the 1MB onboard RAM. At first, I was suspicious of the RAM buffer chips, so I swapped those with another board. No change. I removed and re-installed the onboard RAM with chips from some 256K SIMMs I tested in a IIci. I only found one broken connection between the RAM and the ROM which I fixed with a bodge wire. The issue persisted. After some more testing the board started death-chiming consistently, and UNFORTUNATELY when re-inspecting the board, I noticed that some CPU legs were totally off their pads. At one point I tried to reflow some of the pins that looked corroded, but I didn't noticed that they shifted position. I attempted to fix them, but in doing so, two pads came right off so I stopped. I'm not sure if there was more corrosion than i realized or what.

Luckily, I have two IIsi boards, I was always planning to use one as a parts board, so I guess it will be this one now! I moved the new caps to the other board which I had already de-capped and cleaned. It currently doesn't chime, but does power on with a blank white screen. I'll probably start a new thread for that one when I get around to working on it.