How to repair broken QFP-100 socket for i386SX(solved)

[TheSteve]

I picked up a broken IFR 2.9 GHz spectrum analyzer the other day. It has two known issues - the CPU socket is broken and the LCD doesn't seem to work. I have held the CPU in by hand and powered it up with an external monitor and it appears to work. So before working on the internal LCD issue I need to get the CPU installation taken care of.

The Intel 386 is a 100 pin PQFP package. The CPU is held by the "cap" of the socket. You place the cap/cpu combo into the socket and flip down 4 doors that lock everything into place. The problem is 3 of the 4 doors are broken.
The socket is a 3M 2100-7243-00-1807

-If I could find new clips I could pop them into place, haven't come across a source for them.
-I haven't found any current socket that could be installed in place of the existing one.
-I could go crazy with some zip ties but that would probably just break the socket clips itself.

So what are people's thoughts on how I might repair this?

And yes I was surprised to find a 386 inside this thing, I found the 386 itself wedged between some other PCB's and had to straighten the legs very carefully.

[Nusa]

Do those 4 rivets go through to the other side of the board? And do those 4 holes in the carrier fit over the rivets? If so, you may be able to rig something with wire and springs to hold the carrier in place. Or variations on that theme.

[CJay]

It is possible to solder the chip to those sockets (seen it done) but there were quite a few PC motherboards which used those sockets, laptops as well.

I *think* the same sockets were used in embedded products so a search of industiral surplus might turn up a few.

I've also got a memory that they were used on some of the Cyris 486SLC upgrade boards but they're likely to be hens teeth rare now.

I'd be surprised if the socket wasn't still available from someone like Aries?

[Twoflower]

By 'held the CPU in by hand' you mean you just pushed it down into the socket? If yes, you might check at some metal shops if they can bend you some custom made leaf-springs spanning from one side of the socket to the other, holding into the holes of the socket and pushes down the processor.

An alternative idea would be a 3D-Print of the clamps. The question would if a print would be strong enough to do the job.

But why are three out of four clamps broken in the first way? Got the material brittle by out-gasing the softener? If yes the same problem will happen to the socket itself. Especially the part of the socket that hold the clamps. In this case a replace of the socket may might sense.

[TheSteve]

Sadly the metal posts/rivots do not extend into the PCB.
I assume the clips were broken due to abuse - they were delicate 20 years ago when they were made. They are made of a different material then the socket itself.

I do have a 3D printer and can use Fusion 360 but don't think anything I can print from PLA will be strong enough given the size of the area where it would be to clip to. Also the force needs to be applied at each edge. The original force per pin is 10 grams, with 100 pins that means 1 kg of force.

At this point the leading solution is to trim off the edges of the socket that the locking clips are connected to and solder the CPU to the socket itself. If the socket is trimmed I should have pretty good access.

Being I have had the machine boot several times when pressing the crap out of the socket I am pretty sure the CPU is good. I am thinking the dead internal LCD is related to the video card/frame buffer as external video works. The enable line for the LCD backlight isn't even working.


edit - decided to go for it. I cut the remaining clip bits off, fluxed it up and went to town soldering the CPU to the pins from the socket. Took about 30 minutes of careful soldering, checking and then cleaning.
The results are a winner (so far anyway)

As a bonus the internal LCD came to life as well - maybe fluke or it may be because I removed the video chip from its socket and cleaned it. Hopefully it keeps working.

So far the unit passes all self tests and I've verified the spectrum analyzer and tracking gen are working 0-2.9 GHz.

[CJay]

Brilliant, I'm pleased it's working and now I'm also envious, if the TG option is working that is (it's about all I'm missing on my CMU200)...

[TiN]

Now let's see all the plumbing, shall we? 

[Nusa]

Very good fix!

It seems weird to me that the designers chose to socket most of the large chips, yet the memory modules are soldered in place. And the solder joints look to be the only mechanical support.

And the size of the backup battery bank....wonder how long it's intended to last.

[CJay]

Those memory modules look like 30 pin SIMMs, while it's note necesarily best practice I think there'd be plenty of mechanical support from the soldered connections and it's almost definitely leaded solder as well plus I doubt an instrument like that would be in a vibration environment, I'd be more worried about the PLCC chips 'walking' out of their sockets over time.

[SeanB]

Technically SIP, Single Inline Pinned, which were either soldered to the board, or which were plugged into a 30 position 0.1in turned pin socket. I had a lot of those at one time, a whole 1Mx8each, and even a few that were 4Mx8 as well, double sided at that. Even a few replaced 256kx8 ones. Some HP printers used them as well to upgrade the memory.

[james_s]

I wonder why they socketed the CPU in the first place? That's the first time I've seen a 386sx in a socket, usually those were soldered to the motherboard.

[SeanB]

Plenty were socketed, as it was on early machines an expensive part, well worth the price of the socket to keep it from thermal shock, and later on the socket made it easy to do a speed upgrade mid life by simply using a faster version and changing the program to take advantage of that or correct timing loops for the faster processor, and of course changing the crystal oscillator block, also often in a 16 pin DIP socket, with a zip tie holding it down to the board. I remember the very early ones also ran pretty hot, so a socket made repair easier if it got flaky after a few hundred thermal cycles.

[james_s]

The 386DX systems I saw all had socketed PGA CPUs but the 386sx was the low cost version, and QFP package was intended for direct soldering to the board. We had a cheap Packard Bell 386sx back in the 90s and it had a soldered CPU, as did the others I've worked on. That doesn't mean socketed 386sx machines were not around but I never saw one. That socket must have been an expensive part.

[TheSteve]

There are a crap load of sockets in this thing, the entire thing is custom. The motherboard has 2 custom plugins cards, one for video and one that interfaces to the RF section. The front panel also plugs in. IFR always did make pretty serious hardware - nothing cheap. It does seem strange to me that they used a CPU socket but the ram is soldered in. It does have a crap load of batteries on the MB for backup and they all seem to be good, the clock was still set correctly.
I've fully reassembled the unit and have been testing it, so far it has performed flawlessly. Having the tracking generator is great for radio use. Add cheap return loss bridge or directional couple and it will measure return loss to 2.9 GHz. I've also tuned some filters etc.

Check out the crazy RF section hardware that was needed back in the 90's.

[Nusa]

Re the last picture: If I'd known there was a nice metal frame surrounding the CPU socket, I would have suggested a spring-loaded crossbrace as a quick fix. And maybe a support block underneath.

Still, you ended up with the best solution.

[TheSteve]

Re the last picture: If I'd known there was a nice metal frame surrounding the CPU socket, I would have suggested a spring-loaded crossbrace as a quick fix. And maybe a support block underneath.

Still, you ended up with the best solution.

I considered that as well but it wouldn't have been a good idea as there is no support under the cpu and it would have had to press on the entire motherboard.

[TheSteve]

Wow, 20 years old computer with SSD! History is indeed repeating itself.

Indeed, a 1 megabyte SSD!

[james_s]

I would definitely keep an eye on those batteries, even if they're working fine still now, the issue is that they can start to leak and you may not notice until it's too late.

[wraper]

I would definitely keep an eye on those batteries, even if they're working fine still now, the issue is that they can start to leak and you may not notice until it's too late.

Those Lithium Thionyl Chloride batteries won't leak and are good for many decades unless discharged by external load.
EDIT: Tadiran claims 25 years operating life. In reality will be more.

[james_s]

I'm just nervous having had first hand experience of equipment destroyed by leaking batteries. NiCd on several Amiga computers, one of which was damaged so badly I still don't have it working, and a lithium memory backup battery leaked in a Mac IIfx and completely destroyed the motherboard beyond repair, rusted out most of the metal parts in the case as well.

[helius]

...several Amiga computers ... a Mac IIfx

...Because the companies cut costs by not using Tadiran. Look closely and you can see that their batteries are constructed completely differently from others. There is a rigid polypropylene skeleton that can't corrode away.

[TheSteve]

I've never seen a Tadiran cell leak - this thing was clearly made with no thought of pinching pennies so I am glad they used them.

[james_s]

Some Macs used Tadiran cells, others had Maxell. I *think* the IIfx had Maxell, there were two batteries in it and they both leaked. Perhaps I'm just overly paranoid but in the unlikely event that a lithium cell does leak, it creates huge carnage inside equipment. Even if Tadiran cells hardly ever leak, there's a first time for everything and I would think it prudent to check it periodically.

[TheSteve]

Even if Tadiran cells hardly ever leak, there's a first time for everything and I would think it prudent to check it periodically.

For sure - that will have to be left to the new owner. It is a nice piece of gear but it overlaps with stuff I already have so at some point it will be sold.