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IBM 5150 "Type B" - A story and a blown capacitor C7 - What is it?

1

1200XL M.U.L.E.

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May 9, 2021
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While I wait for my 3.5" floppy drive for my Epson Equity 1+ to be delivered, I decided to buy an IBM 5150 on eBay. It was a low priced "barnyard find" and I understood there may be risks ... but I couldn't anticipate this one.

The computer came with several cards including a floppy controller, a HD controller, a monochrome graphic card, and a CHRONOGRAPH RAMPORT card. I took all the cards except the floppy controller (so I can boot). There was a Seagate ST-225 hard drive and I removed it also. I installed the CGA card from my Epson and connected my monitor to it all. After checking the DIP switches, I turned on the computer and to my happy surprise the computer booted! That means the power supply, motherboard, and floppy drive are generally OK. Unfortunately, I don't have a XT keyboard and I learned the hard way that AT keyboards don't work here.

Anyways ...

Now I try to run the HD controller and the Seagate 225. Oh, not good! The fan in the power supply barely turns and I hear buzzing sounds from the power supply. The hard drive is groaning. I quickly turn off the computer. The HD and the HD controller are out.

Next I try the CHRONOGRAPH RAMPORT card. Ohh ... ohhh ... ohhhhhh! If I could turn back time I would.

I inserted that card into a slot and turned on the computer. The fan spun up but made a slightly different sound. Nothing appeared on the screen. Suddenly, a capacitor in the upper left corner of the card popped and all the magic smoke inside of it escaped! The power shut itself off and I quickly turned off the computer.

I removed the RAMPORT card but now the computer won't boot. Sometimes the power supply fan spins up, sometimes it doesn't. If I remove all the cards then the power supply fan will always spin up. I swap out CGA card for the monochrome card and the fan spins back up. I add the floppy controller back in and the fan still spins. Connecting the floppy drive back kills the fan.

Oh, I felt so bad! What happened? What did I blow out? Can I fix it? I look at the board and I see nothing (yet).

Then I see it after much card swapping.

c7.jpg

I blew up a capacitor!! :( :(

What is this C7 capacitor? I'm OK with soldering and can mechanically replace this but what is it? The back of the neighboring capacitor says this.

abother_c7.jpg

"10-16"

What is this three-legged device? They are all over the motherboard?

Is there anything else I could have broke by plugging the RAMPORT card in?

Any and all help and guidance would be appreciated here.

Thank you!!
 
I will add that your adding/removing of cards and drives did not provoke C7 to fail.

And don't be surprised if another tantalum capacitor fails soon. Have any safety glasses? On newly acquired IBM 5150/5155/5160 motherboards, some people might see, say, three tantalum capacitors fail within the first 10 hours of operation, then after that, no more. You could be lucky, one tantalum capacitor only.

These things fail on cards as well. For example, capacitor C8 on the IBM CGA card.
 
There are C7's all over the board. They are not critical to the operation of the machine; they just help to filter out ripple and noise from the power rail. The ones closest to the power supply connector are the most likely to fail. They don't always explode -- sometimes they silently short and cause the power supply to shut down in protection mode.
 
Thanks for the replies, everyone! :)

Whoever runs the minuszerodegrees website deserves a medal! There is so much useful information there! Whoever you are, thanks!!

So these C7 capacitors can fail spectacularly and they can fail silently. Do they usually fail to an open circuit state or do they fail to a short circuit state?

I'm starting to think that if I am going to replace one of them then I might as well replace all of them. Would that be a safe and prudent course of action or am I overreacting? I counted 14 of them in the schematic hosted by minuszerodegrees. Does that sounds right? I could order 17 to have some spares. The site also lists the Mouser part number.

https://www.mouser.com/ProductDetail/KEMET/T398E106M016AT/?qs=VbGWdtQe/N64GFYncuFWHQ==

Are there any other parts that would be worth replacing in addition to C7?

Thanks!
 
1200XL M.U.L.E. said:
So these C7 capacitors can fail spectacularly and they can fail silently. Do they usually fail to an open circuit state or do they fail to a short circuit state?
Click to expand...
I don't know.

1200XL M.U.L.E. said:
I'm starting to think that if I am going to replace one of them then I might as well replace all of them. Would that be a safe and prudent course of action or am I overreacting?
Click to expand...
In my opinion, it is overreaction.

1200XL M.U.L.E. said:
Are there any other parts that would be worth replacing in addition to C7?
Click to expand...
I cannot think of any.
 
modem7 said:
And don't be surprised if another tantalum capacitor fails soon. Have any safety glasses? On newly acquired IBM 5150/5155/5160 motherboards, some people might see, say, three tantalum capacitors fail within the first 10 hours of operation, then after that, no more. You could be lucky, one tantalum capacitor only.

These things fail on cards as well. For example, capacitor C8 on the IBM CGA card.
Click to expand...

I agree 100%, I can't remember one failing once I start using the computer. Very common on any old vintage computer of any make.
 
The ones that pop are definitely a short circuit state. I've seen them go cherry red before they pop like miniature firecrackers. Wish I had taken a video of it.
 
The three leg design was to ensure they could not be soldered in the wrong way round. The two outer legs are actually connected together with the central one forming the other terminal, so no matter what you do, you can't get it wrong (well, as long as you have bought the right three legged cap, for some reason you can get them in two flavours, +ve centre or +ve both outer legs)

Except of course, three legged ones are hard to find now, so make sure you get your two legged ones in the right way.

http://minuszerodegrees.net/failure/failure - 3lt.htm
 
retrogear said:
The ones that pop are definitely a short circuit state. I've seen them go cherry red before they pop like miniature firecrackers. Wish I had taken a video of it.
Click to expand...

On a Honeywell VIP terminal, I had molten metal fly out of the bottom and leave a burnt trail across the desk as it fizzed onto the floor.
 
Gary C said:
The three leg design was to ensure they could not be soldered in the wrong way round. The two outer legs are actually connected together with the central one forming the other terminal, so no matter what you do, you can't get it wrong (well, as long as you have bought the right three legged cap, for some reason you can get them in two flavours, +ve centre or +ve both outer legs)

Except of course, three legged ones are hard to find now, so make sure you get your two legged ones in the right way.

http://minuszerodegrees.net/failure/failure - 3lt.htm
Click to expand...

I got three-legged components. Link to DigiKey below.

https://www.digikey.com/en/products/detail/kemet/T398E106M016AT/12546513
 
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