Burned an IC on the Seagate ST-412. How to find a replacement IC or at least recognize the right IC?

[floogy]

Hi, I'm relatively new to electronics and vintage computer repairs and soldering.

But I want to try to get into such repairs.

I currently bought an old IBM XT 5160 with an IBM 5153 CGA monitor.

In the beginning they didn't work, but the PSU ventilator spinned up and eventually the Tandon 100 tape drive LED flickered for a moment. The CGA gave a grey light image only.

Then, I don't remember exactly the circumstances, A capacitor near the P8 P9 PSU connectors exploded with an orange flame and some smoke and smells.

That was the moment the hard disk began to spin up and the monitor gave a black screen.

I was a bit shocked and while the hard disk began smoking a lot I was still not able to react instantly. So I eventually shut down the power a little to late.

On the PCB I found an IC cracked and burned up. I'm not sure if it's the culprit of a diode near by or if maybe the dust on the PCB became conductive.


I'm now wondering if I may be able to fix that PCB and if the drive then might present it's data again?

Unfortunately the IC is no longer exactly identifiable. But it seems to be a Fairchild Semiconductor beginning with DM and ending to N.

I looked up with Google image search to find that PCB and identify it that way.

It seems to be a DM74574N, but I can't find such an IC. In the handwritten schematic in the service manual if the drive I also read it as DM74574N, also on other images of that PCB.

But then I found a different one with another font used in the imprint. It was a SN74S74N. Therefore I guess now, that it was probably a S instead of a 5. So a DM74S74N instead of DM74574N. It took me a long time to get to that point.

I'm still not sure, therefore I decided to ask in this forum for help.

Two different PCBs from the manual.
Unfortunately mine is a little bit different. It looks like the PCB from the 'higher intellect ' webpage.

C14 or 6K



On eBay someone sells the Chip with a clear and decent foto. So it looks more like an S then in the Fotos of the PCB.


The Texas instruments SN74S74N. With this font it looks more like a S instead of a 5.


A PCB with the Fairchild one. The S looks more like a 5 to me.


The image of the PCB on the higher intellect webpage that made me think it's the DM74574N instead of the DM74S74N

 

[daver2]

Welcome to VCFED (as this is your first post).

An SN74S74N will work just as well, and you can buy those from a 'real' electronics shop rather than risk eBay suppliers...

The DM and SN is (essentially) the same device from two different manufacturers (National Semiconductors and Texas Instruments respectively).

Can you post a photograph of the damage please. An IC doesn't fail like this unless it has been overvoltaged by a power supply fault!

Do NOT power the machine up again without taking some precautions...

So, The initial fault was a capacitor on the main logic board that burst into flames followed by the IC on the disk drive? Is this correct?

Did you disconnect the power supply FIRST and test it with a dummy load?

Dave

 

[floogy]

I thought the culprit might be rather the diode or some components nearby that IC, because I ran successfully into basic with the disconnected IBM Seagate ST-412. Also I managed to find the right DIP switch positions to get the Tseng ET3000AX deliver the right TTL DB9 CGA signal.

But nevertheless, I'll measure the PSU before starting it up the next time.

Yes, before the initial exploded Tantal Cap the Board was more or less dead.

After that explosion the drive began to spin. Before it was dead.

Because of the shock it burned relatively heavy, die to my slow reaction. Then I disconnected the molex after shutting down the PSU.

I have to process the images, because they are to big for uploading.

Capacitor C58 near P9 on the mobo (culprit for the dead short? Now open)


Burned ST-412 PCB (maybe the diode was the culprit?)


Cleaned with Isopropanol (will measure Thuesday the diode and also the PSU)


Dust underneath (conductive?)

 

[daver2]

Tantalum bead capacitors (aka Smurf Grenades) just go bang!

Yes, it may have been short circuit, and then went open circuit.

However, it could also have been overvoltaged and died then the IC on the disk drive failed after that?

Conductive dust - grasping at straws I am afraid. However, you should always do a visual inspection before powering anything up looking for rubbish like this - and clean it up. Also, look for anything not right from the previous owner - burnt components, cables disconnected or (even worse) plugged in the wrong way round. Bits of metal (nuts, bolts and washers) shorting things out etc. etc. etc.

An IC does not fail like that (unless you have been really, really unlucky). It is most likely the casualty of being overvoltaged.

Dave

 

[floogy]

I added pictures from that IC on my earlier post. What do you think on the 5 vs S?

Also: Can you make sense of the diode type and capacitor type near that IC? I think the resistors I can figure out. Sadly my PCB is a bit different from those in the manual.

 

[T-R-A]

An IC does not fail like that (unless you have been really, really unlucky). It is most likely the casualty of being overvoltaged.

Dave

Precisely, and 74/74S series were some of the "toughest" chips around during their day. You're going to have to find the source of why it failed so catastrophically before trying to put another one in. Also make sure the back of the PCB is cleaned and hasn't delaminated or broken traces from such heat.

 

[floogy]

I think, I'll do some measurements and hopefully I can cope with that despite the fact that I'm a beginner with electronics.

Is there any service manual or schematics out there for the PCB ASSY 20221?

Or someone with the same PCB, to get some data for the parts?

I found that the Manual I provided above wasn't complete.
There is a complete one with the schematics for the ASSY 20221 PCB at bit savers probably.
But I got it from archive . org

seagate :: mfm :: ST-412 :: ST406 412 419 Service Manual 198310 : Free Download, Borrow, and Streaming : Internet Archive

From the bitsavers.org collection, a scanned-in computer-related document.seagate :: mfm :: ST-412 :: ST406 412 419 Service Manual 198310

archive.org

Thank you all for your kindness and help!

 

[sergioag]

I've never seen any chip having that kind of damage unless it was a gross overvoltage. Even 12V on a TTL wouldn't cause that. Something else must have gone very wrong there. If that's the case, it's very likely that all other chips in the system are also dead. I suggest you first check the output voltages of the power supply with only a dummy load connected. Or you can use a known-good power supply and start testing other parts first (I would test the motherboard first). Whatever you decide to do, please DO NOT trust that power supply without any serious testing first.

 

[daver2]

I had a colleague accidentally connect +24V and -24V to the inputs of some 5V TTL buffers the other week.

That took a couple of chips out - but none of them looked as bad as that!

Dave

 

[twolazy]

Im going to guess the chip was cracked before the cap let go... I've seen cracked chips do crazy things when powered, including self ignition!

 

[modem7]

I found that the Manual I provided above wasn't complete.
There is a complete one with the schematics for the ASSY 20221 PCB at bit savers probably.
But I got it from archive . org

So you will have determined:


Those components may well still be functional.

 

[floogy]

Thank you modem7, wow, nice and fine montage! I already figured out, but it was a hassle with no printed manuals only acrobat on the android cellphone.

I got a second non working 5160 and tried that Seagate ST-412.

Unfortunately it instantly burned the same items and cracked the chip. Now I saw it red glowing, like 500°C or the like.

I didn't expect it, because the voltage at the molex and P8, P9 were fine and I think the PSU isn't the culprit, because that 5160 is now in a working condition because I could add a floppy controller. The Tandon 100 is doing just fine.

The PCB was in a way cleaner condition with almost no dust.

Therefore I think a capacitor or resistor may was shorted. I think maybe the cap with 104 imprint and glass housing(?): C14 or the resisor below (now shorted, but not yet measured removed).

I may investigate further end of the week.

 

[daver2]

I have checked the schematic and none of those components (R34, R58, C14 or C19) can be (directly) responsible for the burnt IC 6G.

Even if the capacitors or resistors were open or short circuit...

However, if IC 6G goes 'volcano', then R34 could could get damaged in the process.

Resistor R58 and capacitor C19 are nothing to do with 6G - they are on the output of 7G and act as a short time delay.

Likewise, capacitor C14 has nothing to do with 6G either.

Either there is another component that is faulty, or there is a short circuit on the PCB tracking somewhere (causing 6G to go 'volcano').

Do you have a current-limiting bench power supply?

If so, perhaps you could power the drive from the bench power supply - with a current limit set fairly close to the expected current consumption for the drive. But do not connect the signal cables.

Apply the +5V first and see what happens...

However, before you do that, check for a short circuit between +5V and GND and +12V and +5V with the drive disconnected and using a multimeter.

Dave

 

[modem7]

Either there is another component that is faulty, or there is a short circuit on the PCB tracking somewhere (causing 6G to go 'volcano').

I have traced faults to a chip, and when the chip was removed, a crack was found on the chip's underside. Replacing the chip got the equipment running again.