Author Topic: Value of color coded fuse?  (Read 429 times)

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Offline MattisLind

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Value of color coded fuse?
« on: September 10, 2020, 06:53:47 pm »
Hello!

I am trying to repair an IBM 3279 terminal and as usual for IBM stuff there are no schematics to be found. And since field repair is on the board level the fuse is not in any parts list.



The fuse is violet / yellow in the ends. It is in series with the vertical deflection yoke. The vertical deflection system is fed by 25V. No idea about inductance of the coil especially since there is likely to be a problem with the yoke itself. The resistance is far too low.

What is a proper value for the fuse?
 

Offline floobydust

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Re: Value of color coded fuse?
« Reply #1 on: September 11, 2020, 08:12:03 am »
I've never seen datasheets on fuses like that, or have a clue who makes it.

For that size screen, vertical deflection IC's (i.e. LA7837) are about 2Apk at 24VDC power.
I would guess a fast acting Picofuse or TR5 or similar 2A fuse. Look at the fuse curves (370) and a 2A part clears 5A in a second or two.
 

Offline MattisLind

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Re: Value of color coded fuse?
« Reply #2 on: September 19, 2020, 10:19:46 pm »
I happened to be able to solve this mystery when I came a cross a few old 1965 vintage IBM SMS modules in my father's stash of things.



Exactly the same type of fuse. Searched a bit and found Ken Shiriff's excellent SMS card database where this DJL card was listed!

The schematic showed a fuse named GFA 1,5 A.



Google "GFA Fuse" gave a matching datasheet!. Seems to be a quite standard Bussmann fuse after all!
 

Online Alex Eisenhut

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Re: Value of color coded fuse?
« Reply #3 on: September 20, 2020, 03:09:56 am »
Wait, you have an IBM 3279 terminal?
*Except AC/DC adapters on eBay. Avoid them all!
 

Offline MattisLind

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Re: Value of color coded fuse?
« Reply #4 on: September 20, 2020, 06:39:41 am »
Wait, you have an IBM 3279 terminal?

Yes, I do. The intention is to connect it to the Hercules mainframe emulator. If I get the damn thing working.

I had to reverse engineer the PSU to understand it so that I could fix that one. The PSU now seems to work just fine. Then i tried to run the entire terminal but a 200 ohm resistor in the vertical deflection circuit smoked and this fuse blow.

Since posting the first message in the thread I have learnt that IBM field service manuals state that the resistance of both defelection coils should be less than 2 ohms. I measured them to be 1.9 for horisontal and 1.2 ohm for the vertical which probably is allright then.

So I still have no idea what the root cause of the blown fuse is. Unfortunately the IBM board has five square aluminium covered chips which it seems to be impossible to find a cross for. So reverse engineering it fully will not be possible.

My next step is to apply 5V and 8.5V to the card and study parts of the board to see if I can understand vertical deflection better.

A schematic would be great but I doubt I ever find one..
 

Online Alex Eisenhut

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Re: Value of color coded fuse?
« Reply #5 on: September 25, 2020, 05:48:51 am »

So I still have no idea what the root cause of the blown fuse is. Unfortunately the IBM board has five square aluminium covered chips which it seems to be impossible to find a cross for. So reverse engineering it fully will not be possible.


So cool. Do you mean these

https://en.wikipedia.org/wiki/IBM_Solid_Logic_Technology
*Except AC/DC adapters on eBay. Avoid them all!
 


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