SMD diode identification on Blizzard SCSI-KIT IV help please!

ctrlbreak

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I'm looking for some help IDing a diode (I think?) on my Blizzard SCSI-KIT IV board. I blew it up 20 years ago (I know!) before I packed away my amigas - I think due to a short with metal case component (it's along time ago!). Now they're mostly all up and running again but I need to try to get this card working again if possible.

Here's the component (not from my board - this one isn't blown!)

Looks like a beefy diode, used for protection, but mine is no longer on the board and I can't find any pics that positively ID it.

Help appreciated please! :)

Thanks in advance!

Blizzard_board.jpg
 

imqqmi

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It could be a zener diode, which can have a specific voltage since there's a capacitor in parallel with it. If there's a series resistor going to it, it's another clue that it could be a zener diode.
If you can find out to what it's connected to, an IC for example you can work out what voltage it was using the datasheet of that IC. Zener diodes are often used as a simple voltage regulator.
 
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miggymad

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Judging by the visible component markings in the picture, it does look to be a 1SMAF4777A zener diode.

Spec sheet attached - see last entry on page 2.
 

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ctrlbreak

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Thanks all. Seems like a v high voltage part for the use case if it is the one miggymad! But it does seem to be a zener diode so perhaps it is that one

Anyone got any tips for how to clean up the board for replacement? Was thinking of isoproponol clean then flux and bit of solder to try to clean up the pads
IMG_6004.jpeg
. Doesn’t look like they were blown off and the vias seem ok (pics of my actual board here):
 

Watson

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You may want to check those traces nearby, if not done so already, to be sure they're not broken. (y)
 

imqqmi

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It's unlikely a very high voltage zener, the 1SMAF4777A is rated at 330V, the tentalum capacitor next to it is a low value as well, probably 10uF 6V.
If you lookup the specs of scsi1 it mentions a few voltages that should be there on the connector, I think 4.3V was one of them.
If the zener connects to that pin then you have a likely candidate for a replacement. If you get it wrong you might damage connected peripherals or get unexpected behaviour. Best to check where the diode is connected to by tracing out the PCB traces and then decide on the proper value.

As for cleaning, yes some denatured alcohol should do the trick in cleaning, then some flux and fresh solder.
 

ctrlbreak

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Ok, thanks for all the help and pointers all!

I did some tracing and some research and it seems like this diode is connected between the Vcc (5V) rail and a pin on the scsi connector for termination power that some devices use. Sounds like blown diodes is possible if there is some back-voltage or other issue on this pin and the idea is to protect the controller circuitry… let’s see if they was successful!

Also found another pic of a slightly different board revision with an alternative scsi controller chip that’s using a through-hole zener for the same purpose.

IMG_6018.jpeg


Given it’s on a 5V rail what would be a suitable voltage diode? I presume I should go higher… but how much?

Also I cleaned up the board and think there could be damage to one trace - can’t quite tell if it’s just smoke damage on the surface or the trace is blown away - will have to test it… and also potentially deal with that lifted pad too!

IMG_6014.jpeg
 

imqqmi

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Lookup the pin number and see what the pinout says the function is. If it's the pin that's supposed to supply 4.3V (pin 25 termpower) then I'd get a 4.3V Zener diode. If the diode is there to drop 5V down to 4.3V a normal diode with a voltage drop of 0.7V should also suffice.
If there's no series resistor between 5V and the diode, then it's likely a normal diode. You could try a standard 1N4001 or something equivalent. If you put a zener diode at 5V and ground you'll risk burning it up. It needs a series resistance. A normal diode doesn't need it.

So scenario 1, normal diode voltage drop of 0.7V (note the polarity indicated with >|):
5V -->|Diode-- 4.3V pin 25

Scenario 2 with Zener (note the polarity indicated with S<|, zener has a squiggly shape ):
5V ---resistor---S<|Zener--GND
.......................|
................4.3Vpin 25 (ignore the ... because spaces are truncated)

In the end you should be able to measure 4.3V at pin 25.

If it's not TERMPOWER then we're back on square one ;)
 
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