Finding a replacement for D1 on a first generation Macintosh LC

[MrGasS27]

Hi everybody

This summer I broke the diode D1 on my first gen Macintosh LC while testing some prototype boards, since then I need power from the molex plug in order to fire up my BlueSCSI internally, while a BlueSCSI attached to the external port works fine with termination power.

I already published this issue on a facebook group and somebody pointed out that D1, marked 43XH on my motherboard, could be a 200mA / 40 V diode.
I downloaded a reverse engineered Macintosh LC schematic from here , but D1 is undocumented.
After few researches on the SMD codebook, I discovered that 43 leads to BAS40 and, accordingly to the schematic, BAS40-6 could be a potential candidate because it effectively reflects the characteristics described above, but I would like an opinion from this forum before I turn my LC from a pizza box to a firework box.

For the sake of documentation, I'm attaching few pictures that I made in these days.
Does someone figured out what the exact replacement for D1 is?

Thank you a lot and happy new year

 

[marcelv]

looks like A3X diode (NOT 43X H) the H is a date code

 

[MrGasS27]

looks like A3X diode (NOT 43X H) the H is a date code

Thank you a lot, I took better photos of D1 and yes, I misread the first value, it's an A3X indeed, I suspected that H was a date code.
A3X seems to correspond to MMBD2835LT1G / SMMBD2835LT1G, the only problem is that I can't find this component on sale, I'll do a better research.

I'm also attaching a photo of D4 because I will probably blow it up.

 

[marcelv]

picture of the relevant part of my lc board

 

[MrGasS27]

picture of the relevant part of my lc board
View attachment 50832

Thank you again for you precious help.
The only problem now is that I need to pay 20€ only for shipping an A3X / MMBD2835LT1G / SMMBD2835LT1G here in Italy, because of this I'm looking for an equivalent component.
A similar component that I can easily buy is the BAR43A, which has a forward current 100mA like the A3X but a 1V forward voltage drop and a 30V reverse voltage breakdown, whereas the A3X has a 1.2V voltage drop and a 35V reverse voltage breakdown.
I'll do more researches.

 

[MrGasS27]

Replaced the A3X with two 1N4148 just to see if something would change but no luck, the problem stays, the BlueSCSI works only with molex power.
However, it works like before.

 

[max1zzz]

D1 is part of the power supply circuit for the EGRET so is not the cause of your issue
D3 is the one which generates term power (it's up by the external SCSI connector) it your BlueSCSI works on a externally it must be working so it seems likely you have a trace break between the internal and external ports

Do you have continuity between pin 25 on the external port and pin 26 on the internal?

Edit: Just looked at the BOM, I have marked BAW56 as a suitable replacement for D1. (I wasn't able to positively identify what the original part was) Will update the schematic with a note that BAW56 can be used to replace D1

 

[MrGasS27]

Well, I must say sorry to everybody for wasting your time and, at the same time, I want to say thanks for helping me anyway.
I must say that I found this "journey" really funny and, as always, it was useful as every other experience.

Diode D3 is fine, I've continuity between termination power pins, external pin 25 and internal pin 26.
A3X has an hole but it's absolutely fine, in fact I tested it with my multimeter after desoldering it, I've 0.6V if I put the positive probe on pin 3 and the negative one on pins 1-2 and nothing if reverse the probes.

In fact, my only problem is that oddly I've no GND contact between the 50 pin SCSI ribbon cable and the BlueSCSI: as you can see, in this photo I'm bringing GND to the bluepill with an alligator cable and the berg adapter unconnected.


At the same time, if I connect the BlueSCSI to my homemade 50 pin -> 25 pin adapter, everything works.

Now I'll solder the original D1 back on the motherboard and I'll try to solve this stupid problem.

I hope this makes you laugh just like I did after discovering this.
Thanks again to everybody, this forum rocks as always.

 

[max1zzz]

Glad to hear you got it working

 

[MrGasS27]

Just discovered that unlike the IIci, IIsi, Quadra 700 and other Macs I have, the LC DOESN'T have any GND point on the 2-50 row

 

[MrGasS27]

I soldered back the original A3X and now the hole disappeared, I think that the plastic melted during the soldering process or something like that.

Well, mystery solved.

 

[max1zzz]

Just discovered that unlike the IIci, IIsi, Quadra 700 and other Macs I have, the LC DOESN'T have any GND point on the 2-50 row

Yep, it only grounds the odd pins on the the connector