MegaSTE power LED: Mixup in the PCB layout?

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czietz
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Joined: Tue May 24, 2016 6:47 pm

MegaSTE power LED: Mixup in the PCB layout?

Postby czietz » Sun Aug 05, 2018 9:25 am

Hello,

one cosmetic thing that annoys me with my MegaSTE is the extremely dim power LED. At first I assumed this was simply due to old age. But then I measured the current: only about 2 - 3 mA, which is much less than one can expect according to the schematic: http://gossuin.be/images/schemasatari/MEGA%20STE.pdf. According to that, only R290 (120 ohm) is in series between +5V and the power LED, so there should be around 20 mA, which is a typical current for LEDs.

I measured with the continuity tester and found that in fact R421 (1 kohm) is in series with the power LED, while R290 is connected to the jumper W401 where R421 is supposed to be used. Like so (click on the picture to see a much less blurry version):
megaste_power_led_layout.jpg


I always assumed that a previous owner had rewired that resistors for whatever reason. However, today I planned to fix this and had a look at the bottom side of the PCB: that mixup is already in the PCB layout! No traces have been cut and rewired. The resistors were populated according to the BOM, but the layout doesn't match the schematic.

Now I wonder: is this an error in the PCB revision of my MegaSTE only? Is it always the case? Wouldn't that mean that a lot power MegaSTE power LEDs are incredibly dim?

I'm now thinking about either swapping the resistors or replacing the LED with a low-current type that already shines brightly at 2 mA.
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Silly_Pony
Captain Atari
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Posts: 226
Joined: Sat Jul 02, 2011 7:18 pm

Re: MegaSTE power LED: Mixup in the PCB layout?

Postby Silly_Pony » Sun Aug 05, 2018 1:47 pm

Count yourself lucky, the power LED on mine doesn't even work anymore. But I don't remember it being particularly bright.

At least it's not like STFMs where every time you open one up you find a different patch wire.
1040STe, 1040STf


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