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strangepapyrus
Newbie

Joined: 24/06/2025
Location: United Kingdom
Posts: 2

Posted: 11:43am 21 Jun 2026

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Hi,

Electronics is a fairly new hobby for me and this is my first post here. Please could anyone offer some advice to help me out? Thank you.

I decided to have a go a building the Picomite VGA and have been partially successful.

I ordered the PCBs to the UK from JLCPCB and the other parts sourced from various suppliers.

The VGA works, but not the keyboard. I have 5 volts out from the port so the MOSFET level shifting circuit must at least be partially working? The system boots, I can see all the post boot info on screen and a cursor, but absolutely nothing from the keyboard.

Thinking I must of done something wrong somewhere, I had a second board, so built a second picomite from scratch, including a second pico ....... and exactly the same. There does not seem to be power to the keyboards, apart from an initial flash of the leds on my Fujitsu keyboard at boot, CAPS LOCK, does not illuminate when pressed for example, and no output onscreen.

I have tried different keyboards. I have newer Fujitsu keyboards (x2) and an older Compaq keyboard (x1) - all PS2 wired, no adapters. I have tried soldering on different PS2 ports and different MOSFETS from different suppliers. I've tested the resistors and all seem good. I've tested the output from the PS2 port. I've tried newer and older firmwares. I've swapped the picos around and all to no avail.

The power supply is one I used to use on a raspberry pi, 5V 3000mA output.

I am starting to lean towards the PCBs being bad as I'm a bit stuck as to what's going on, so if anyone could offer any advice I'd be extremely grateful.

Thanks, Martin

matherp
Guru

Joined: 11/12/2012
Location: United Kingdom
Posts: 11516

Posted: 11:49am 21 Jun 2026

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What firmware version are you using? Before anything else type "OPTION RESET VGA DESIGN 1" and then try again.

robert.rozee
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Joined: 31/12/2012
Location: New Zealand
Posts: 2529

Posted: 12:05pm 21 Jun 2026

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hi Martin,
welcome to the forums.

what are the colour bands on the four resistors near the 2N7000 transistors? from the photo they appear to be brown-black-black-gold, which would indicate a value of 10 ohms each. they should be 10,000 ohms each, which are instead coded as brown-black-orange-gold (the gold indicates 5% tolerance parts, which is unimportant).

you do appear to have inserted the 40-pin blue header (located between the keyboard and VGA sockets) with the long ends of the pins the wrong way round, but this does not matter too much at the moment. otherwise, you have done a very nice job of the assembly and soldering

cheers,
rob :-)
Edited 2026-06-21 22:15 by robert.rozee

phil99

Guru

Joined: 11/02/2018
Location: Australia
Posts: 3293

Posted: 12:33pm 21 Jun 2026

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Quote ... the long ends of the pins the wrong way round

Rather than trying to de-solder the entire header it may be possible to push the pins through one at a time.
With the board upside down on a firm surface re-melt the solder and press down to 1mm above the PCB. The blunt end of a skewer, or better still a carpenters panel-pin punch should do the job.

As Rob said they do look like 10Ω resistors. If so that could damage the Pico if powered up for long. I think Peter has set 8mA current limits for the pins but it will still be heating the chip.

strangepapyrus
Newbie

Joined: 24/06/2025
Location: United Kingdom
Posts: 2

Posted: 03:08pm 21 Jun 2026

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Oh my goodness. What a plonker. I need better glasses and missed that K in the list of parts in the design construction notes.

I have indeed put 10 rather than 10,000 ohm resistors in!

Thank you all so much for the help and suggestions. I really appreciate it.

Sometimes forums aren't very kind to noobs, so thanks for not being like that.

I'll get some of the right resistors in and get back to you with hopefully some success!

All the best,

Martin

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