VCC is Earthed on my control board

I’ve just completed my Shruthi build, it powers up and display lights up - but it doesn’t see Midi. My Midi kit is confirmed as working.

I’ve noticed that more than one pin on OK.1 has a connection to GND on the 8x1, here’s the resistances -

Pin.5 0
Pin.6 10k
Pin.8 0

And Leg.4 on Midi 2 is also earthed at 220. I’ve removed Midi 2.

My reading of the schematic says the VCC must be getting earthed, but I can’t see where. I’ve been over the board with a magnifying glass and can’t see any solder blobs touching and I’ve pulled the display off to have a good look behind.

Any suggestions on possible weak points on the VCC or places to look?

Or am I barking up the wrong tree?

Thanks in advance

Dave

Pin5 is normal - it’s the ground pin of the chip.
Pin8 is not normal.

Are you checking with the 6N137 inserted or removed?

There’s a 100nF cap near the 6N137. Is its top pin connected to ground or not? (it shouldn’t).

Hello,

I’ve been testing with OK.1, IC.1 and IC.4 removed.

The 100nF cap - do mean C1? That bit of soldering is looking good and is not in contact with any other blobs or pins.

Another question - if you check continuity between the +5V and GND pins on the board to board connector, is there contact? I’m curious because if there was a short between GND and +5V, the synth wouldn’t have booted.

That’s been puzzling me too but before I removed the screen it lit up and I was able change settings in the menu using the pots and switches.

One thing I wasn’t able to get working was the test tone.

Anyway…

There is continuity between +5v and GND, but there’s 1.8k of resistance in it. There’s no 1.8k resistors in the BOM and I gave R21 a few turns just in case but the resistance remained the same.

Its a complex network of many many Rs between GND and Vcc, so don’t expect it to change significant just b makin a few turns on one trimpot…

It is normal that you get resistance between +5V and GND. The voltmeter is pissing a probe current through your circuit, which is flowing through the pots, the contrast trimmer, and so on, so you get a corresponding voltage and the meter deduces a kind of “global” resistance for the whole thing…

“The voltmeter is pissing a probe current through your circuit,…” … sounds painful :wink: