Probably shorted Shruthi - Possible fixes?

I think I managed to short my digital control board.
The board was working as described in step 11 of the build instructions.
After I soldered the LCD it did not work anymore.
No LEDs are lightning up and 7805 on the filter board (a SSM2044) is getting really hot.
The filter board itself seems to be still ok, as I can power another control board. (This one has bad MIDI ports, so I can not test full functionality, but it boots and displays, etc…)

I noticed that a pin of C8 was touching one of the black blobs on the back of the LCD. (But they ain’t conductive, right??)

I also might have touched 7805 with the pins of the resistor network.

Any tips which components on the control board could be damaged so I can exchange them?

Or how I could find out?

I have a multimeter at hand, but don’t dare to connect the control board to the filter board for too long…

My first guesses would be to change IC2 as it appears to be the first one connected to the resistor network…

Thanks for any help!

Best,
peek

Have you checked continuity between +5V and GND to confirm it’s really a short?

After doing some tests with my multimeter, comparing the two boards I have, I noticed the following:

Pin 1 of the resistor network (RN) is connected to both 5V and Ground.
Pin 1 of RN is connected to pin 8 of 74hc165 (IC2).
Pin 1 of RN is connected to pin 8, 9 and 12 of 74hc595 (IC3).

This is not the case on the working board.

So I guess I fried at least one of the ICs?

I will start with exchanging IC2 (and when I’m on it, put a socket there…)

Best,
peek

And yes, there is continuity between +5V and GND.

Sorry, it is pin 16 and 13 of IC2 that are connected to the RN…

Your measurements do not really imply that IC2 and IC3 are fried - they are direct consequences of GND and VCC being shorted.

Stuff on the back on the LCD module touching the board is rarely a problem - unless you have used an unusual LCD module.

Does removing all the socketed ICs help?

Do you see any potential bad solder bridges on the LCD pins?

Okay, thanks!

I removed all socketed ICs (including the Atmega) and 7805 is still getting really hot!

I use a regular LCD module. (It came with my pcbs!)

The resistor network seems to be fine, I still measure 10K.

Because of the LCD pins, I removed the LCD. As i thought it was part of the problem.

Actually i wanted to take the 165 from a working control board with bad midi ports.
But i just found out about the test tone function which makes it possible to use the shruthi without midi (maybe it would be good to put this information at step 11 of the build manual).
So i’m not really willing, to destroy my only “working” board if i’m not sure it makes sense.

Any way i can find out, which component i really fried, before changing it?

Sorry, detected a short is not easy - a micro-ohm meter can help but it’s a rather specialized bit of equipment. If you suspect a chip is fried, try replacing the 165 and the 595, but keep in mind that it could also very well be a solder bridge between two pins.

So i’m not really willing, to destroy my only “working” board if i’m not sure it makes sense.
that would make no sense at all.
If you suspect that one of your non-socketed chips is fried, you just replace it with a new, bulk one, and leave your working board alone. On this kind of circuit board (industrial through-hole board), unless you’re very well equipped, de-soldering a chip generally means destroying it by overheating it, and sometimes even damaging the board.

You stated that removing your LCD solved part of the problem? What’s the behaviour of your board now that you’ve removed it?

@pichenettes: i checked again, no solder bridges (the board was also working before putting on the lcd)

@micmicman: good, i will order new chips. (lesson learned: order spare parts right away next time)
the behaviour of the board is the same after i removed the lcd: no leds light up, 7805 get’s really hot, really quick…

uhh, next time i will take warnings in red boxes in build descriptions more seriously :wink:
i guess i was just too excited to finally try it out…

The digital control board top side ICs can be socketed with low profile turned pin sockets, both my Polivoks and 4PM has them that way.

indeed there is enough space in the standard case for a normal socket. all my machines (expect the very first) have sockets on the 165 and 595, although i never broke one…

so i desoldered the left over pins from the removed lcd and the short was gone.
strange, because i checked the pins for bridges, obviously not thorough enough…
the short between gnd and 5v was gone, as was the heating up of the 7805 on the filter board.
leds still didn’t go on.

so i exchanged the 74ls165 with a new one.
now led 8 flickrs very briefly and thats that.

any ideas??

Bigger WallWart?

If you suspect one of the cmos parts are bad, simply clip the 16 pin (+5V) midway up the leg, if is bad you will be able to tell and then you can chop it out, if not, solder it back together…