Ramps won't boot with stepper drivers

Hello,

I have a ramps 1.4 controlling a prusa i3 variant - worked well for awhile. 12v, dual extruder, heated bed with SSR for PID. I've added a servo for auto bed level and it was working very well but I was getting jitter between movements. I made a small noise trap but apparently had a short in it somewhere - when I plugged it into servo 1 it blew out the arduino's voltage regulator. Powering servo via arduino and the jumper between 5V and VCC next to the reset switch.

To further complicate things I removed the voltage regulator and replaced it with a 7805 for temporary use. I stupidly rushed and shorted pins 2 and 3 of the regulator. Something else blew up, don't know what.

The situation now - I have a new mega 2560 r3. Via usb or 12V it will boot and work as expected unless stepper drivers are connected. If I connect even one driver (using stepsticks) it will show the initial boot screen and then fade off; the more stepper drivers connected the worse it gets. LEDS come on as they should but quickly fade with LCD.

Here's what I've tried and done:
new 2560
replaced d1 and d2 - mine were apparently 1n4007 but I replaced with the original prescribed 1n4004 but I would think 4007 in lieu of 4004 should be fine?
pulled polyfuses and wired automotive fuses in their place - F1 (MFR500) with 5A and F1 (MFR1100) with 10A.
Saved all the old stuff because I don't think it was the issue

verified continuity from D1 to Vin and measured output as 12V

I haven't messed with the mosfets because the n channel only ones I have on hand are 2N7000G which are only rated for 200mA - everything else is p channel. I do have a transistor tester (https://www.mikrocontroller.net/svnbrowser/transistortester/) that can test mosfets so l might pull them tomorrow and test them out.

I have some unused/backup stepsticks but I was wary to connect them just in case my board is frying them

My current suspicion is resistors - all were within tolerance except R3, R5, R6, R8, and R10 which read 20k (should be 100k according to the schematic). These appear to be the pull down resistors on the MS1 line from the drivers. I wanted to confirm that these could be the issue before I start tearing the board apart to get to the pulldowns. I also I don't have any 100k SMD resistors on hand (although I do have through-hole and may have some stuff I can salvage from)

anything else to try or measure? I have a standard DMM, kind of crappy digital o-scope, a ton of spare parts that may or may not be useful, and the aforementioned transistor tester on hand if any of that would be of use.

edit: More info - appears that some stepsticks are more of an issue than others - with only the graphical LCD and power connected the machine starts up fine. With the stepstick I used for E0 in any slot it remains fine. Stepsticks prev. used for y and z start up but push the contrast way up? Stepstick prev. used for E1 and X cause boot loop. Behavior occurs regardless of slot.

Edited 4 time(s). Last edit at 01/23/2016 05:31AM by quixotic120.

Sounds to me that you probably shorted 12V to 5V and blew all the stepsticks.

You should definitely use an external 5V regulator. The one on the Arduino will overheat if you try to power a servo from it. The way people use RAMPS now, it needs to be redesigned to include a higher-current 5V regulator on the board instead of relying on the one on the Arduino (or the whole design replaced by something better).

Edited 1 time(s). Last edit at 01/23/2016 07:52AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks! That's probably it. Before I throw new drivers in: can I test the old ones somehow and should I be concerned regarding the resistors that are not reading correctly?

For the ext 5v would it be sufficient to tap the 12v from the power supply with something like a 7805 and send the emitter to the 5v pin next to reset in lieu of the jumper? I have an nte960 on hand that should be sufficient for this.

A 7805 should do nicely. Don't forget to provide an input capacitor, needed to keep the regulator stable - see the 7805 datasheet for recommendations.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Oh smart - good call. Thanks!