Mesa 5i25/7i76 Stepgens 3 and 4 stopped working

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05 Sep 2020 15:53 #180808 by Uthayne
Recently started the machine after a 2 week break where everything was working fine to find out that both of my Z axis (Stepgens 3 and 4) no longer work. I have troubleshot it down to what I believe is the 7i76 at fault. I confirmed that the drivers are functional as I swapped inputs from stepgen 0 to each 3 and 4 and it will move both of the independent Z axes.

Came across this post here:https://forum.linuxcnc.org/38-general-linuxcnc-questions/37999-problem-with-a-mesa-7i76 and per Peter's direction I monitored the voltages on each STEP-, STEP+, DIR-, DIR+ pins before and after running linuxcnc. Below are the results
All voltages referenced to ground

OFF       PIN          ON

337mV     STEP0-     4.1V
4.18V     STEP0+     424mV
335mV     DIR0-      4.1V
4.18V     DIR0+      420mV

342mV     STEP1-     4.1V
4.18V     STEP1+     424mV
342mV     DIR1-      4.1V
4.18V     DIR1+      424mV

341mV     STEP2-     4.1V
4.18V     STEP2+     424mV
343mV     DIR2-      4.1V
4.18V     DIR2+      426mV

14mV      STEP3-     4.5V
38mV      STEP3+     3.7V
342mV     DIR3-      4.1V
4.18V     DIR3+      424mV

4.6V      STEP4-     4.5V
4.6V      STEP4+     2.3V
4.6V      DIR4-      4.5V
3.0V      DIR4+      2.3V

Stepgens 3 & 4 are definitely out of line here. DIR3-/+ seems to be ok though.

Couple of questions: Does the ~4.18V showing on the "good" stepgens seem high enough? The 7i76 is getting its 5V from the 5i25 with W2 in the left position. External 5V power has never been supplied while in this configuration. The +5VP pins read about 4.5V.

I only need 4 concurrent stepgens; can an additional stepgen be flashed to the aux connector on the 5i25 temporarily until this issue is resolved?

Thanks in advance

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05 Sep 2020 17:17 #180812 by PCW
4.18 seems OK if they are loaded, they should equal the 5V supply voltage (4.5V)
if unloaded

My guess would be that the 74ACT541 (that drives the stepgen + outputs 0..3)
has been damaged somehow along with the 74ACT04 that drives the stepgen
4 outputs
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05 Sep 2020 23:14 #180872 by Uthayne
Thanks for the quick reply!

After closer inspection, the 74ACT04 chip is definitely toast. I'll be replacing those two and see if it fixes it.
Attachments:

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10 Sep 2020 19:14 - 13 Sep 2020 06:15 #181666 by Uthayne
Was able to replace U12 and U13 and it solved the problem! Thanks a lot

Edit: while the voltages checked out with the 7i76 unloaded in both states with linuxcnc off and on, it has stopped working again and exhibits the same voltages as before on stepgen 3. It worked for a few seconds, then it acted like a loose connection only sometimes working, and now it has stopped completely.

Double Edit: Seems like I have bigger problems, as stepgens 0-3 are no longer working.
Last edit: 13 Sep 2020 06:15 by Uthayne.

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18 Sep 2020 21:43 #182685 by Uthayne
I removed and resoldered both ACT540 and 541 and Stepgens 0-2 are working again. Apparently I'm not great at soldering TSSOP packages as I had a few pins not connected.

I checked the resistances on all of the resistor networks and the network attached to stepgen 3 had one of the four resistors out @ 2.9kOhm. Unfortunately, when I ordered the other chips I got the wrong package size for the resistor networks.

What is the correct package size or part number for the 27 Ohm resistor network connected to the stepgens?

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18 Sep 2020 23:23 #182689 by PCW
1206 = 4x 0603

say Panasonic EXB-V8V270JV
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