Mesa 7i77 board - field power shorted on TB2

I started hooking up my mesa 7i77 board today and I am finding that all the pins on TB2 (which is the field power connector) are shorted. I did go through the instructions and I left switch W1 set to the left which should connect VIN to field power; but this should not be connecting pin 8, which is ground directly to the other pins. I am letting the 6i25 board supply the 5Vdc power the 7i77 so switch W5 on the 7i77 is to the left, and switches W1 and W2 are up on the 6i25 board; but that should not matter since the boards were not connected and the computer power was off.

I hooked up my 24 Vdc power supply with positive to the bottom pin (pin 1) (7i77 oriented with 25pin connector to the left) and 24 Vdc negative (ground) to the top pin (pin 8). At this time the 7i77 is simply sitting on the wood work bench. I have lots of wires hooked up to the encoder, drv, and I/O connectors, but all these wires go to an interface board which is simply a bunch of DP37, DP25 and DP15 female connectors which eventually will mate up with the wiring harness on my mill. None of these wires are connected to actual components at this time. I turned on the power supply and it refused to start. Turned off the power, disconnected the 24 Vdc positive, and the power supply turned on. Touched the 24 Vdc wire to the power supply's positive Vdc terminal, and got sparks. Turned everything off and checking with an ohm meter and I get a direct short between all the pins on the TB2 connector (1-2,1-3,1-4..2-3,2-8,...7-8). Yes, I checked all the pins and all possible combinations. Keeping the ohm meter connected to pins 1 and 8 (which are power and ground and I do not think these should be shorted), I unplugged all the other connectors (encoders, drvs, i/Os) and still get a direct short between pins 1 and 8. I tested all the pins on TB2 (yes it is the orange connector), all eight of them, and all the pins are shorted together.

Anyone have any idea why there is a direct short between all these pins? I would ask if this is normal, but I do not see how it could be normal because then I could never apply field power to this board. Is this happening because I do not have power applied to the 7i77 board from the 6i25? If so, is this normal practice to short my power supply if no power is applied to the 7i77 board? And how does anyone explain pins 6 and 7? I can understand pins 1 - 4 reading as being shorted since they are all four labeled as field power. I can even understand pin 5 which is labeled as VIN being connected to pins 1-4 since I did set the W5 switch; but I do not understand why pin 8 which is ground is shorted to pins 1-4. I also do not understand why pins 6 and 7 are shorted since they are labeled as NC; they should not be connected to anything.

Any help would be appreciated.

A power/ground short on TB2 most likely means the 7I77 has been damaged either by reversed/AC or higher that 36V input
to its field power source

Well I guess on Monday I will have to contact Mesa because I know my power supply does not produce anything chose to 36 volts. In fact I tested it with a voltmeter prior to hooking it up to make sure I got the polarity correct coming out of the power supply and I adjusted it to exactly 24 Vdc. I know the polarity was right because that was why I got out the voltmeter and the wires were hooked up correctly. Shorting between the power and ground also does not explain why Pins 6 and 7, which are suppose to be connected to nothing, also are shorted to all the other pins.

Its possible its some kind of infant mortality, But its pretty unlikely

Pin 6 and 7 are connected to Ground on Rev. B cards

Well it is a physical fact that my power supply can not produce more than about 27 Vdc even if the adjustment was turned all the way up, and I know it was set at exactly 24 Vdc before I hooked it up. While I am no electronics genius, the 7i77 instruction manual is pretty simple to follow especially when I had to read it a couple of times to figure out what was meant by VIN and field power. It wasn't until I looked at the wiring page for the TB2 connector that I finally understood what the manual was talking about as far as the W5 switch and the connector callouts are pretty simple. Unless there is some other switch that needs to be set to get the board to work, this board arrived DOA with this problem.

I do have one question about the earlier remarks. You talked about reversed /AC. As far as I know there is no AC power on this board, and in fact the only source of AC power, which would be the PC computer, was not even turned on. Where would this board even get AC power?

jwsigler wrote:

You talked about reversed /AC.

I think he means connecting "reversed Power" OR "AC Power" on the field power input will damage the card,
--Per

As far as I can tell, there is no AC power being applied to the board which is what is confusing me. I assume AC means the standard definition which is Alternating Current. Is there something that I am missing? As far as I can tell from reading the manual, the only power going to this board is 5 Vdc which comes from the 6i25 board via the PC power supply, and then 24 Vdc from an external power supply. Neither of these voltages are AC, so I do not know how a reversed AC is occurring. The issue is kind of academic right now since I contacted Mesa and they had me send them the board and they are going to repair or replace it. I do not know if the reference was to the 24 Vdc power, and the AC part was just a typo, or whether there is suppose to be some other power connected to the board.

We received the card and found that CR2 (the Field power supply reverse protection diode) was shorted
The diode was replaced and full functional/loopback test was performed.and passed

Its possible that the diode failed but its more likely that reverse polarity or AC power was applied to the field
power terminals at some point...