Lost My X-Axis

[astr]

I have a Series I Bridgeport that was originally run under BOSS 5. I converted it to Mach3 using a Hillbilly's BOB. I still use the original BOSS 5 power supply and driver boards.

The other day, I was finally going to run my first true milling operation and was doing a dry run to check out my G-code when the X-axis abruptly stopped. Y and Z continued to function OK. I assumed that I blew one of the output transistors so I pulled the ones for the X-axis and checked them - all OK. I checked the DC fuse (FU12) and the AC fuse (FU15). Both OK. I tried swapping the SMD boards between the X and Z axis - X still won't work. Finally, I measured the voltage at FU12 (X-axis), FU13 (Y-axis) and FU14 (Z-axis). The Y and Z-axis read 37 to 39 volts, the X was at 3 volts so I concluded the problem was in the X-axis power supply.

My next step is to check the rectifier bridge that is associated with the X-axis but I can't seem to find anything in the manual that tells me where these rectifiers are located. I can see lots of rectifiers on and below the big heat sink that the output transistors are mounted on but which are which?

My questions are: Am I on the right track? Where are the X-axis rectifiers located? Is there anything else that I should be checking?

Any help will be greatly appreciated.

[machintek]

To get to the axis bridge rectifiers: I remove the tape reader in the upper portion of the logic board cabinet. Just below the tape reader, there is a horizontal bank of capacitors. Remove the screws holding this entire unit. You may need to remove some of the wires from the terminal strip. As you pull this forward there is a black heat sink on the right side in the back of this rack. There are 4 VERY LONG screws holding this heat sink to the rack that will take forever to remove. The bridge rectifiers are on the inside of this heatsink. You will appreciate that you removed the tape reader if you have one there.

George

[astr]

Thank you George. Doesn't sound like a fun job! I think I'll try testing the rectifiers remotely (from the transformer end and from the fuse end) before I commit pulling all that stuff. If I determine that the rectifiers are bad and have to pull all that stuff, it may be a good time to also remove the unused computer rack and associated power supply and do a general interior clean up. Thanks again for your help. I'll post up the results of what I find.

Andy

[astr]

The saga continues: I checked the bridge rectifiers that supply power for the X-axis. All OK.

Checked the AC on transformer T2 for the X-axis. Looks OK.

Pulled fuse FU12, thereby disconnecting the X-axis power supply from the stepper and associated drive circuitry. DC voltage popped back up to arround 40 VDC on the power supply side of the fuse holder. Conclusion: power supply probably OK, have a short on the stepper side of the fuse.

Assuming that the motor was OK (I had checked the windings earlier), then the most likely culprit would be a shorted driver transistor or a shorted protection diode. I had checked the driver transistors earlier and they tested OK so I focused on a shorted diode.

A little more trouble shooting isolated the problem to the X2 line which has a protection diode associated with it. Looking at the small printed circuit board that contains 12 diodes mounted down below the transistor blocks on the back door of the control cabinet, I found a diode connected to a lead labled X2. Pulled this diode and tested it. Bingo!, It was shorted.

The diode is a 1N4722 rated at 400V peak reverse voltage and 3 amp average forward current and 300 amp max surge current. Somehow, I suspect I must have exceeded either the 400 volt or 300 amp surge ratings.

What should I replace this diode with?

[astr]

Finally got a replacement for the bad diode. I found some 6A10 diodes rated 1,000V peak reverse voltage and 6 amp average forward current and 400 amp max surge current. I found them on ebay at $1.00 each so I got 20 so I could replace all of them on my mill if I needed to.

I installed the one repalcement and everything is back and working. As George stated in another thread, I need to check and readjusts my stepper static DC current to less than 8 amps. Lastly, I need to get the T2 voltage down to 63VAC. IIRC, mine currently is at around 75 VAC. My line voltage is 490 VAC, a bit on the high side.

George, thanks for all your help.