Another busted Trailblazer 251 with only 290 hours! Anybody have bad PC2 boards?

[denwhit]

I have the identical problem another posted described with this machine SN KH422829. Have power at the convenience plugs but no weld power. I will begin trouble shooting next week as the tech manual and experts on here have suggested. The previous owner told me he did "jump start" the welder a few times. From reading, I think that is the death move for these machines. The main reason for this new thread is I'm wondering what everybody does with their bad PC2 and other boards? I was an electronic technician in my working life and I might be able to fix a board by troubleshooting down to a component. I'll post my progress when I get started. Thanks in advance.

[denwhit]

Went backwards

Well, today, I took the covers off and decided to check some diode readings, look at the brushes, etc. While taking a wire off CR 2, the lead came out of the diode. So, off to the electronics store. Luckily they had one. Replaced that and started it up. Now, instead of 120 VAC at the convenience plug, I have 24! I rechecked the replaced diode and the wires, checked the brushes, looked all around but could find nothing different. How do you perform this step: "Disconnect leads 23 and 24 from brushes and check continuity across slip rings" If I read this correctly, you'd now be reading from one brush through the rings to the other brush. What kind of reading in ohms should I get? Do you stick the commutator stone through the brush holder when stoning the rings? Where do you get one to fit the brush holder? Where does one buy a stone for this job? Thanks for any help in advance. May have a good Onan motor for sale here shortly.

[duaneb55]

On slip rings I use and recommend just 320grit or finer sandpaper applying just enough pressure to clean them.


Specific component failures on Trailblazer Power and Control boards vary but optocouplers, transistor gate resistors, zener diodes and IC chips are among the list. Anyone repairing these boards needs to keep in mind a component failure on one can cause one or more component failures on the other in applications that involve both Main Control and Power boards.

[denwhit]

On slip rings I use and recommend just 320grit or finer sandpaper applying just enough pressure to clean them. Specific component failures on Trailblazer Power and Control boards vary but optocouplers, transistor gate resistors, zener diodes and IC chips are among the list. Anyone repairing these boards needs to keep in mind a component failure on one can cause one or more component failures on the other in applications that involve both Main Control and Power boards.

I was hoping you would respond as you are the expert! I'm thinking my problem might be with the slip rings because I did take the brush bracket off and look at it and just reinstalled. I can't think of anything else I did to cause the voltage at the convenience plugs to go from 120 to 24. Thanks for your advise on the 320 grit sandpaper. I'll use that. What resistance should I get measuring from ring to ring? Are you open for more questions as they arise and offering more help, if so here is my email address; [email protected].

[duaneb55]

I appreciate the compliment but I'm no expert by any stretch of the imagination. I'm more the blind squirrel that finds a nut once in awhile.


If you're now only getting 24VAC at the auxiliary outlets my guess is you're only getting 12VDC flashing voltage (battery +) across the brushes.

If that's the case, I would check for a blown exciter output fuse F1 and/or failed exciter bridge rectifier SR3 for starters.

[denwhit]

I appreciate the compliment but I'm no expert by any stretch of the imagination. I'm more the blind squirrel that finds a nut once in awhile. If you're now only getting 24VAC at the auxiliary outlets my guess is you're only getting 12VDC flashing voltage (battery +) across the brushes. If that's the case, I would check for a blown exciter output fuse F1 and/or failed exciter bridge rectifier SR3 for starters.

SR2 and 3 are good, fuse F1 is good, I cleaned the rings and the brushes are making good contact. Had about 24 ohms between front ring (from control panel) to middle ring and about half that from middle to back ring.

[denwhit]

LEDs on PC2 are not lit

Should two of these be lit?

[duaneb55]

Yes, LED #2 and #3 should be lit when the board is energized.

Did you check DC voltage between the center and rear slip rings?

[duaneb55]

And your resistance reading between the center and front rings is a little low and should be 31ohms but the exciter rotor would be good at 12-12.5ohms.

[denwhit]

And your resistance reading between the center and front rings is a little low and should be 31ohms but the exciter rotor would be good at 12-12.5ohms.

I'll check those when I get back from a four day short vacation. When I get back, I'll take all of those resistance readings. Is anyone looking for a low hours 20 hp Onan motor? I'm about to give up. If it's PC2 or the windings, I think it's totaled and will part out. Thanks for your help, I really appreciate it.

[duaneb55]

I wouldn't throw in the towel just yet.

Enjoy the vacation!

BTW, if you can't find me here on the Miller sight, I spend most of my time over on WeldingWeb http://weldingweb.com/forumdisplay.p...&s=&daysprune=

[walker]

FWIW, mine turned out to have a broken wire in one of the wires going down to the slip rings. The terminal in one of the molex connector cracked. Since the other wires were fine, that one stayed in there, but didnt excite the generator.

[denwhit]

And your resistance reading between the center and front rings is a little low and should be 31ohms but the exciter rotor would be good at 12-12.5ohms.

Had a minute this morning to check those out and I get 30.8 and 12.1 ohms.

[denwhit]

FWIW, mine turned out to have a broken wire in one of the wires going down to the slip rings. The terminal in one of the molex connector cracked. Since the other wires were fine, that one stayed in there, but didnt excite the generator.

I hope mine turns out to be that easy (cheap fix). What reading, voltage or resistance, showed the problem?

[Can You Weld This For Me?]

Have power at the convenience plugs but no weld power.

Just want to triple confirm, you do have the output contractor switch "ON" correct?