Mep003a blowing resistor on bridge rectifier

[Robbied46]

Hi all!

im new to the site and am very impressed with the knowledge of the members. I'm having a problem with the resistor on the bridge assembly blowing on start up. I have been careful not to hold the start switch too long and the problem persists. Now for a little background. There is no problem with starting the gen and it produces power as it should. I have noticed that after I replace the resistor and restart, the voltage spikes above where I had it set and remains high until the resistor blows. As soon as the resistor burns out, the voltage returns to normal and everything appears to be ok. Could this be a vr problem? I'm thinking my next step is to make sure resistor R3 and R16 are properly adjusted. Any help would be greatly appreciated as this has left me scratching my head.

 

[Keith_J]

Check voltage at terminal board A1, connection 8 with the main switch in the run position, engine off. It sounds like it is getting 24 volts there in the run position, it should only be there when the engine is cranking. This flashes the field and engages the starter, it is fed by terminal 27 on the master switch. Could be improperly assembled.

 

[Robbied46]

Thanks for the insight Keith. I checked the voltage as you said. There was no voltage at connection 8 until I started to crank. When I released the start switch, voltage went back to zero as expected. This is where it gets weird. If I leave my meter connected and start the unit, I get battery voltage while cranking. It drops off when the start switch is released but then shows -59.9 volts while running! I'm using a digital fluke meter. It's as if the ground becomes positive. This reading continues until the unit is shut down. Output volts and hertz are normal.

 

[Speddmon]

There is your problem...you are getting WAY too much voltage from the DC regulator. It is bad!! Replace or remove that and see if your problem doesn't go away

 

[Robbied46]

Thanks for the help Speddmon. I removed the blower housing to get access to the dc regulator. I unhooked the three wires and even pulled the fuse. Still on start up, the voltage pegs the meter at 300 when it was adjusted to 220. I attached the resistor on the bridge with alligator clips so I could disconnect it quickly. As soon as I disconnect the resistor , the voltage drops back to 220. I disconnected the wires at TB3-5 and TB3-6 and restarted and got no output. I reconnected them, hooked the resistor back up, restarted and the voltage went back to 300 until I disconnected the resistor again. I sure appreciate any ideas as to what to try next. In the mean time, I will continue to study the tm.

 

[Speddmon]

I'll look into the TM's a little deeper when I get some time later tonight at work. For the time being, are you sure that the resistor is the only bad thing on the bridge board. Maybe you have a shorted diode as well.

 

[Robbied46]

Thanks so much for the help! I will check the bridge board for problems again in the morning. Using the diode check function on my meter, I checked each individually before and got good checks. I was not really happy about the results I obtained according to table 6-6 so I will do it again. The tm said the readings should be really low one direction and high when reversed. While the readings were higher in one direction, both were high. For instance, one direction would give approximately 3 M ohms and the other direction 12 M ohms. I was surprised to see readings this high. Is something wrong or would that be normal?

 

[Keith_J]

3M ohms in one way and 12M ohms in the other is a blown diode.

 

[Speddmon]

Like Keith said, you've got some bad diodes as well.

You could possibly have a bad CT/CVT assembly as well, go through the TM and check the resistance readings against the specs in the TM. I think you might find some really funky readings.

 

[Keith_J]

I doubt the transformer has an issue. It is just windings in an iron field.

The primary side windings are the output of the generator, when the load increases, the magnetic field increases which increases the voltage and current on the secondary. This increase helps to stabilize the output voltage, much like a Lima MAC generator which has no electronic voltage regulation. I believe Onan called this a Magniciter (tm). Marathon-Lima uses Magnetic Amplifier Controlled (MAC).

 

[Robbied46]

I did some more checking today. The diodes on the bridge all test good. The M ohm readings occur when not using the diode test function. The meter voltage is too low that way. CT1 checks good. CVT1 has problems. All H connections test good but not the X connections. X1-X2 reads 39.1 ohms , X1-X3 reads 39.1 ohms, and X2-X3 reads 1.6 ohms. It looks like all three should be in the 1.6 range. Also, I noticed a crack on top of the linear reactor. What does this do and is there anyway to test it? Does anyone have any idea as to where I may be able to get these componets ? Let me say thanks guys for all your help. It has been invaluable.

 

[Speddmon]

The secondary sides of the CVT is exactly where I thought you might find issues. I would say that needs replaced because once you let go of the start switch the AC voltage from the CVT is rectified through the bridge board, and is controlled with the VR to keep the field excited.

The linear reactor is nothing more than a noise filter for the CT assembly. Since all of your metering and such goes through the CT assembly, noise will play heck on your readings. It should also have some effect on the power quality of the output...that's one reason why these gen sets put out such clean power.

 

[Robbied46]

I ordered a new cvt1 transformer and linear reactor from Delk's today. I'll let everyone know how it turns out. Thanks for all the input and great advice

 

[Robbied46]

I received the cvt and linear reactor from Delks today and am hoping to do the install tomorrow. All I can find about the linear reactor in the tm says if there is physical damage, it should be replaced. The original one from my gen has a small crack in the top of the housing. The one I received from Delks also is cracked but they believe it to be a good one. If I read the resistance between any of the 3 pairs of terminals on the original, I get 2 ohms. The one from Delks reads 12 ohms. Does anyone know which one may be the best one or know of a proper test for this component?

 

[Speddmon]

I can't tell you right off hand what the readings should be for that reactor. I have one at home I could check for you, but it won't be for a few days...my schedule it too tight currently to be able to get to it.

BUT, since it is basically nothing more than coils around an iron core, I would think the 3 coils should have very close to the same resistance. As long as they do either one would probably be good.

 

[Robbied46]

Issue resolved ! Changing out cvt1 seems to have done the trick. I can't thank Keith J and Speddmon enough for all their help! I would have had no idea of what to check. As a footnote, milbatteries.com has the adapter trays for converting 6tl's to the optima 34 for $14 each and will sell to individuals.

 

[Speddmon]

Congrats on getting the issue fixed. I love to hear when someone finally fixes a problem with these generators, and hopefully learns something in the process.