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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan 7.5JB-3CR/9804T - Welcome 'red' to the family


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  #101  
Old 08-01-2017, 09:51:22 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

BStrutton, something you said does not compute with me. Could you explain what you mean by : "--Fired it up and now I'm getting 140 on 1 leg and down to 0 on the other. Investigating revealed that I must have sheared off the 'inside' brush on that PIA 8'clock position holder. --".???

Reason I say this is because the revolving field rotor is powered by a simple DC voltage in the vicinity of 40 volts. So if you are reading 140 volts on one leg of the stator windings and nothing on the other, it seems to me your problem is with the stator not the magneciter.

As for the higher voltage, I would suggest you first ring out the stator 120/240 circuits and then go and investigate the relatively higher output voltage.

Just my two cents.

Leon
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  #102  
Old 08-04-2017, 11:49:32 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Leon,

What I meant was that post-cleanup of the slip rings, I was measuring with my meter on the wires downstream from the junction box, I was getting:

140v (about 20% over normal) when measuring between 1 of the hots and ground.

Zero-ish volts measuring between the other hot & ground.

I re-checked my brushes because I had taken them off while cleaning up the slip rings and had a bad feeling. Sure enough I had not seated the ones in the brush holder at the 8'clock position and had sheared the inboard-most brush in that position.

An update, I replaced the bad brush today and that seems to have taken care of the odd overvolt situation -On the first leg that was reading 140v above, it is now reading 121v.

Unfortunately on the other leg I am now reading about 3v.

If it sounded like it, I was not originally trying to indicate that I thought the trouble was magniciter related -at this point I have no idea. The only thing I know for sure is that I did check for continuity between the slip rings and none from slip rings to shaft and both checked ok.

I've been trying to really dig in on the magniciter FAQ and search a bunch of other posts this week whilst waiting on the new brush but honestly find myself struggling to get my head wrapped around the 'big picture'.

I think what I need is a 'readers digest' version of how things flow, start to finish, maybe without getting into all the more detailed saturation stuff. The manuals, which I've been through extensively, seem to jump right into the magnaciter stuff but I'm not sure I even understand the relationship of the rotating field to the brushes to the magniciter, etc. I can't get a good picture in my head of how it all fits together. If someone could help put that into perspective, I think I'd be far ahead in troubleshooting further..

Thanks to all for the words of wisdom on this project -it has been an interesting diversion thus far!
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  #103  
Old 08-05-2017, 12:46:55 AM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

You might want to ohms check between your output "T" wires. T-1, T-2, T-3 and T-4. Make sure you have no opens. Just a thought.
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  #104  
Old 08-05-2017, 02:26:12 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Measuring downstream from the junction box???? To me that is very confusing.
You have not explained your junction box. Nothingbutdarts has the right idea. Find the four wires he mentions. Temporarily disconnect ALL for leads from anything so that you can make the following continuity measurements. Then measure the resistance or at least look for continuity between T1 and T2. OK? Now do the same between T3 and T4. You should get pretty much an identical reading. OK? If not, stop right here. Something does not compute. You must resolve this dilemma. What are those readings?

If these readings check out, STOP! And reconnect T2/T3 ( the neutral) back the the chassis before attempting to start the machine. Note, if your machine is wired for 120/240 volts, then you should measure ~120 volts between T1 and neutral and between T4 and neutral, with T2/T3 designated as the neutral. If all OK so far, then you should get ~240 volts between T1 and T4. It is as simple as that, unless of course something is broken and/or your machine is wired differently or someone messed around with it. Let us know what gives. Note: If you are measuring 121 volts on one leg, you should see the same @ NL on the other leg!
It would help if a fellow Staker could tell us what your JB factory installed option #9804 represents. You make have some sort of a unique design?

Last edited by Leon N.; 08-05-2017 at 02:29:43 AM. Reason: Add info
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  #105  
Old 08-05-2017, 08:13:35 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Leon & nothingbutdarts. I believe that I have success at long last. It's been almost a year and a half but I just saw the JB produce 240 volts for the first time!

Thanks so very much for the help along the journey.

Here are a couple of pics of the junction box the previous owner had installed where the T2 & T3 were joined with a green wire running to the chassis and where T1 & T4 were joined to the black and red wires in the whip that exit the junction box for connection to 'whatever'..

https://www.dropbox.com/s/x37nx1qvt6...onbox.jpg?dl=0

https://www.dropbox.com/s/q09esythqg...Block.jpg?dl=0

I took y'alls advice this afternoon and disassembled the 'junction box'. Unlike the rest of the generator, it was pretty clean inside with some pretty decent looking connectors. I did find a little corrosion inside one of them but oddly enough it was actually on the T4 leg that had been measuring out ok (on the whip external to the box).

Took the fluke & ohm'd out T1-T2 and got continuity and around .2 ohms. Same on the T3-T4 side.

I also found that there was a pair of fairly skinny solid wire coming out of the box (white & green) that had been joined to the neutral and to T4. It's been so long now that I've lost site of where those went (maybe the hobbs?) but regardless, I taped them off, reconnected T2/3 with the green wire I've got going to chassis.

Put the meter across T1 & T4 and fired it off and had a great degree of relief when I saw 240v on the meter.

Now that I've made it past the seized motor that we started out with and actually have it seemingly producing the correct output, I feel like I'm ok to start putting more time / money into getting it cleaned up / restored / modded the way I want it. Beforehand I was kinda in a holding pattern as I didn't want to piss money away.

Thanks again for the assist.
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  #106  
Old 08-07-2017, 04:44:13 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Time to get serious with the cleanup... Nasty what a mice/oil/dirt combo will deposit after so many years! Pulled the skid off to have a good go at it and see what lies beneath.

Couple of hours with some plastic scrapers, a can of gunk and some hot water and pressure at the car wash and it's much more recognizable...

By the time I was done scraping sludge, that white cardboard box was 3/4 full -yuck!
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Last edited by BStrutton; 08-07-2017 at 05:07:35 PM.
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