MEP-803a Short Circuit

[jamawieb]

Having a short circuit fault on a recently acquired 803a. When I first got it started the short circuit fault light was on, so I hit the reset and everything seemed fine. I hit the AC interrupter closed, with nothing attached to the lugs and everything was running perfect (power to lugs). I hooked the lugs up to a 240volt 7.5kw heater that I use for load testing, it tripped the short circuit fault as soon as I closed the AC interrupter. I tested the heater on another generator and it worked perfect, so it's not the heater.
I replaced the short circuit/overload relay K8 with a brand new one and it still trips the short circuit fault. I also worked the reconnection switch and AV/VM switch several dozen times to try to clean the connections (also used contact cleaner on both switches). I've seen some post about the burden resistor's (R10,R11,R12,R13) could be the culprit but looking at the wiring diagram, it looks like the burden resistor's bypass the short circuit/overload relay. I didn't want to de-solder the burden resistors to test, if they were not part of the equation. The TM says to test them without de-soldering but I'm not getting correct readings (I did the same test on another good unit and got the same results from the test). I'm hoping someone with more experience can help me with this problem.

 

[Guyfang]

You have to do this at night, or at least where you can make it dark. Start up the set, with the load hooked up. Open the control panel door, look at the burden resistors. Turn off the lights, and switch the S-5 AC interrupter switch to the closed position. See a spark where the resistors are? Sometimes you can see it, and sometimes not. But I have done this several times and that how I found the culprit!

If you de-solder the resistors, make sure your soldering iron is HOT before you start. Don't hold the tip on the connections long, and when you re-solder them, same as above. The standoff terminals that the resisters hook up to, are very prone to break off when you overheat them. When ever I had bad burden resisters, the short circuit light came on.

 

[jamawieb]

Thank you so much Guyfang, I'll try that and report back. I've been trying to find a source for the burden resistor but I can't find an exact matches for the part number. I've found some with higher ohm readings but nothing like OEM. Apparently the HLM-20-10Z7R50J resistor was discontinued this year. Anyone have a source for the resistor?

 

[Haoleb]

I don't know the specs on those resistors but you should be able to get whatever you need from www.mouser.com As long as you know the resistance and wattage that is really all you need. I believe those are wire wound resistors.

 

[jamawieb]

Here's a pic of the resistors, they are a little different. The HLM-20 have been discontinued and replaced with FSOT-20. They are 7.5ohm 20watt with 1% tolerance. The only thing I can find is 5 or 10ohm 20watt with 10% tolerance.

 

[Haoleb]

These should work.

7.5 ohm 20w 1% tolerance. Still a chassis mount but different so you'll have to tap a couple of holes to mount it.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtlubZbdhIBIDTODbDtFnrRo2vN02o1xgA=


5 week lead time. I would imagine someone around here has some of these kicking around from a spare parts unit if you didn't want to wait.

Here is another option, 7.5 ohm 1% but it is 25W. These are in stock.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtbXrIkmrvidDNaDpN5VXc5w3KOGABl0Lc=

 

[jamawieb]

So after many calls and searching, I finally found the direct replacement http://www.mouser.com/search/ProductDetail.aspx?r=71-FSOT2011E7R500JE. Vishay changed the manufacture number, the only difference is the new resistor has a 5% tolerance instead of a 1% tolerance. But these should fit directly in the spot of the old resistors.

Today, I tried Guyfang's experiment of finding the bad burden resistor in the dark but it did not work for me. So I started de-soldering the resistor's and found R11 is in fact bad. So, as soon as I have a replacement, I'll report back. Thanks to everyone who helped.

 

[jamawieb]

To give everyone an update. I took R11 out of a good unit and now the generator is producing power as it should.
So if anyone has a short circuit fault, more than likely it will be a burden resistor and you do in fact have to de-solder the resistor's to test them correctly (although the TM does not tell you to)

 

[Daybreak]

Howdy,
Well, you got it fixed. You will have the whole panel board shortly.
I left the wires extra long. You can use that extra wire to hook the gauges to the panel lights.
MEP-802A/803A gauge lighting

 

[jamawieb]

I had some free time today and figured I would make sure that was the only problem with the short circuit fault. LOL! Now, when I get the board in, I've got to repair the unit I took the resistor from. Thanks so much for the help!

 

[Guyfang]

do not think the R-11 is the only place this can happen! I have found lots of other places that the wire harnes rubbed through and up pops the short circuit. Never assume!!!!!!

But good work getting to the problem and fixing it.