Mep-002A Bridge rectifier R1 burnt

[Ray70]

Hello folks, I was wondering if anyone has had any experience with a burnt R1 resistor on the A4 bridge rectifier board in an 002 / 003?
I recently picked up another nice 002 from a nearby estate auction, on a tip from another SS member.
The owner told me he acquired it while stationed at Ft. Drum back in the 90's
It has sat at his home ever since and never used.
It was apparently a refurb with 25 hours on it that was drained ( totally dry, clean empty dry tank, empty fuel filter housings without filters ) of fuel before being sold to him.
He let it sit in a storage tent and never touched it.
After going through all my initial inspections I opened the AC reconnect box, since that is a well known location for luxury rodent condo's to be built.
Sure enough, a big mouse nest in there.
After cleaning it out I found a couple wires with damaged insulation, but more importantly I can see that the A4 rectifier is pretty black and burnt at the R1 resistor, so much so that the ceramic is missing near the center of the resistor and the PCB looks like raw fiberglass cloth around R1.
I've got very little experience with the A4 card, but I believe it is part of the gen excite circuit.
I have spare A4 cards as well as spare AC reconnect boxes, but before I replace A4 and start this unit up, does anyone have an idea what would typically cause R1 to overheat?
Being that this unit was refurbed with little use since ( as evident by the non-original numbering tags placed on all the VR , A4 and other TB wires ) should I be suspect of an issue with the Field exciter or its wiring?
Also does anyone know if operating the generator incorrectly ( ie. idling the engine ) only stresses the VR, or is it possible R1 became overstresses due to a low RPM idling?
I'm looking for ideas on what to check before initial start up , to prevent further damage to the machine or damage to the replacement A4 card.

 

[peapvp]

@Ray70 it is part of the exciter / VR circuit and usually that indicates a excessive current draw, most likely by VR Board. I would check the exciter coil first for resistance.

 

[Ray70]

Thanks Peter. I took the plug off the back of the AC reconnect box and it appears the exciter stator winding is about .065 ohms. Looking at the TM I believe they are looking for 3.6 ohms +/- 10% using a wheatstone bridge.
Assuming the wheatstone is similar to a DMM I think I need to dig deeper into the exciter.

Reading an older thread from Triple Jim, it sounds like excite voltage flows from S1 to the stator, then returns to ground through R1 on the A4 board as long as you are holding S1 in the crank position, and disconnects once the speed switch opens. So this sounds like R1 is only in play during the initial field flash at startup.

Think I'll do a little more searching to see what else I can find regarding the exciter stator's normal resistance.

 

[peapvp]

Thanks Peter. I took the plug off the back of the AC reconnect box and it appears the exciter stator winding is about .065 ohms. Looking at the TM I believe they are looking for 3.6 ohms +/- 10% using a wheatstone bridge.
Assuming the wheatstone is similar to a DMM I think I need to dig deeper into the exciter.

Reading an older thread from Triple Jim, it sounds like excite voltage flows from S1 to the stator, then returns to ground through R1 on the A4 board as long as you are holding S1 in the crank position, and disconnects once the speed switch opens. So this sounds like R1 is only in play during the initial field flash at startup.

Think I'll do a little more searching to see what else I can find regarding the exciter stator's normal resistance.

Yes Ray, that’s correct, the TM was written when the only DMM’s where made by HP and Tektronix as 19” Rack mountable bench test equipment weighing in at a whopping 100 pounds….

 

[peapvp]

@Ray70 so far so good, however the DC Resistance of the Exciter Stator Coil is 36 Ohm……
Peter

 

[Triple Jim]

@Ray70 it is part of the exciter / VR circuit and usually that indicates a excessive current draw, most likely by VR Board. I would check the exciter coil first for resistance.

Ray is correct that resistor is supplied by the 24v battery, not the regulator. As he said, it should carry current only when the switch is in the start position. Either you're right that the exciter filed resistance is very low, or for some reason it was carrying current to the exciter field full-time, rather than being cut off when the start switch was released.

 

[Ray70]

After checking a known good 002, either I'm measuring it wrong, or the DMM Vs. wheatstone bridge measurements are completely different.
I also got .062 ohms when checking another gen, so perhaps I'm doing something wrong with my measurement.
I do have a few other machines I can check as well as check if I'm doing something wrong.
looking at the board's condition I would not be at all surprised if it was carrying current all the time, so a check of the wiring may be in order!

 

[peapvp]

Ray is correct that resistor is supplied by the 24v battery, not the regulator. As he said, it should carry current only when the switch is in the start position. Either you're right that the exciter filed resistance is very low, or for some reason it was carrying current to the exciter field full-time, rather than being cut off when the start switch was released.

The Resistor R1 Ray is referring to is on the sub-assembly A4. The Battery has nothing to do with it.
This is a current limiting resistor and excess heat wear indicates a problem with exciter coil itself.
The exciter coil is terminated on TB3-5 P67B16 and TB3-6 P60G16
Remove both wires from TB3 and test resistance: 36 Ohm +/- 3.6 Ohm
If Ok, then problem is VR Board (normally blown Power Transistor). Check also CR2.
From my experience, when the Power Transistor on the VR blows, then there is no voltage produced unless you hold the S1 switch in "Start" which could overheat R1, but you would have to hold it in Start for several minutes if the exciter coil is 36 Ohm for this to happen.
Peter

 

[Ray70]

Got it Peter, I know what to do next. I will give another update as soon as I have another change to look at it.

 

[Ray70]

Turns out the exciter coil is good. 34.6 ohms. Last night outside in the dark I apparently put the DMM on the diode setting, which is right next to the 200 ohm setting!
But when measuring correctly, both at the plug on the back of the AC reconnect box and TB3-5/6 both yield 34.6 ohms.
So it sounds like the VR or an incorrect wire connection are the next likely suspects.
I am very familiar with changing all 3 transistors on the VR and have transistors and several spare VR boards on the shelf, so that's the easy part.
Once I fix up a few mouse-damaged wires I will be ready to attempt to start it up and see what happens from there.

 

[Triple Jim]

The Resistor R1 Ray is referring to is on the sub-assembly A4. The Battery has nothing to do with it.

This resistor is responsible for flashing the field during starting. It is connected to terminals 5 and 6 of board A4. During cranking the exciter field gets 24v from the master switch through diode CR2. The other end of the exciter field at terminal 5 is grounded through the resistor, via terminal 6 and the starter disconnect switch S7. Unless something is grounding the resistor full time (which would overheat it), it carries current only when the master switch is in the start position and the starter disconnect switch is closed, which is only when the engine is not yet up to speed. Note that the board labels + and - of the schematic are reversed (wrong).

Some early generators had the resistor grounded full time, so that you could flash the field any time the engine was running and the master switch was turned to the start position. The change was probably made to prevent the resistor from overheating if the switch was held in the start position for an extended time.

Terminal 4 of the board supplies the full output current of the bridge to the exciter field. The regulator sends current through CVT1's control windings (C1 and C2 terminals) to control its AC output to board A4. The output of the regulator is highest when it's trying to lower the generator output, and lowest when it wants to raise it. That's why this particular generator, unlike other generators, does not stress the regulator when the engine is below 1800 rpm.

 

[Triple Jim]

So it sounds like the VR or an incorrect wire connection are the next likely suspects.

If the regulator is sending excessive control current to CVT1, which is one mode of failure, then it could, as you say, be the problem. If this is the case, if you disconnect the wire from terminal 17 of the regulator and tape its end so it doesn't contact anything, you will prevent the regulator from sending control current to CVT1, and the generator output voltage will go up to about 170-0-170 (340). If this doesn't happen, the problem is somewhere else.

Swapping regulators is the sure way to get the answer, but maybe posting this will help someone else who reads this later.

 

[Ray70]

Ok, so I will begin by starting the machine and seeing what happens, keeping an eye on A4 and the generator output. Then I will disconnect #17 off the regulator, isolate it and see the result.
Thanks for the guidance!

 

[Guyfang]

I have seen several R-1's burnt. Every time it was someone trying to Troubleshoot a problem, and held up the S-1 in the start position for forty forevers.

 

[Ray70]

Status update:
Last night was an interesting time, I taped up the mouse-damaged wires in the AC reconnect box and went ahead with the initial start-up.
This set has been sitting for well over 25 years.... with the notorious PSU pump in it! Cranked it up and she fired up almost immediately, but.... would not stay running after releasing the start switch. Turned out the temp switch was bad, so I jumped the connector and it fired up and ran on 1 cylinder for a few minutes, then came to life on both cylinders. With that said.... Guy's comment about people holding the start switch and burning up R1 makes a lot of sense since the temp switch was bad.
Now for the more interesting finds:
AC voltage was present but fluctuating on the gage and not reading correctly. I was getting 120V on the gage while in the 120V L3-L0 setting but about 208 when AM/VM switch was set to 240 L1-L3 position.
Then after switching between 120 and 240V setting I got nothing on the gage in the 120V setting. This often indicates a dirty AM/VM switch and sometimes issues with the AC reconnect switch.
Stuck my Kill-a-watt into the courtesy outlet and got 239V ???
Verified the lugs were at 239 between L1/L3 so the VR was working correctly.
As I'm looking inside the AC box to see if I can follow the wires from the courtesy outlet the engine coughed for a second, threw a puff of black smoke and suddenly the magic white smoke billowed out of the center hole in the CT/CVT1 assembly.
Luckily I was able to shut it down within seconds of the smoke screen!
After looking at the schematic, the AC outlet is powered directly from the main ac select switch, so I suspect there is an incorrect wire connection causing power to be present on the neutral side of the AC outlet, or possibly a short somewhere in the AC box.
So at this point my curiosity wants to first see what's going on with the courtesy outlet, then see what caused the smoke, but I'm also thinking of either swapping out the AC box with a spare and diagnosing the defective AC box later, but also thinking this set may be a good candidate for an SX460 VR conversion, digital gages and an overall "civilianized simplification" of the electronics.
I have swapped CVT1 once before in my life and probably will not do that again! I'll swap the AC box and save myself tons of 4 letter words.
I guess the good news is that she runs and the PSU pump is working, since the current asking price for an IP seems to be more that the entire generator is worth fully serviced and tested.

 

[Light in the Dark]

I guess the good news is that she runs and the PSU pump is working, since the current asking price for an IP seems to be more that the entire generator is worth fully serviced and tested.

One of the reasons I am done with the air cooled sets... just too costly if there is a problem these days. That and the request for such items are low here in the Northeast (for me at least). I'm glad to see you are moving through this with relative ease though, all things considered.

 

[Ray70]

Well, a bit of good news tonight. Just swapped out the AC box and everything seems to be back to normal. Voltages are all correct and no stinky smoke pouring out anywhere anymore.
Looking closer at the original AC box, it looks like all the heavy AC wires in the thing are discolored. Instead of white they are mostly all more like a light tan color, indicating excessive current / heat at some point.
Also being the machine was reworked shortly before being sold, every wire in the AC box has been removed, labeled and reinstalled. Some are definitely routed differently than originally ( up over the main switch body, rather than under it but it is difficult to tell just yet where the wrong connection is located, but the wires from the outlet to the terminal block TB2 are correct, so I'm really leaning towards a misplaced wire or more on the AC select switch itself.
The gages in the machine were changed ( had newer style from an 802/803 but the HZ gage reads low and bounces around a lot and apparently last night the AC gage died. At first it was working incorrectly, then it stopped. Tonight I confirmed the gage is getting 240V but reads nothing. So tomorrow I'll swap out a couple gages and get ready to load test.
It's also a bit sluggish starting when cold compared to other 002's I have and it's about 40F right now, so the glow plugs and their control circuit probably need to be checked as well.

 

[Guyfang]

The new/used AC box. Did it come from an MEP-002A or a MEP-003A? They are kinda interchangeable, but not 100%.

 

[Ray70]

AC box is off an 002. I know the circuit breaker is different, as well as the number of loops through the CVT but I also notice the schematic in the 003 TM shows the ac outlet being wired right to the AC select switch, but the schematic plate on top of the 002 shows the wires going to TB2, which they do.
No idea if the 003 TM schematic is correct or not, but I know the plate on the 002 is correct.

Today I ran the machine at 80% load for about 2 hours without any hiccups other than the HZ gage and / or transducer went from erratic and incorrect to just plain inoperative. So for the gages I've got digital gages and custom made adapter rings already in my stash as well as newer style transducers and spare gages, so it's a toss up, maybe I'll fancy up this unit and go with digital gages, maybe even throw on my spin on filter kits, not that I plan on keeping it. I already have my keeper 831, 002 and 003 all setup in a ventilated shed. All hooked up with a transfer switch and a synchronizer from an 802 power plant I had.

 

[Guyfang]

If I remember right, the R3 needs to be adjusted, when a box is swapped? I am going to bed now, but will take a look see in the morning.