MEP803A Resistor - Diode Assembly

Can someone explain to me what the resistor diode assembly in the control section of the genset do? I saw a genset that may have bad resistors and diodes, discolored and an ashy substance on them.

What do they control/what function do they serve in which circuits?

They provide balancing loads for the over current system.

From top to bottom...
R10 thru R13 are the burden resistors for the over current circuit and the %Load Meter circuit.
R14 is part of the field flash circuit
CR2 is part of the field flash circuit
R15 is a current limiting resistor for the Battleshort and AC Interruptor indicator lights
CR1 feeds power to the the Dead Crank switch
V5 is a surge protector for the 24 volts in the control cube

Looking for a source for these parts:

48 XDFZZ XB XBFZZ 91637 HLM-20-10Z7R50J ...RESISTOR, WIREWOUND 4
49 XDFZZ XB XBFZZ 91637 HLM-20-10Z-20R0J ...RESISTOR, WIREWOUND 1
50 XDFZZ XB XBFZZ 91637 HLM-10-10Z1301J ...RESISTOR, WIREWOUND 1

53 PAFZZ PAFZZ PAFZZ 80131 1N5406 ...SEMICONDUCTOR DEVIC 2
54 PAFZZ PAFZZ PAFZZ 03508 V68MA3B ...RESISTOR, VOLTAGE SE 1

I found a source for 53 and 54. Need 48, 49, and 50.

These are the wirewound resistors on the resistor/diode assembly. As an alternative if someone has parts, PM me.

Do not know if they are bad or not untill I test.

Thanks

Drop in replacemets for them are available. Crazy expensive though.
If you don't care if replacements are drop ins you can get less expensive wire wound resistors and modify the plate as needed to mount and wire them.

A replacement for #48 is a FSOT2011E7R500JE. They are available from Mouser Electronics.
https://www.mouser.com/ProductDetai...CPRwebPKtkfhQJyJNJP1Y4QMMNcBPyDfVIp1fLMwycA==


A replacement for #49 is a FSOT2011E20R00JE. They are available from Mouser Electronics.

https://www.mouser.com/ProductDetai...ebPKtkYqxHft%2bk9QLJMAXcH/apsHp%2bv6sGzBNcg==



A replacement for #50 is a FSOT1011E1K300JE They are available from Mouser Electronics.

https://www.mouser.com/ProductDetai...CbIc7okaw2EOl7QoWHV/3rjKZfZXux11u6ElbXcxfjA==

Any particular reason that these can not also be used. I am not an electronics component expert, heck I can barely not confuse diodes and capacitors and transistors except that they are round, flat, and tubular, sometimes......

https://www.mouser.com/ProductDetail/71-RE70G7R50

https://www.mouser.com/ProductDetail/71-RE70G20R0

https://www.mouser.com/ProductDetail/603-SQP10AJB-1K3

https://www.mouser.com/ProductDetail/576-V68MA3B

https://www.mouser.com/ProductDetail/78-1N5406GP-E354

Just looking for alternatives.

Take care un-soldering and soldering the components. The plastic stand offs are very sensitive to heat. If you hold the iron on the posts too long, they get soft and break off. Test before replacing. If it ain't broke, don't fix it.

Those are the much lower cost types I was suggesting that could be used instead of the pricey ones.
They look good to me.

I have a feeling that there are at least the diodes and the varistor are going to have to be replaced. Have to find the cause first...





Looks like someone put the heat to these. Have a feeling that the genset or the generator part at least may have some damaged components.

Looks like both CR1 and CR2 have been replaced based on the solder blobs and corrosion on their leads in the pics.
V5 looks baked too. Ought to be red in color.

My thoughts too. Need to start diagnosing the problems. Considering these are all generator related, I need to check all the generator leads before I even attempt a start.

Took the cover off and examined everything including the burnt electronics smell (there was none) and found this to be the only thing that appears to be out of order (not normal looking). So it appears that the 2 diodes and the varistor may be bad. The varistor is red colored. This is a very clean set. Some dust probably due to the missing door. Never saw one that actually had all the radiator bleed hoses and the lift hoop.

So CR1, CR2, surge protector. I will pull off the back plate of the generator and run all the chapter 4 depot relavent test tomorrow on the generator. More reading of the tm's tonight.

Pictures of inside the set. Also better pictures of the burden resistors and diodes.

More:



Looks really clean except for that disaster at the burden resistors.

As I said, I opened the back of the generator today and did the depot testing on the generator itself to determine if there is something that may have caused this issue with the diodes. Here are the results...





Exciter F1 to F2 = 13 ohms

F1pos to CR1neg = 6 ohms
F1neg to CR1pos = ~

F1pos to CR2neg = 6 ohms
F1neg to CR2pos = ~

F1pos to CR3neg = 7 ohms
F1neg to CR3pos = ~

F2pos to CR4neg = ~
F2neg to CR4pos = 6 ohms

F2pos to CR5neg = ~
F2neg to CR5pos = 6 ohms

F2pos to CR6neg = ~
F2neg to CR6pos = 6 ohms

Exciter Rotor Leads: (CR1 to CR2, CR1 to CR3, CR2 to CR3, CR4 to CR5, CR4 to CR6, and CR5 to CR6) .6 ohms

Generator Rotor Leads: 2.4 ohms

Generator Stator Leads: (T1 to T4, T7 to T10, T3 to T6, T9 to T12, T2 to T5, T8 to T11) .35 to .4 ohms - This is the best I could manage. I tried with two different digital multi-meters and neither registered anything. I threw them out because I have been having problems with them for awhile. So I went to my analog multi meter and I just barely got the movement of the needle. It would huddle about .4 then slowly move to .45. I measured three different times to make sure that I could be as accurate as possible. I need a multi-meter with X10, X1, and X-1. It is almost impossible to get a accurate reading on an analog meter for such a tiny resistance.

Q1 to Q2 = 1.1 ohms

Based on table 4-1 in the TM, all these values seem to be within limits so I think that the generator seems to be OK.

I could not get the end bell off. The cover was no problems and the four nuts came of easily. But I could not for the life of me get to the two bolts that were holding the stator to the end bell. So I was never able to take the end bell off but I was able to work around it to get the measurements above.

I put a new air filter, oil filter, and fuel/water filter on the set, added new 15W40 diesel oil, 5 gal of fresh diesel (the genset was dry of all fluids), filled the radiator, put a couple of batteries in the set and started checking the motor and DC circuit. I need a new fuse holder for the 30A fuse. The top half does not stay on the bottom half with the fuse installed. Also need the small fuel filter as well.

The box wrench for the lugs is for lugs a lot bigger than the lugs on this set. Maybe a 100KW set? About twice as big.

I have head the aux fuel pump, the primary fuel pump and the starter engage the flywheel (accidentally when I moved the crank switch in the wrong direction - doh!). The motor moves freely as does the generator.

So tomorrow I will do the fuel filter and the fuse holder, test the diodes and varistor along with the burden resistors. If everything checks out and I can not find any thing wrong with the engine, I will try to crank it over and see what happens.

Well, today I replaced the fuse holder for the alternator. Checked it over once again and gave her a shot. She turned over a bit then caught and fired right up. Let it run for awhile then decided to check the output terminals to see if there was anything and all is OK here as well, as I suspected per yesterdays generator check.

She is spitting some carbon snow flakes but seems to run fine.

This generator was sold for scrap on gov liq at an auction from "Defense Reutilization & Marketing Office" in Jacksonville, FL. I took a chance and went to $3200 (I know, insane not knowing if it even ran, turned over, or did anything) and won but still could not believe I went that high. I was my final bid and if the other parties interested bid again, I was out. Everything seemed to be taken care of in the pictures on the sale site and there seemed to be no visible damage anywhere except for the convenience outlet plug cover.

So so far, I have put an air filter, oil filter, left the used fuel water separator filter (looked brand new), and I have on order the small fuel filter. I put 4 qts of 15W40 in her and filled the radiator with water for now as I am going to flush before I fill with water and rust inhibitor. And I will replace the diodes and varistor when I get the ones I ordered from Mouser.




Item Manifest




Total cost so far on repairs/items is about $29 for the air filter, $5 for the oil filter, $6 for the fuse holder, and $16 for oil. These are all expendable and would have been replaced anyways. So $56 is not a bad deal to get the genset running.

Next is the load test and that will tell whether I got a good working unit.

https://youtu.be/bndo85NI7F4



Seems I have better luck at the scrap yard then from a refurbishing seller.

So why replace the diodes? If it works, don't fix it.

I actually put a bid on that auction when it popped up. Was going to see if I could get it at the same price I got my last one for out of Albany MCLB.

But I knew that wasn't probably going to happen...considering how the prices have sky rocketed over the past 3 years or so.

Glad to hear you got it and made it a runner.

Guyfang said:

So why replace the diodes? If it works, don't fix it.

Click to expand...

Take a look at the butcher soldering job. It was butchered. Leads were cut and resoldered and the solder joints are questionable. The varisror was 67 cents and the diodes 54 cents each. Yes I will replace them. Sometimes even if it works, it has to be fixed or tweaked. Replacement of these parts is easy and it will look so much better.

kavesman1 said:

I actually put a bid on that auction when it popped up. Was going to see if I could get it at the same price I got my last one for out of Albany MCLB.

But I knew that wasn't probably going to happen...considering how the prices have sky rocketed over the past 3 years or so.

Glad to hear you got it and made it a runner.

Click to expand...

You are right about the skyrocketing prices of these units. Sorry I stepped on your toes on the bidding. What did you pay for your one out of Albany?

On this one, I got lucky.......

I put varying loads on the genset for 3 hours. Put 40% then 65% then 85% loads on it. All ran OK with no issues.

There is something that I noticed today but it came home this way. I do not think the motional transducer is operating properly. The starter pinion gear is not disengaging when the genset starts and reaches 900 rpm.

I tried to measure for dcv across the transducer. I think I recall reading that there should be about 3vdc or so. Still looking into it. Anyways there was no dcv across the terminals. When I removed it there was some junk on the magnet so I brushed that off and I am getting about 150 ohms across the transducer.

Any ideas on this one?