MEP-804A suspected voltage regulation issues.

[Rexit]

First post and it is a big one. Thank you in advance for taking the time to read it.

My dad recently got an 804a generator with 2 hours on it. It still has the break in oil in the engine. The injectors weren't getting any fuel and he gave it an attitude adjustment with a slide hammer until it started working again. I wasn't present for that part... Engine runs great now. The problem is he started it up while things were stuck wide open and it took him 30 sec to get it stopped. During this time, some electronics got fried. During startup (while cranking), the frequency and voltage are right where they should be. After cranking stops, the engine runs great but the voltage and frequency drop off.

Seems similar to https://steelsoldiers.com/threads/mep-804a-voltage-regulation-circuit-question.194301/

I suspect the voltage regulator and have tested the mosfets. One mosfet shows dead short across all terminals and the other seems normal. I plan to replace both of these. R2 also appears to have some heat damage and the resistance is coming in at about 2k instead of 510 ohms. The TM says its 55 watts which is funny cause that would make it the size of my arm. I suspect that it is 1/2 watt based on size. I plan to replace that as well. The 4 diodes d1-4 look dark and the board looks slightly bubbled there but it might just be the protective coating on the board making it look that way. I tested them for resistance in both directions and all 4 are 300k+- 20k ohms in both directions (could be because of the configuration in the board?). Diodes are cheap so I plan to replace them as well.

My plan so far:
Replace both IRFP450 transistors q1 and q2
Replace the 4 diodes d1-4 (I think I read something about upgrading them to handle voltage spikes better) I can't read the component markings on them until I remove them.
Replace R2 with a new 510 ohm 1/2 watt resistor

Since this machine has 2 hours on it. No modifications have been made. I'd like to do any suggested electrical mods right now before I put everything back together. I have only seen info about the MOV modification for the lower model number generators 802 etc. Does anyone have information on suggested mods like the MOV mod for the 804 model?

Thanks!
Dave

 

[Daybreak]

Howdy,

Military MEP generator reset information

Howdy, This Technical Bulletin has all the information regarding what happens to a Tier I and Tier II reset. It's a good read to understand what happens and what gets changed, inspected and/or replaced. Covers 2kw to 200kw

www.steelsoldiers.com

I believe you would be on PDF page 33

 

[Rexit]

Howdy,

Military MEP generator reset information

Howdy, This Technical Bulletin has all the information regarding what happens to a Tier I and Tier II reset. It's a good read to understand what happens and what gets changed, inspected and/or replaced. Covers 2kw to 200kw

www.steelsoldiers.com

I believe you would be on PDF page 33

Thank you very much for taking the time to respond. I've just finished reading that reset manual. I will be doing the fuse mod described in section 5.3. I did not see any information related to installing the MOV for any models in the reset manual. Do you have any information related to installation of a MOV on the voltage regulator of the 804?

 

[Guyfang]

Remember. There are 2 different types of volt regulater systems. You can NOT mix the Libby and TRC parts. So make sure you are putting in an R2 that matches the other parts of the system. Look in the TM forum, for a listing of systems and parts.

 

[Rexit]

Remember. There are 2 different types of volt regulater systems. You can NOT mix the Libby and TRC parts. So make sure you are putting in an R2 that matches the other parts of the system. Look in the TM forum, for a listing of systems and parts.

Thanks for the heads-up Guy. I have the TM https://www.steelsoldiers.com/attachments/tm-9-6115-464-34-pdf.609454/ it does not specify libby or technology research corp anywhere in the manual but the schematic on page 338 (3-274) appears to be correct. I'll sift through the rest of the TMs again to see if I can find anything. This one seemed to be the only one with regulator schematics. I'll double check the values on the resistor by reading the bands before a replacement is made.

The manual I'm currently referencing also does not appear to have any part numbers for transistors or diodes which is unfortunate because there are no markings on my mosfets q1 and q2. My transistor (mosfet) part numbers came from skimming the forums where (IRFP450) was the replacement part used. Specifically this post https://www.steelsoldiers.com/threa...gulator-fuse-fu2-blowing.173675/#post-2163889 about repairing a TRC board (which matches mine).

I am fairly confident in my ability to swap out electrical components directly, I did a lot of that in my youth. What I am unsure about is where this specific regulator would benefit from the MOV(s). I am assuming between terminals 10 and 11 since they are the ones that feed the h-bridge (diode bridge).

 

[Guyfang]

Attached is the volt reg. compatibility list.

Army parts TM's do not show part numbers for parts in the VR, simply because its not a repairable item.

The only schematics are in the -24 TM.

 

[Daybreak]

Thank you very much for taking the time to respond. I've just finished reading that reset manual. I will be doing the fuse mod described in section 5.3. I did not see any information related to installing the MOV for any models in the reset manual. Do you have any information related to installation of a MOV on the voltage regulator of the 804?

Howdy,
The MOV is not from the Manufacturers. It is not in any TM. The MOV came about by members here.

 

[Rexit]

Howdy,
The MOV is not from the Manufacturers. It is not in any TM. The MOV came about by members here.

Thank you for the reply daybreak. Do you have any input as far as the location for the mov on an 804? I'm assuming the 2 pins that feed the h bridge (10/11) are the ones that it should be on if it's purpose is to protect that part of the circuit. I really did try to find this information via the search feature but the only references I could find were for the 802 and 803. I'm sorry if my questions have been answered before. I truly cannot find the information.

 

[Rexit]

Well, I'm back. I desoldered 1 leg from the diodes and the questionable resistor and tested them. They all tested good. The plastic coating on the boards just makes the components look dark and the board look blistered in places. I swapped out both of the mosfets for new ones because one of them tested bad and I figured changing both would be a good idea. I popped in a new klk-3 fuse. Hooked the regulator back up. Cleaned the contacts on the VM-AM with crc-contact cleaner (what I had) and a worn out wrist. Cranked the engine for 10 sec. Overvoltage light came on while cranking. Stopped cranking. Gen shut its self down.

Did the following:

4.1.2 - Testing. (Regulator. PIN: 19870 For MEP-804A Generator Set).
a. Shut down generator set.
b. Open output box access door.
c. Note position of voltage reconnection terminal board and set FREQUENCY SELECT switch to 60 Hz position.
d. Start generator set and turn VOLTAGE adjust potentiometer to ensure the adjustment ranges on TABLE 4-1 are
met, depending on position of voltage reconnection terminal board.
e. Shut down generator set.
f. If no voltage or Iow voltage was indicated, or voltage adjustment range could not be achieved, perform the following
steps. Otherwise, AC voltage regulator (2, FIGURE 4-1) is serviceable.
g. Disconnect wire 141 A from terminal 1 of AC voltage regulator.
h. Set multimeterfor DC volts and connect positive Iead to wire 141A. Connect negative lead of multimeter to terminal
3 of AC voltage regulator. Start generator set and operate at rated frequency. Move and hold MASTER SWITCH
in START position. Multi meter should indicate between 4 and 12 VDC.
i. Shut down generator set. Isolate wire 141A.
j. Set multimeter for AC volts and connect to terminals 10 and 11 of AC voltage regulator. Start generator set and
operate at rated frequency. Move and hold MASTER SWITCH in START position. Multimeter should indicate 110
to 160 VAC.
k. Shut down generator set.
l. Disconnect wire 137A from terminal 5 of AC voltage regulator. Set multimeter for ohms and connect positive lead
to wire 137A and negative lead to terminal 4 of AC voltage regulator. Move VOLTAGE adjust potentiometer to full
counterclockwise position. Multimeter should indicate no more than 2 ohms with FREQUENCY SELECT switch
in 60 Hz position or 50 Hz position. Move VOLTAGE adjust potentiometer clockwise slowly while observing multimeter. Multimeter should increase smoothly to approximately 20,000 ohms.
m. If steps h, j, and I are as indicated above, AC voltage regulator is defective and must be replaced.
n. Connect all wires previously disconnected.
o. Close output box access door


Values from steps h,j,l are, 10 vdc, 150 vac, 2.2 ohms-17k ohms in 50 hz position and 3.4 ohms-17k ohms in 60 hz position.

All values are withing spec except for the resistance range of the voltage adjustment pot in step l being slightly out of tolerance. Step m, says that this means the regulator is bad.

I'm not sure what could be wrong with the regulator at this point. The diodes test good, the mosfets are new (I even lapped the heatsink they were attached to since it was not flat to help with cooling), and no components on the board show any visible signs of damage.

New TRC voltage regulators for the 804A seem to be about impossible to find on ebay and $2000 though green mountain is far too much, I've seen a post or 2 about converting the 400 hz ones from the 814a but have not seen any follow up posts mentioning their success. I'm confident that my soldering skills are capable of this as long as I have a component list that is accurate. I've also heard people suggesting aftermarket voltage regulators. I'm less confident in my ability to do this unless someone knows of a 1 for 1 replacement and how to wire it.

Anyone have any advice?