MEP-002A are all Control cubicles interchangeable?

[Ray70]

Hello everyone,
I'm brand new to posting here, although I've been reading articles and learning a lot for a few months.
So cut me some slack if I mess anything up due to my inexperience!
Anyway, I was wondering if anyone knows of any interchangeability problems between Libby Corp. and J.R Hollingsworth control cubicles on MEP-002A's? I recently bought my 3rd -002A which ran but would only put out power while holding the start switch, the field will not hold a flash no matter how long I hold the switch. My first step was to swap the control cube with a known good one, which worked. I thought the problem with the original cubicle was some cracked solder joints on the voltage regulator board, so I fixed that and installed that suspect cubicle on the other generator that the test cubicle came from and it was works. So now both units are working fine. Last night I swapped the cubicles back to their original generators and that unit is now doing the same thing, not flashing the field. Tonight I'm going to swap cubicles again and see if both generators work again. If they both work again, I suppose the easy solution would be to just leave it be, but out of curiosity I would like to know what is going on because I was under the impression that the parts are completely interchangeable. Anyone ever seen any interchange problems before? Or do I just have a connection problem or something going on here. BTW, the 2 cubicles have different voltage regulator PN's and 1 is Libby ( the one that is acting up ) the known good one is a JR H.

 

[Munchies]

Its all the same. Watch for 400hz and 60hz control cubicles. Just check the jumpers on the VR, and the frequency transducer

 

[Ray70]

So last night I did a little more testing and found that when I swap the cubicles back to their original generators, the Libby unit once again will not produce power unless you hold the start switch indefinitely. Next I tried another known good Libby control cube on the suspect Libby generator and it acted the same way. Next I put a known good AC Output control box on the suspect Libby generator and everything works as it should. This leads me to believe there is a problem with a component in the AC control box that only affects the Libby control cubicles. What ever is wrong in the AC box, does not affect the JR. H control cubicle for some reason. Do any of you folks know what the electrical differences are between the 2 different VR part numbers? And does anyone know where I should look next? Possibly the bridge rectifier or the CV-T? Since I have known good parts available, I will part swap until I find the root cause, then trouble shoot that component, but any advise you guys can provide is greatly appreciated.

 

[Triple Jim]

Do any of you folks know what the electrical differences are between the 2 different VR part numbers?

There are essentially no electrical differences between the 72-5020 and 72-5338 AC voltage regulators. The changes are of things like resistor power ratings and board layout, but nothing to change the operation of the circuits.

 

[Ray70]

That's what I figured, I wouldn't expect any operational differences between the two. Now that I try to retrace my steps I think originally I did put the JR.H VR into the questionable generator's Libby cubicle and it did not work. It just seems very strange that the JR.H cubicle as a whole works on this generator but neither of the Libby cubicles will work, but all 3 cubicles can be mixed and matched and all work on other 2 generators. So depending on what I find after a little more parts swapping, maybe a VR conversion to a Powertronics or Flight systems or other might be a better way to go, seeing that it sounds like these regulators do away with the reactor and CV-T and possibly the rectifier bridge too.
Could any of you knowledgeable gentelmen briefly explain the functional operation of the whole VR, reactor, CVT and rectifier assemblies as they relate to control of the generator field? Maybe that would help me pinpoint the problem? To describe the problem again, the generator only produces power if I continuously hold the start switch, thereby sending power to the field, as soon as I let go, power goes out. Swapping the AC box seems to fix the issue.

 

[Triple Jim]

CVT1 puts out 3-phase AC at its terminals X1, X2, and X3. That AC is rectified by six diodes on board A4, and sent to the exciter field.

The regulator controls how much of this 3-phase AC is produced by CVT1, which has control windings connected to its terminals C1 and C2. Transformer T1's output is rectified by two diodes on the regulator board and sent through these control windings, and then on to ground through Q3 of the voltage regulator. The circuitry of the regulator board controls the exciter field current by turning on Q3, sending more current through the control windings of CVT1 when it wants less generator output, and by turning off Q3 when it wants less control current to produce more generator output.

By the usual thinking this regulator works backwards, working harder when it wants less generator output, and working less hard when it wants more generator output. If the regulator board fails by not carrying current at Q3, the generator output voltage goes high, like 150V instead of 120V. If it fails with Q3 shorted, or otherwise turns on Q3 fully, there can be so much control current that the generator output falls to near zero. Of course there are many other possible causes of the output falling to zero, but few that make it go too high.

 

[Triple Jim]

The flashing circuit applies 24V to the exciter field through the resistor on the diode board A4 during starting. According to the schematics, the 002A will get flashing voltage as long as the master switch is in the start position, but the 003A has the flashing voltage cut off by the engine speed switch. If the generators are wired according to the schematics, the 003A cannot get its field flashed after the engine is running. I don't know if any of this is related to your trouble.

 

[Ray70]

Jim, thank you for your help. The info you provided along with info in the thread ( which I just noticed ) about the nice regulators you make... Plus the schematics and test procedures I printed out of the TM should get me started on a more detailed analysis of the problem. Al I need now if a few free hours to look at it!
I think I need to look at the bridge rectifier, CVT-1 and the voltages coming into the VR.
Have a good weekend....

 

[Triple Jim]

You're certainly welcome. I'm sure you will, but please keep us posted.

 

[Ray70]

Well, no time to work on it this weekend, but I brought it (the AC box) to work with me this morning and I think I see the problem...
Bridge rectifier checked out fine. 1.86M on each diode in 1 direction and opecircuit in the other direction. 55 ohms on the resistor.
BUT... the CVT1 readsings are out of whack!
H1-H2= 3.8
H3-H4= 2.8
H5-H6= 15.8M
X1-X2= 2.6
X1-X3= 2.5
X2-X3= 2.5
C1-C2= 9.6

So according to the TM, both the primary and secondary windings all look a little high, but H5-H6 looks like its screwed!
I'm no electrical engineer, but I assume this would cause erroneous signals to the VR board.

 

[Triple Jim]

It's certainly not going to work right if H5-H6 is open-circuited like that. To check diodes you really need either a meter with a diode testing mode, or to pass a small current through them with a battery or power supply and a resistor, and measure the voltage across them. You should see battery voltage in the direction they don't conduct, and about 0.6V in the direction they do conduct. With a 12V battery, a 1,000 ohm 1/4 watt resistor would work OK.

 

[Ray70]

Well, back to fixing this -002A... After a couple failed attempts to score a parts machine at a reasonable price from GL, Craig A Tull hooked me up with a replacement CVT. I got it installed in my AC connection box today. It wasn't too bad to swap out, the hardest part is the heavy wire going to the reconnect switch from the center hole in the CVT. Its nearly impossible to get to the bolt, even with the switch and terminal board unbolted.
If all goes well I will take it home ( I swapped out the CVT at work) and test it tonight. With any luck this will solve my "no power" problem. Then maybe put 1 or 2 of the 5K units up for sale and try to upgrade to a 10K

 

[Ray70]

Problem solved! the replacement CVT has fixed the problem. So, to recap, the machine with a Libby AC box would not put out power unless I hold the start switch indefinitely, but it would work normally with a JR.H AC box. The CVT had an open in the windings at H5-H6. Replacing the CVT has fixed it. Thanks to everyone for their help and especially Craig for taking the time to yank out his old CVT for me.