MEP-004A no output!

[testdepth]

Looking for assistance on troubleshooting to determine if the problem is definitively the A11.
Symptoms are the typical, no voltage after start up. While holding the start switch in the start position voltage will build to about 50vac but immediately drops to 0 as soon as start is released. Removed the A11 cover and inspected all components and nothing burned or obviously broken.
Disconnected all amphenol connectors, sprayed with CRC contact cleaner dried with lp air, reconnected, did same for all connectors.
Questions:
1 - Is there a definitive test to say the problem is the A11 and not somewhere else?
2 - Anyone have a 50/60 cycle A11 for sale?
3 - Does anyone have a “theory of operation” description for the exciter/VR they could share to attempt a component level repair. I do have a DVM and o’scope if all else fails!

thanks,

Mike

 

[Guyfang]

The -34 TM tells you how to test and repair the A11. Have you downloaded the TM's and maybe looked at them?

 

[Scoobyshep]

If it comes to it, We successfully replaced the regulator/exciter assembly on a 005.

 

[testdepth]

The -34 TM tells you how to test and repair the A11. Have you downloaded the TM's and maybe looked at them?

I do have and have read the section on troubleshooting the A11. I’ve actually got a variac on order to conduct those tests. I was trying to determine if there is anything external to A11 that could cause the symptoms. I don’t want to end up buying a module and not have that be the problem!

 

[Guyfang]

Normaly the problem is in the VR. A few screws and a few canon plugs. Look at the VR card. sometimes you can see burn marks. Its always nice if you can throw a known good one in, and system check.

First, you need to troubleshoot. And that starts at the begining. Have you tested at F1 and F2 to see if you have excitation?

 

[testdepth]

the exciter coils measure about 2.0 ohms.
After disconnecting the wires going to the exciter coil I measure 24 vdc after engine starts while holding switch in start. It appears to me that tells me the problem is definitely in the A11, and likely in the VR circuit.
My next step would be to add the fuse mentioned in another thread to the primary of T2 and then start running the voltage checks listed in the manual.

 

[testdepth]

My variac I ordered came in so I continued the troubleshooting as follows:
1 - Sensing ckt test. Apply 95 vac to J9 A&B. Measure across R15. Expect 22 vdc; Actual 22.3vdc - GOOD
2 - Power ckt test. Apply 208 vac to J13 L&M. Measure across J13 R&S. Expect 48vdc; Actual 50.2 vdc - Probably Just Fine.
3 - Boost ckt test. Apply 12 vac to J13 A&B. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
Apply 12 vac to J13 E&F. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
Apply 12 vac to J13 J&K. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
4 - Measured fwd and reverse impedance across CR1-CR6. 5 of 6 measure the same fwd and reverse ~ 4.5M ohm. the other measured .4M in one direction and ~4.5 M ohm in the other direction. Might be OK but will replace all 6 as soon as I can find replacements.

Now my main concern is what caused all 6 diodes in this 3 Phase full wave bridge rectifier to blow?
Also what else is toast? I've looked the VR card over pretty well and don't see any physical evidence of component failures.

Anyone have any thoughts on how safe it would be to replace the diodes and then give it a try? I'm thinking that just running the test shouldn't fry the diodes but I'm not overly confident in that!

Mike

 

[Guyfang]

My variac I ordered came in so I continued the troubleshooting as follows:
1 - Sensing ckt test. Apply 95 vac to J9 A&B. Measure across R15. Expect 22 vdc; Actual 22.3vdc - GOOD
2 - Power ckt test. Apply 208 vac to J13 L&M. Measure across J13 R&S. Expect 48vdc; Actual 50.2 vdc - Probably Just Fine.
3 - Boost ckt test. Apply 12 vac to J13 A&B. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
Apply 12 vac to J13 E&F. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
Apply 12 vac to J13 J&K. Expect 9.3 vdc. Actual 12.5 vdc. FAIL.
4 - Measured fwd and reverse impedance across CR1-CR6. 5 of 6 measure the same fwd and reverse ~ 4.5M ohm. the other measured .4M in one direction and ~4.5 M ohm in the other direction. Might be OK but will replace all 6 as soon as I can find replacements.

Now my main concern is what caused all 6 diodes in this 3 Phase full wave bridge rectifier to blow?
Also what else is toast? I've looked the VR card over pretty well and don't see any physical evidence of component failures.

Anyone have any thoughts on how safe it would be to replace the diodes and then give it a try? I'm thinking that just running the test shouldn't fry the diodes but I'm not overly confident in that!

Mike

If it were my machine, thats what I would do. Fix what I find wrong, and put it back together. Test.