Something appears to be loading it, hence the LED flicker?
Is the micro getting warm to the touch?
Max.

No, not at all as far as I can tell.

 

It could be the 78L05 but not common to be low, usually goes to 0v out. You could carefully lift the output pin and measure the current?
Or simply replace it!
Max.

 

I'll try replacing RG3, but I don't have a replacement at the moment. Newbie thinking out loud: if 78L05 not common to be low, could it be fine itself but receiving too weak a signal? If so, where would one look? I'm not very good at tracing, yet. Thanks.

 

It would be more of loading issue, if you lift the output (5V side) and insert current meter it should be lower than 1amp.
Max.

 

And if it isn't lower than 1amp, what does that indicate? Thanks.

 

It means it is being loaded somehow, about the only thing it supplies is the micro and its port loads, LED etc etc.
Could be a hard problem to pin down.
Max.

 

Thanks. I ordered some components. Will check back with findings in a week or so.

 

So, I replaced the 78L05 at RG3. Unfortunately, I got the same reading from the 78L05, the same very dim LED, and of course no voltage at the motor terminals. Perhaps something is wrong upstream from the 78L05. If anyone has any suggestions, please chime in. Thanks.

 

If you are getting 10vdc - 15vdc on the input of the 78L05 then it is not upstream, but loading of the 78L05, more likely.
Max.

 

The input on the 78L05 is a low ~5.5V, same as before replacing it.

 

What i would do is first trace and map out the circuit prior to the 78L05, and then power up and do some definitive tests.
It is slightly harder with SMT devices.
Your board version appears different from the reverse engineered version, I have come across at least 6 version of these boards, although basically similar in many respects.
Max.

 

Well, that's touching the limits of my current skills. I'll see what I can do. Thanks for all your feedback, Max.

 

OK, I finally got some time to work on that board.(MC2100LT)
In short:
I can run the motor on a car 12V battery. It pulls 10 amps on start and then 7 A on run
So the motor is good.
The console is working.
I have 103 VDC on the DB1 rectifier.
I have a steady LED when power is on.
The LED is rapidly flashing when the console is issuing command ( motor disconnected)
So it seems that the console and logic of the board is OK.

When I start with the console (with motor disconnected) I immediately get 150 V on the pins going to the motor. Changing the setting(slow-Fast) does not change the voltage.
When I stop with the console, the voltage is gradually going down to 0V. (probably because of the big condenser)
When I try the same with the motor connected, the board goes in error mode with the LED flashing slowly.

Question:
Is the Q2 (IRFP250) an ON/Off or is it controlling?
What if the role of Q1 (S4025L) Is it the controlling SCR? How to test if it is working properly
Thank you

 

The motor is normally accelerated/decelerated by the processor building & decaying the HV DC through the SCR.
Max.

 

You mean the Q1 ?

 

Yes.
I should add it does not control the motor per-se, but ramp the voltage up and down at the start and end of a command.
Max.

 

So If I get immediate full voltage it could be the culprit

 

It's possible.
But generally there would be +ve all the time.
Max.

 

OK, then the controlling circuit from U4 (TD3052)

 

I replaced the Mosfet IRFP250 once more.
Now I can control the voltage going to the motor ( using a light bulb for the testing).
Everything seems to be working fine, except that last time I tried that with the motor the Mosfet burned! I am hesitant to plug the motor again. I ordered 2 replacement and this is the last one!