Electronics > Repair
Looking for circuit diagram GSI Galvo servo amplifier (Minisax)
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_Wim_:
Hi All,

I am trying to repair a GSI Lumonics servo amplifier board (Minisax Series, exact type number 000-3014006). It is part of a polychrome IV monochromator.  It worked correctly before, but died when trying to ignite the Xenon lamp (I am not using the original polychrome power supply, as I only have the light unit with a damaged internal ignitor). The xenon lamp did ignite with its new power supply, but unfortunately this causes the galvo driver to die...  :(

Symptoms:
- no movement of galvo mirror when inputting a control signal
- no torque on galvo mirror when trying to move the mirror manually
- encoder feedback of galvo mirror works ok (when moving the mirror by hand, the position signal and error signals are correctly updated on the test interface connector)

I have checked already the following:
- power supplies are ok (+24V and -24V)
- on board + and - 7V supplies are also ok
- nothing obvious is damaged, no abnormal hot spots with thermal camera (although one LM337 runs quite hot at 100°C)
- enable is tied to GND
- ready signal is not tied to GND (indicating the servo does detect the issue)
- LM3886 amplifiers seems to work ok (they create movements of the mirror when probing on their input pins)
- LM3886 amplifiers do not get any input signal (so the fault is upstream of the amplifiers)
- checked all + and - inputs of various opamps to verify zero volts between the pins and outputs not railed

I have attached the user manual should anybody need this in the future. I know there are some replacement boards available on Ebay (quite expensive  :o), but would like to find the fault as it is likely to happen again with the xenon igniter.

So if anybody would have some schematics (of any GSI galvo driver so I at least have an idea of the general principle used), this would be useful as this board is a pain to work on due to the board stacking...
Harry_22:
Hi!

Have you checked fuse F2 as stated in the manual?

2. On power up, the scanner does not “torque up” hard to the center position, with a 0V command
signal. (There is a soft “electronic spring” which centers the rotor when the servo is disabled or not
tuned.)

a. Be sure that SERVO_ENABLE (J2-5) is tied low.
b. Check for a blown fuse (F2) on the SAX baseboard.
c. Make sure the AGC switch (S2) is in the proper position. See the table in the setup section of
this manual for the correct position. If not, it may be an indication that the driver was not
tuned at the factory.

Seems you have an error and thus Servo_Ready is high.

5.1 Fault Detection
The MiniSAX contains protection circuitry that will disable the servo loop and set the SERVO_READY
flag to a high logic level if any of several monitored error conditions are detected. Once an error is
cleared, the servo will automatically reset in a controlled power-up sequence that enables the servo loop,
resets the integrator, and then enables the command input and the SERVO_READY flag. The MiniSAX
protection circuitry monitors SERVO_ENABLE status, scanner over-position, AGC out of regulation, and
supply under-voltage. A fault condition is reported under the following circumstances:
• The SERVO_ENABLE status will cause a fault condition if it is at a TTL or CMOS HI logic level. (The
SERVO_ENABLE is normally HI.)
• A scanner over-position fault is detected when the position voltage exceeds ±3.5 volts.
• An AGC out of regulation condition is detected if the AGC voltage level exceeds 11.8 volts or drops
below 2.2 volts. The primary purpose of this feature is to disable the servo if the scanner position
feedback cable is not connected, or if position detector fails.
• A supply under-voltage condition exists if either supply voltage drops below 13.2V.
_Wim_:

--- Quote from: Harry_22 on June 30, 2024, 09:00:45 pm ---Have you checked fuse F2 as stated in the manual?

--- End quote ---

Hi, thanks for your reply. Yes, sorry I forgot to mention, but I checked all fuses also and AGC is in the correct position (as it worked fine before it died).

However, I do not remember "ready" being pulled high, was just floating and not being pulled to ground. I will check again this evening, now leaving for work.
Harry_22:
I had no doubt that you checked the fuses but had to ask.

Now I see two approaches.
The first is to restore events before failure and understand what could have gone wrong and what caused the breakdown.

The second is a routine check of the circuit operation.
This is a feedback circuit. Your angle sensor is working but there is no turn signal. The loop is open somewhere or another error prevent circuit from operation.

Could you make a detailed photo of each board from the both sides.
_Wim_:

--- Quote from: Harry_22 on July 01, 2024, 08:38:16 am ---Could you make a detailed photo of each board from the both sides.

--- End quote ---

Attached the board pictures. The soldering marks are from soldering test wires so I could probe hard to probe places. The OPA4134 on the input (that works as a instrumentation amplifier) and the op285 that I think closes the feedback loop were also replaced by a temporary test opamp to see it they were at fault.

Today I discovered that the left LM3886 (closest to the elko's) has 0.5V of its minus input signal (pin 9). I am not sure if I checked this minus input before, could be I assumed this was tied to GND. But more importantly,  it does not create any movement any longer when being probed (the right LM3886 still does create small movements). Not sure if I messed up something else with all the testing,  but certainly worth investigating. I ordered a couple of LM3886 to give this a try.
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