Looking for circuit diagram GSI Galvo servo amplifier (Minisax)

[_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  ), 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_]

Have you checked fuse F2 as stated in the manual?

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_]

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

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.

[_Wim_]

The first is to restore events before failure and understand what could have gone wrong and what caused the breakdown.

The galvo grating mirror movement was working correctly when I bought the monochromator on Ebay. I bought a "safe arc warner power 75-300" power supply to power up the Xenon lamp (I first tried with a Xenon power supply I had available, but the ignition circuit inside the lamp unit was not working). The safe arc power supply has its own ignition circuit, so I removed the broken one from the lamp unit. Upon start-up, the Xenon power supply had 5 or 6 ignition tries before the lamp powered up. My best guess is the first ignition tries were arcing over somewhere and causing damage to the servo amplifier board.

[Harry_22]

Please check this diode.

[_Wim_]

Please check this diode.

This was checked, it is a zener or around +-16V (works as a voltage regulator together with a BJT at the other side of the board)

[Harry_22]

The manual describes the following test procedure to check the functionality. Could you try it.

Remove jumper W1 to open the loop and apply power.
There are two circuits to monitor. Check them both.
- Pin 4 (Position) of Test Interface (see Page 14).
- Pins 7 an 8 of Signal Interface Connector (see Page 12).
Rotating the mirror shaft should cause the position voltage to fluctuate between +3V and -3V (probably this is between pins 7 and 8. See Page 37 of the manual)

Regarding LM3886 amplifier. The voltages on inverting and non-inverting inputs must be equal. If you measure 0.5V at pin 9 then on pin 10 must be also 0.5V.

Please also check the following areas.

PS
Monitor the signals on both pins W1 wile rotating the mirror.

[_Wim_]

The manual describes the following test procedure to check the functionality. Could you try it.

Remove jumper W1 to open the loop and apply power.
There are two circuits to monitor. Check them both.
- Pin 4 (Position) of Test Interface (see Page 14).
- Pins 7 an 8 of Signal Interface Connector (see Page 12).
Rotating the mirror shaft should cause the position voltage to fluctuate between +3V and -3V (probably this is between pins 7 and 8. See Page 37 of the manual)

Regarding LM3886 amplifier. The voltages on inverting and non-inverting inputs must be equal. If you measure 0.5V at pin 9 then on pin 10 must be also 0.5V.

Please also check the following areas.

PS
Monitor the signals on both pins W1 wile rotating the mirror.

Hi, thanks for your reply. Those possible bridges look indeed suspicous when looking at the pictures. I am not at home, so can't check today, but will have a look tomorrow evening. As to the input of the LM3886, the + pin was at GND potentinal, so the 0.5V on the minus input is certainly not ok. Mute pin was at approx -3V, so that should be ok.

[_Wim_]

Remove jumper W1 to open the loop and apply power.
There are two circuits to monitor. Check them both.
- Pin 4 (Position) of Test Interface (see Page 14).
- Pins 7 an 8 of Signal Interface Connector (see Page 12).
Rotating the mirror shaft should cause the position voltage to fluctuate between +3V and -3V (probably this is between pins 7 and 8. See Page 37 of the manual)

- Tested first on pin 4 of the test interface. Signal was larger than +-3V but followed nicely my mirror movement
- Tested on pin 7: lots of high frequency noise/oscillation (+-2.5MHz) but signal track mirror movement. Measured with grounded probe, check with ground from signal interface and with ground from power supply, same result.
- Tested on pin 8: identical result to pin 7
- Tested with differential probe (HP 1141A) between pin 7 and pin 8: same result, oscillation remains present.
- Tested by powering with a lab power supply instead of the switch mode I was using, same result (consumption +-3.5 Watt on +24Vdc, and +-4 Watt on -24Vdc)
- forgot to test between both pins of W1
- checked the easily probable opamps if I could find one with the same oscillation signal, no luck, so it must be one of the opamps that is hidden by the boards on top. Will test this weekend by soldering probing wires to all of this.

Please also check the following areas.

No solder bridges were found, was only some minor flux residue and it looked WAY better in real than on the pictures 

[Harry_22]

Hi!

I prepared the layer diagram for PaintNet (please see attachment).
There are several 78L00 and 79L00 in SOT-89 as well as LM317, LM377. 7V reference U5. Check them also.

https://drive.google.com/file/d/12g5bB9MpHA1xPD8NZRaeOuxLwTiKXTSC/view?usp=sharing

[_Wim_]

Hi!

I prepared the layer diagram for PaintNet (please see attachment).
There are several 78L00 and 79L00 in SOT-89 as well as LM317, LM377. 7V reference U5. Check them also.

https://drive.google.com/file/d/12g5bB9MpHA1xPD8NZRaeOuxLwTiKXTSC/view?usp=sharing

All voltage regulators measured  OK. The 7V reference was also fine. Measured also with the jumper W1 open an closed:
- open: 0.5V on one pin (changes when I move the mirror by hand), +-7V on the other pin
- closed: +-7 volt on the jumper.

It really seems the LM3886 is at fault, which could make sense it the damage occurred due to high voltage arcing, as the servo cable runs close the the Xenon lamp cables. The replacement LM3886 has not shipped yet (Ebay seller from Germany), so I still have to wait patiently to test if it is really all what is wrong...

[_Wim_]

The LM3886 arrived and is installed. Now the galvo is moving again  , but always to the same direction and goes full stroke  . I am now thinking the other LM3886 might also be broken (as the replaced one also showed movement initially), so I ordered a second one (should have done so immediately, but hindsight is a wonderful thing...)