LIBRA-UL-R repair on the Phobos Gate Opener System


LIBRA-UL-R controller

After more than two years of operation, the controller stopped working of the Phobos BT L system at the end of 2016.  I wrote the company via the website multiple times, but received no response.  From inspection, it was clear that the R23 power resistor (above the bridge heatsink in the previous image) was burned up.  I could not tell the original value, so I had few options.

Analyzing the schematic
Prior to removing the board, I checked the power supply voltage at the white terminals at the transformer, and measured 33Vac (labelled as 25V on the sticker).  Taking the board to my workbench, I was able to power up the board with a 24Vac transformer and could then measure voltages and trace the schematic.

photo of board
Photo of the solder side of the board.  The main driver for the relays is U20 in the middle of the photo (pin 1 is top-right).

I only traced the Power/IO section of the board.  This is the section most likely to fail and besides, it would be nearly impossible to find a replacement for U6, the main chip of the logic section.  

Power Supply Section
Referring to the schematic, we start at the bottom of the page for the power section.  Incoming AC power is first filtered by C1 and RV1 and then applied to JP9 terminals #11+12 as the Aux output, and also D1 for full wave rectification.  This creates the B+ bus, which is used to power the motor.  At an input power of 28Vac, B+ will measure about 26Vdc.  D20 isolates B+ from the C+ bus, which is filtered by the C3 2200uF electrolytic capacitor.  Due to the filtering, this bus will measure higher, or around 36V in my case.  After that, this bus powers the red LED (LD1), and a 28V zener preregulator formed by R30 and D24.  This in turn feeds the 5V regulator.

Motor Control Section
I will focus first focus on how Motor2 is powered.  Current for this is switched by K2 and K3.  When these relay coils are unpowered, the motor is shorted, causing the motor to act as a brake.  Throwing K2 (power from U20-15) will cause B+ application to terminal #4 on .  If then Q4 is also turned on (via MTR_DISABLE from U6), current will flow through the motor and use R4 as the current sense shunt.  Per the note in the Project Log below, the K2 is for the CLOSE function.  The opposite occurs when K3 is thrown, and the motor runs in the opposite direction.  The circuitry inside the motor and its limit switch is a guess, but I am pretty certain this is correct.  It causes the switch output to go to B+ when that switch is closed, which pulls down the open collector output to U6.

However an interesting and initially puzzling action occurs before the motor runs.  That is when the remote button is pushed.  The first resulting action is a 30 msec pulse low on the U20-12 pin (Sensor Check).  Via Q22, R23 and D25, this applies C+ to both terminals of the motor and Q4 (neither K2+K3 are actuated yet).  Initially, Q4 is OFF, but in the middle of the 30 msec pulse, MTR_DISABLE goes low for 10 msec, and that causes about half an amp of current to flow in Q4.  This results in about 50mV across R4 for 10 msec, which is routed to the op-amp at U21.  My best guess is that this 30 msec pulse is a sensor check. It allows the U6 controller to tell if the system is up against a limit switch, and if the current sensor is working.  As an experiment, I shorted the base-emitter junction of Q20, causing the lack of the 50mV pulse.  The result was that the motor move was immediately aborted, and none of the relays actuated.  This matches what occurs with my original controller with the blown R23 and Q22.  

When Q22 is ON, the instantaneous power on R23 is 17W.  This resistor is rated for 1W per my inspection.  It is unusual for these power resistors to fail shorted (they overheat and open), so my guess is that Q22 failed shorted in the original controller.

Actuation of Motor1 is similar, except power to it is not applied until about 920mseconds after Motor2 runs.  This delay allows proper phasing of the gates.  The Flasher output is composed by diode ORing all four motor output terminals with diodes D27 through D30, and applied to #9.  Current from this feed is returned through steering diodes D31+D32.

The interface from the logic section to the Power/IO section is nine logic lines.  Two of them are the MTR_DISABLE lines mentioned above.  The remaining seven get buffered by U20.  Momentarily jumpering the lowered numbered pins to 5V causes a relay to throw and a motor action to occur.  These pins and functions are:
  1. Motor 2 at +25V (Measured at JP9-3 to JP9-4) / OPEN
  2. Motor 2 at -25V / CLOSE
  3. Motor 1 at +25V (Measured at JP9-6 to JP9-7) / OPEN
  4. Motor 1 at -25V / CLOSE
  5. Enable Sensor Check (active low)
  6. Small relay K8 (probably area illumination output on JP9)
  7. Small relay K4 (probably "Safe" output on JP9 to power light beam devices)
One thing that you can do is jumper these to 5V one at a time and see what power output does not work.  Note that these logic control lines into the ULN2003 all have a capacitor to return allowing you to find them easily.

Per the technical manual, the max working current to the motor is 3.5 Amps, and 1.5 Amps nominal.  So a good dummy load is about 20 ohms.  In the case of my board, repair was not possible so I purchased a new one for $211 (link below).  With the new (working) controller in hand, I could better trace the schematic and could now read the value of R23.

Logic Section
The main processor is the H8/3672 from Hitachi.  From the hardware manual:
This LSI is equipped with ROM, RAM, an 8-bit timer (TMR), a 16-bit timer, a watchdog timer (WDT), two types of serial communication interfaces (SCIs), a 10-bit A/D converter, and I/O ports as on-chip peripheral modules. This LSI is suitable for use as an embedded processor for high-level control systems. Its on-chip ROM is flash memory that provides flexibility as it can be reprogrammed in no time to cope with all situations from the early stages of mass production to full-scale mass production. This is particularly applicable to application devices with specifications that will most probably change.

A search shows this chip is not available in retail distributors and needs to be purchased in bulk from sources in Asia.  The markings on one of my chips is:

The first line is probably the actual part number, and the second line the generic number. The third line may be some kind of production or batch code, and I am guessing the last four digits is the date code, which would make the date of manufacture the 38th week of 2011.

The main chip is connected to a four-digit numeric LCD display with 18 interface pins.  I have not been able to find much about it, but it looks like a simple direct multiplexed interface from the processor to the display segments.  With 7 segments, a decimal point each, and four digits, the minimum number of pins needed is this sum, or 12.  During a move, the display appears to indicate the peak current level of the motor and should be multiplied by 4.5x to get the current in Amps.  The value is held while the move is ongoing, even if the limit switch is hit.  The display then goes blank once the move is completed or cancelled.

Repair Case #1
I was sent a broken unit in August 2020 and was able to repair it.  It failed in the same way as ours and I could verify that Q22 was shorted.  This would have led to R23 burning open.  I was unable to tell much about Q22 and the markings are "AL W72".  Based on the resistors around the control pin, I am pretty sure this was originally a MOSFET.  However, if you calculate the Vgs, it is a very high value, and I think this would have caused long term stress.  This is probably a vulnerability in the design.  I addressed that in the repair by changing the resistors around the transistor.

Transistor Q22
Damaged Q22 transistor.

I decided to build an interface board to help test the controller, which is shown below.  It allows me to easily tell if the motor has power and the polarity (red/green LEDs).  It also allows me to easily start and stop the motor as you can see in the video.  There are also equivalent buttons for the travel limit switches.

Libra Tester Board
The Libra Tester board.  Start/Stop button is on the right.
Red / Green LEDs indicate polarity of the motor power and
the switches on the left are the travel limit switches.

Video of repaired Libra.

MOSFET repair
Photo of new MOSFET installed onto the back of the board (middle right).
Note the old FETs is the size of the ones in the bottom left.
New one is much bigger and stronger.

Repair Case #2
Information will be posted in the future on this repair case.

check back

In the light of the Libra being phased out and replaced by
the (much more expensive) Thalia, I think it makes
sense to repair these boards.  Send me an email
if you want to have me do it.

Project Log
K2 repair


Notes on the types of Phobos systems


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(c) 2020 Edward Cheung, all rights reserved.