TideLog Posts Tagged “faulty regulator”

This is another wear related symptom, and often occurs on power down of an old system after a power cut. It is again to do with the input regulation circuit (the main resistor, diodes, and rectifier transistors bolted to the keypad chassis). If your Optima starts OK on battery, but not on just the mains, the cause of this is the CPU isn’t getting enough power to start up from the AC to DC rectification stage. The start sequence goes visually like this:

  1. Power is applied, the regulators get up to working voltage, and start supplying power to the CPU.
  2. The LED’s all come on, briefly, as the CPU boots up, doing its self test of itself, and the NVRAM, containing your code and exit/entry timers.
  3. Within a few milliseconds of 2 above, once the CPU has started, the LED’s go out, and the alarm now goes into a full alarm condition, leaving just the Power LED on, and any open Zone LED’s. If no zones are open, just the power LED is on.

If the LED’s all stay on with no more activity or sound, the CPU isn’t starting correctly, because the voltage to it is insufficient coming from the AC to DC rectifier stage. Allowing the alarm to start here going into full alarm, would cause too much current inrush, and voltage drop to sustain keeping itself running, due to the strobe/bell and 13v PIR’s drawing power when there isn’t enough.

The transformer puts out 16.2v AC. If the voltage at your battery charge terminals with no battery connected is less than 14v, (the last one I did was 10v) the whole system is being starved of power. The two transistors that are bolted through the keypad chassis need to be replaced, the big three-legged things top right of this picture with the holes through the tags:

Optima-XM-board-faulty-regulators

I always replace both to make sure, as they can be quite stressed out at such an old age, and be breaking down under load, as does the 47 ohm battery resistor. I also check the capacitors accompanying them. The leftmost transistor is a Toshiba TA7805S Positive Voltage Regulator which seems to be the battery regulator, I’ve uploaded the datasheet to Tidelog HERE. The second (rightmost) transistor is an ST Microelectronics LT8I5CV, for which I cannot find a datasheet, and I assume is the AC rectifier stage’s main DC regulator.

I am attempting to find suitable modern equivalents for these regulators, so if any electronics guys out there can help, I’d be most grateful, as finding info on these 15+ year old components is tricky! I’m running out of working ones to cannibalize off old unrepairable Optima boards! A good alternative to the Toshiba TA7805S is the Panasonic AN7805F, the datasheet is on TideLog, HERE, for you to take a peek at, if you understand electronics 🙂

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