TideLog Posts Tagged “no sound”

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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Techwood are another “faceless” brand of TV that are Vestel made, they are sold at Morrisons, we have done a few of them. Recently a customer brought her 32″ Techwood TV to our workshop, saying there was power, but no picture or sound, and the LED blinks. Normally on a Vestel mainboard the LED only blinks during Over-The-Air firmware update, so she left it with us, and we took a look under the bonnet.

Techwood-32832-under-the-bonnet

It uses a Vestel 17MB25-3 mainboard, similar to my 16″ Linsar 16LVD4. Another common cause of a blinking LED on a Vestel can also be the inverter. Often on these separate inverter TV’s, if the inverter chip, a MOSFET, or indeed the coil (some have 1 some have 2), are faulty, the error reporting circuit built into the main chip on the inverter relays this info to the processor and the software goes into protection mode, preventing the set from booting up, causing the flashing LED.

A blinking LED in any other case than firmware update is mostly a power supply issue, but here’s our checklist:

  • Check microfuse FS105 on main board, near CAM module.
  • Check SMD fuse FS106 (4A) for open-circuit. Check diodes D881 and D893 (UF5402) for short-circuit.
  • 24V rail might be short-circuit, disconnect inverter board supply to prove. Check and replace dual N-channel MOSFETs IC803 and IC804.
  • Replace IC830 (FAN7711 and SMD capacitor C925 (1nF).
  • Check for short-circuit between pin 6 and pin 8 of IC830 (FAN7711). Replace IC830 and SMD capacitor C833 (100nF).
  • Check and replace SMD transistor Q839 (BC859).
  • Check D893 and D891 (UF5402) in centre of PSU for short-circuit.
  • Voltage at regulator U122 varies between 4V and 9V instead of being stable at 8V. Replace U122 (LM1117ADJ) on main board.
  • Check C961 (470uF/35V) at top of PSU.
  • Check D893 and R1036 (2.2K) on PSU.
  • Replace IC830 (FAN7711).
  • Check D893 and its feed resistor. Replace MOSFETs Q813 and Q814 and IC830. Replace C828.
  • Replace C892 (100uF), C801 (33uF), C840 (33uF).

In our case, the voltages coming out of the power supply were fluctuating badly, so we knew the mainboard wasn’t the original fault. The power supply is a Vestel 17 PW26-3. So, out came our service manual and power supply schematics, and we set to work.

Vestel 17pw26-3 PSUNo obvious bulging capacitors, no burst vents, and no burn marks anywhere. The PSU microcontroller was operating, and the clock waveforms were fine. The bridge rectifier system and components were OK. The problem was the output capacitors just before the output sockets, their voltages were up and down.

Replacing C802, C934, Q828, D882 & D883 and some various resistors fixed the problem, and the TV was back up and running. The power supply even emitted less no-load whine than it did before! A lot of people might say Vestel are junk, but the thing I love about them is they’re off the shelf parts and components, fixing them is a joy. You’ll often find a lot of Vestel TV’s with all manner of screen sizes, big and small, being driven by the same boards and power supplies. The ProView panels can also be replaced with dual lamp screens from a Sony Vaio laptop if you need a 15.4″, 16″, or 17″ panel 😉

Here’s a fun fact: The 17MB25 board can drive panels from 12″ all the way to 36″ full HD, even if the TV is marketed as just HD Ready, that’s often just because the panel is a small one, so the board software goes into HD Ready mode to drive it. Connect a 32″ to it and the multiplexer goes into full steam 1920×1080 🙂

Vestel? Turkish? Delight? Yeah, I would say so, cheap, cheerful, and easily fixed. Take note, Sony 😉 Funnily enough Greg has a Luxor branded Vestel lined up for me with similar symptoms, I suspect it’s going to be the same routine as this one.

 

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