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TDA 7625 Questions

Discussion in 'Troubleshooting and Repair' started by dicky98, Jun 22, 2015.

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  1. dicky98

    dicky98

    14
    1
    May 18, 2015
    Hi guys
    I'm now trying to repair a high power (1.5W) RGB Laser

    The original fault I found was a blown (blackened) fuse and two s/c rectifier diodes on the PSU HT side

    Having replaced the diodes the laser then powered up for a second and blew the fuse again (just popped this time)

    I then disconnected the feed to the main PCB and powered up the PSU again with a new fuse. The cooling fan worked so I checked the voltage to the main PCB it is +12 0v- 12

    So I thin powered up the main PCB from my bench supply. The current limit was kicking in and the PSU was supplying about +5/-5V on current limit. When I increased the limit a bit something suddenly changed and the negative rail stopped drawing power (well it went to about 30mA), the positive rail went to about 0.8A and the unit powered up

    All lasers working, and the diffusion grating motot rotating and selecting 'patterns'. The laser diodes kinda flashed on for a second and then and off with each pattern slection.

    I noticed however the projected patterns did not rotate as they should - because the X-Y morror motors were not moving

    I had a quick look athte main PCB and saw what looked like an output device... in fact it looked like a stereo amplifier chip.... and indeed it was. A TDA7625

    datasheet here http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXtzzzq.pdf

    I then tried moving the XY mirrors with my finger. They indeed did freely move and I very quickly discovered that a 1500mW laser can actually burn!! OUCH

    OK so now using a PCB tool I discovered that once the mirrors are roughly in range of the intended beam direction (firing through the defraction gratings - the lasers switch on. If i then keep waggling either of the mirrors a little bit the beam will stay on. Once I stop moving the mirror the beam shuts off after a second. I take this as a good thing as it suggests the microcontroller is alive and well and that the XY motor feedback circuitry is working.

    Both motors check out at around 5 ohms. I then used my bench PSU to power each motor in turn and I can make them move either way by reversing the polarity. All good so far yep?

    I then used my scope on the TDA7265 and noted there is no drive going to the motors, but when I move a mirror with my pcb tool, the corresponding amplifier input swings +ve or -ve which suggests to me that the microcontroller is sensing the position change and trying to move it back.

    I also noted that the Mute/Standby pin on the TDA7265 is at 11.5V (supply voltage is +/1 12V which according to the datasheet the TDA7265 is in standby

    However I kinda thought maybe what has happened is the TDA7265 had a short circuit output on the negative rail to ground, which is what blew the PSU diodes and the fuse in the first place, then once I connected up my bench PSU which has a bit more grunt, the TDA7625 drew more current then went open circuit on the output - hence the sudden current drop. On this thought Ifigured maybe the microcontroller put the TDA into standby when the motors did not move on power up. However this did not tally with the fact it looked like it was trying to drive the mirrors when I moved them

    Anyway today a replacement TDA7265 arrived and I fitted it. It made no difference.

    My next approach was to track down what is driving the TDA Mute pin .... and I have some pics of the circuit attached

    Now my question is this

    Shouldn't the transistor driving the TDA mute pin be on, therefore have voltage low on the collector which connects to TDA pin 5 via a 10K resistor and therefore switching the standby function off (TDA7265 outputs powered up)?

    The transistor has 0.49v on the base and the emitter is connected to ground (0V) There is another transistopr of the same type 'J6' which has 0.02V on the base and 6V on the collector - however this can't be driving the other transistor base as the diode just to the right is reverse biased. There must be something else connected to the transistor base to pu 0.49V on it. The thing is, both transistors look OK in circuit (both have the same voltage drop B-E B-C on my multimeter which makes me think they are both OK. But then what else could explain the high voltage on he transistor collector unless it is faulty even though it meters out OK? Anything??

    I drew some red lines so you can see how this all connects up. R17 is 10K

    If there is no voltage drive to the TDA7625 mute/standby pin does it just float high and go into standby on it's own? I have no experience of these amplifier chips

    I am thinking now, because the micrcontroller actually tries to move the motors when i waggle the mirrors that it 'thinks' the TDA7265 is not in standby, otherwise why would it do that?. So some problem in the standby circuitry itself?? Maybe that transistor is faulty. I am tempted just to connect a low value resistor TDA pin5 to 0V to force it out of standby and see what happens. Good idea or bad?

    Oh the transistors are marked J6 and I think this may be the datasheet http://www.s-manuals.com/pdf/datasheet/s/9/s9014_galaxy.pdf

    Cheers for reading this and for any advice offered

    Rich
     

    Attached Files:

  2. dicky98

    dicky98

    14
    1
    May 18, 2015
    I've traced this out a bit more just to get an idea of what is going on

    See pics. The circuit does not seem to make sense unless D7 is some sort of zener? I definitley drew it the right way round according to my test meter the anode goes to base of Q4

    Anyway Q4 should be turned on according to the voltage readings? But it isn't. Unless 0.49v isn't quite enough. The B-E junction on my multimeter says 0.670V which is much the same as Q8. Does the base have to read that in circuit for Q4 to turn on?

    I'm not rightly sure what drives the base of Q8 but as it looks like it needs to be off to turn Q4 on and power up the TDA that is not really important to trace the circuit out further just now I think :confused:

    I did have a think about why the standby pin needs to be controlled on the TDA7265 in the first place..... now my thoughts here is that the X-Y motors can stop in any position (unlike a speaker coil) so you wounld't want a DC potential across the motor when it is stationary otherwise that would be like seriously clipping a waveform on a speaker = damage to the coil and/or amplifier output stage? So maybe forcing it out of standby to see what happens would not be a good idea? Does that make any sense? I am a bit out of my experience with this one guys.

    My head is hurting - Advice please
    Rich
     

    Attached Files:

  3. dicky98

    dicky98

    14
    1
    May 18, 2015
    Had one more play with this.... tried to remove the Q4 transistor and only succeeded in breaking it into bits. So I replaced it with a 2sc1815 - I just soldered the legs to the pads as they were in the correct BCE order

    This made no difference - still has 0.49v on the base abd does not turn on.

    Found a 47uF 16V electrolytic is connected from cathode of D7 to ground. Don't know for sure if it is ok or not but can see it charges up when I connect my ohmmeter across it. I suppose if it was leaky it may cause too low a voltage on Q4 base - assuming D7 is a zener?

    Now I think I reached the limits of my abilities to figure out what is wrong

    Revised circuit diagram attached - with the addition of the electrolytic it's starting to make Q4 look like a soft start circuit for the TDA7265 and kinda negates this idea I had that Q8/Q4 can rapidly switch the TDA in and out of standby when the XY mirrors are stationary/moving hmmmm

    Cheers
     

    Attached Files:

    Last edited: Jun 22, 2015
  4. dicky98

    dicky98

    14
    1
    May 18, 2015
    OK sorted it after a lot of head scratching... stupid me!

    Connected the control board back to the original psu and it worked. Tried on my bench psu was and realised it was not turned up quite high enough - set it for around 12.5V and it also works

    Rich
     
  5. dicky98

    dicky98

    14
    1
    May 18, 2015
    Well here it is all back together and working in my workshop :)



    Cost me £10 for the faulty unit and about £7 in parts to fix it - plus more head scratching than was strictly necessary!!!

    Anyway these are still available new for best part of £500 so it will make a great centrepiece for my mobile disco at a very attractive price

    Cheers
    Rih
     
    Arouse1973 likes this.
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