Altech DSD4121 SAT TV power supply service manaul

[CdDvdFlash]

I have a friends SAT TV box from Altech model DSD 4121 but I'm looking for a service manual for the power supply, a PJ321R Rev.0 power supply. I need the service manual to check and replace the capacitors as when it is disconnected from the main board the voltages are way too high but with a load the voltages are above normal for 3.3v, 5v and 12v, the unit is still working but not fully as it has 4x ---- lines after "boot" on power up and wont work past "boot", C1 is within its 47uF if the tolerances are 10% (its at 43.5 uF) other than that thats why I would like to have a service manual to go over the power supply, I read someone said R3 and R4 @ 1M Ohm usually go open after a power outage or spike and that these resistors provide power to allow U1, the PWM controller, to function

[CapLeaker]

Re: Altech DSD4121 SAT TV power supply service manaul

I think that problem is more likely a corrupt flash than a power supply issue. You can check the ripple on the caps going across the legs of each capacitor with you DMM set on AC Volt.
The only thing I find weird from the get go is that a switching power supply does not vary on voltage much, specifically going higher than it should be. It can happen but for way different reasons and the voltages wouldn’t be stable. Even if there is a small cap in the feedback circuit that went bad, it should go lower in voltage. Maybe upload a straight shot, high resolution picture of you PSU.

[stj]

Re: Altech DSD4121 SAT TV power supply service manaul

bad optoisolator or bad reference like a 431 could cause the outputs to be higher

[redwire]

Re: Altech DSD4121 SAT TV power supply service manaul

The power supply needs a minimum load, with it disconnected from the main board, it might go unstable and voltages above normal.

I have also found an old, weak opto-coupler can cause regulation to drift upwards for output voltage. The TL431 should measure 2.50V across it unless it has aged and drifted. The electrolytic capacitors are critical as well.

You'd have to post pics to see how the regulation is done.
The 1MEG startup resistors will not affect regulation, it will just go dead. If you suspect mains spikes/surges then the SMPS IC might be damaged but I've never seen them go hyperactive, they just pop.

[CdDvdFlash]

Re: Altech DSD4121 SAT TV power supply service manaul

Apologies for the long reply but ive taken some pictures for you of the power supply at the moment in my testing tools collection I only have a DMM and soon later I'll buy a capacitance meter and an ESR meter for jobs with caps

[lti]

Re: Altech DSD4121 SAT TV power supply service manaul

I see a cracked solder joint on R16, which is connected to the TL431.

[petehall347]

Re: Altech DSD4121 SAT TV power supply service manaul

might it be that its not drawing enough current to lower the voltages ?

[momaka]

Re: Altech DSD4121 SAT TV power supply service manaul

might it be that its not drawing enough current to lower the voltages ?

I'm thinking the same thing.

Well, if you trace out the output / secondary side rails, you can see that there is only one voltage rail that provides feedback to the primary side. Not sure if that's the 3.3V, 5V, or 12V rail, but it's only one of them. I'm willing to guess it's probably the 5V rail. Then the 3.3V and 12V drift to whatever values, based on load. This is flyback topology PSU, so cross-regulation isn't going to be very good no matter what (as there is no coupled output inductor for the 3 rails.)

My only other suggestion is to check that small electrolytic cap C14. It seems to be there for compensation of the 431 shunt regulator... but since it's only a single cap and not a cap-resistor + cap network, compensation is also going to be not so great.

So if C14 is fine and all of the output caps are OK and the 431 shunt + optocoupler are OK, then the "high" output voltages may actually be normal without a load. Really only ONE rail should be spot on to what it should be, since only ONE rail provides feedback. The other two are left to drift about, based on load on that one rail with the feedback.

**EDIT**
I see now - it's the 3.3V rail that provides feedback only. So 3.3V should be pretty close to that value, and 5V and 12V are left to drift around it.