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Sony Dvd power supply problem

Thread starter #1
my sony sr700hp dvd player has no power..
I opened it and check output voltage.no voltage found
primary section looks fine...its on tny176pn ic(u1).
no shorts found.
I changed u2 and u3(tl431) by guessing.
no result.
what could be the problem?
thank you!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#3
There are usually no circuits available, the PSU is a replaceable module, not intended for repair.

I suggest you download the datasheet for the chip (if it's available), and fault find from that.

Common problems are likely to be S/C rectifier diodes, or faulty electrolytics.
 
#5
Bit hard to see from the image ... but are they 2 x 100k resistors above the IC?

I don't know the official term for them, but often the 'start-up' resistor/s go open circuit and cause a no go. Always check any high value resistors in the primary if no other obvious faults.
 

vtech

Active Member
#7
Bit hard to see from the image ... but are they 2 x 100k resistors above the IC?

I don't know the official term for them, but often the 'start-up' resistor/s go open circuit and cause a no go. Always check any high value resistors in the primary if no other obvious faults.
They are bleeder/startup resistors and indeed rather common to open up when there is no other obvious faults. In this case, they are actually 150K in parallel. R2, R3
Looks like the posted pictures are directly from a Brazilian version of Sony's service manual(copy @ elektrotanya) and if you scroll further down, you'd actually see a corresponding schematic! While connecting lines are not straight but it is done on the same schematic program and complete.
As it was mentioned, it is very unusual to have any sort of power supply schematic for a consumer DVD!
https://elektrotanya.com/sony_dvp-sr700hp_sm_es.pdf/download.html
 

tomizett

Active Member
#8
I also agree that startup resistors (if it has them) are a not unusual failure.

First things first, though - if the fuse intact? A blown fuse will probably indicate a shorted switching IC (or rectifier, but you say you have checked that).
Secondly, are you getting volts (about 340 if you're on 240V mains) on the primary capacitor?

Is it absolutely, completely dead, or is there any sign of voltage on the secondary side, or any noise generated when it's powered up? If the secondary rectifier is short the controller IC will be shutting down in an over-current condition. Bad secondary caps can also cause similar problems.

I'm sure I don't need to say this, but please be extremely careful when working on circuits like this - they can very easily be lethal.
 

tomizett

Active Member
#9
Ok, just taken a look at the schematic. Looks like R2, R3 are part of the snubber circuit and so probably not the direct cause - although worth checking as a faulty snubber could have killed the switching IC (U1).
Another thing that can cause poor startup (just running in little bursts) is a problem in the auxiliary power supply circuit - D5 open (unlikely) or its associated winding, or C12 gone bad.

There's really not very much to it - I'm sure you'll be able to fix it, assuming the parts are available.
 
Thread starter #11
yes this is the circuit diagram.
I think primary section is working.its getting high voltage through u1.I predicted optocoupler and shunt regulator ic bad and changed them..d10 is ok its getting around 28v.but no output voltage.something is blocked.
output voltage should be +5+5,g+12,g.should I change cy1?
 
Last edited:
#12
You are getting high voltage _to_ U1, not through it. It's effectively a switch, controlling the current through the transformer winding.

It also uses the high voltage while it's switch is open to produce its own low-voltage supply & cannot work without that.

As I said earlier, check for voltage on pin 2 (relative to 5/6/7/8).
I'd guess somewhere roughly around 10 - 18V. Wrong guess - see the edit below.


If there is no voltage there, the IC is dead - or something in C9 / C12 / 15 is leaky and pulling it down.


Edit - going back to the data sheet, pin 2 is at 5.85V in normal operation.
At anything lower than that, it will remain in shutdown.

Also, a higher voltage from an external source forcing current in to that pin causes the device to shut down, as an overvoltage protection system.

If you see a momentary voltage across the capacitors on the secondary when you connect power, that overvoltage trip could be the cause.

See the section in the data sheet about the "BYPASS/MULTI-FUNCTION (BP/M) Pin"

Edit 2 -
Just zoomed in on the schematic and noticed that R15 & C12 etc are marked as "NC" - if they are not on the board then the IC is in pure stand-alone operation and should only ever have 5.85V on pin 2, from its internal regulator.

If C9 has failed or drastically changed value that could prevent the IC functioning properly.
 
Last edited:

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