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Sanyo DS24425 Dead after quick power down/up - Solved   Goto page 1, 2, 3  Next  
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SteveS



Joined: 22 Aug 2010
Posts: 13
Location: North Carolina

PostPosted: Sun Aug 22, 2010 9:59 pm    Post subject: Sanyo DS24425 Dead after quick power down/up - Solved Reply with quote

Sanyo: DS24425
Chassis: 24425-07
Family/Group: NA7D
Mfg: Feb 06

This TV was in good working order until I moved it about 2 weeks ago. After first power up, I noticed some discoloration, so I powered down and up again to get the degauss circuit to run. I pressed the power button once to power off, and then on again in less than 1 second (I hit the button by accident.....) The TV sounded like it was turning on (big Chunk, and humm) but the screen never went back on. The TV has not worked since. Have tried unplugging and powering on etc.

Pressing the power button causes the big chunk, and humm sound again, and I can hear the relay for what I think is the degauss circuit engage. After I think the 3rd attempt, It does not even do this unless the unit is unplugged for some period of time, then plugged back in again.

The symptoms I have are consistent with the "power fail" circuit described in the service manual, but it does not list any causes.

When the set is OFF, but plugged in, I can hear a faint ticking noise around the flyback area of the circuitboard.

I am planning on measuring the test voltages described by the repair manual, but I don't know what the next step is. All caps look good, and the fuse is good. The remote control will also attempt to power the unit on the same as the front panel power button.

I am tempted to replace the flyback, but want to make sure that's the issue before I order one up.

Thanks for any help you can provide. This is a great forum, and I am happy to have found it.


Last edited by SteveS on Thu Sep 02, 2010 7:16 am; edited 1 time in total
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JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Mon Aug 23, 2010 8:29 am    Post subject: Reply with quote

Start with power supplies:
Raw B-Plus
Stand-by B-Plus
SMPS secondary sources.
SteveS



Joined: 22 Aug 2010
Posts: 13
Location: North Carolina

PostPosted: Mon Aug 23, 2010 8:38 pm    Post subject: Reply with quote

Measured some voltages tonight.

At various test points I am missing some of the low voltages.
Tracing the circuit back to the transformer, I see one half of the coil with 30V at D623, the other half of the coil which feeds a 9v and 12v regulators has only 8v on it at D624.

I am measuring DC after the diodes, so my best guess at this point is that the diode feeding the 9V and 12V regulators is bad. I guess it could be the cap also.

I'm not familiar with how diodes fail. The diode in question looks just fine, no cracks, no scorch marks etc. The service manual calls for an exact replacement on this device due to x-ray protection.
The part number is: RU3YX 100V 2.0A
It is D624 on the circuit diagram.

The Cap is C626 (1000U 16V) and it also looks healthy, no bulges vent is in tact, and no electrolyte leaking from it.

What should I do next?

Service manual with full schematic can be found at:
http://www.fixdevice.com/smanuals/tv/sanyo/ds24425.html

JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Tue Aug 24, 2010 5:33 am    Post subject: Reply with quote

You're barking up the wrong tree. One page above main schematic is a voltage chart. Measurements are with set off or on?
Q627 emitter = Power on: 13.3 volts; Power off: 8.0
IC681 terminal # 3 (I think chart has pin 1 & pin 3 reversed) 13 volts into a 5 volt regulator IC should give 5 volt out.

SteveS wrote:
At various test points I am missing some of the low voltages.

Which ones are missing?
SteveS



Joined: 22 Aug 2010
Posts: 13
Location: North Carolina

PostPosted: Tue Aug 24, 2010 6:23 pm    Post subject: Reply with quote

Ok, Thanks.

You are correct, the diagram and the chart have the pins reversed.
I measured 8V into pin 1, and get 5V out on pin 3.

The Hold-Down Protection (page 11) will cause the set to power off after 10 seconds, which is what I am experiencing. I started there it's starting to look like that's a symptom not a cause.....

Hold-Down circuit voltages:
TJ6 = 0V should be 5V
D002 = 0v should be 15V
D249 = .1V should be 5V
D806 = ~28V should be 30V

With TJ6 at 0V I traced that back to TJ5 which looks like it
should be 9V. I observed 0.47V (Q487-e) Traced that back to TJ2
which looks like it should be 12V. I observed 7V (Q627-c).
Traced back to L627 (Q627-E) which should be 13.3V (power on)
I observed 8V.

Here are the voltages I read out this afternoon:
Part: Expected:(on/off) Observed:(on/off)
Q486 B 9.9 .64V
Q486 C 11.6 1.34V
Q486 E 9.2 .47V

Q627 B - 12.7v/8.0v 7V/9V (off is higher)
Q627 C - 13.2v/0v 7.4V/0V
Q627 E - 13.3v/8.0v 8V/6.9v

Q681 B .7v/0v .7v/0V
Q681 C 0V/6v 0V/9-10v fluctuating
Q681 E GND/GND GND/GND


So here's where I'm confused. Q627-E is the closest point to the power transformer where the voltage appears to be below spec (8V not 13.3V as specified) There's only a coil, a cap and a diode in there. I am guessing the coil is good, since the other half is showing the correct voltage (30V@TJ3) but I'm not 100% sure here. The coil measures .3 Ohm between GND and pin 12 (feeds Q627-E) and .4 Ohm at pin 13 which feeds TJ3. I measured .4 ohm between pins 12 and 13.

I'm not sure what to do next. I'm tempted to remove one leg from L627 and measure the voltage there. I would expect 13.3V if the diode, cap and xfmr are all good, correct?

Again, thanks for the help and this forum. I'm so glad I didn't just try swapping the flyback blindly Embarassed
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JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Wed Aug 25, 2010 3:55 am    Post subject: Reply with quote

The part confusing you is the "Off/On."

Look at it this way: "Off" equals stand-by (TV plugged into AC but Power button NOT pushed yet); "On" equals run (TV plugged into AC, Power button has been pressed and set's running). Of course, this presumes everything is working normally.

Have we checked the HOT yet? Q402 - meter on diode check - measure each pair of leads (collector to emitter/collector to base/base to emitter).
Measure each twice, reversing the polarity of the meter leads for the second reading. Base to emitter SHOULD be the only pair to give a very low reading.
SteveS



Joined: 22 Aug 2010
Posts: 13
Location: North Carolina

PostPosted: Sat Aug 28, 2010 4:14 pm    Post subject: Reply with quote

I think I have this figured out, my observations that got me to this point are below the break.

I think I have a bad Q486, which is fed from Q627.
The voltages according to the service manual:
Expected Observed
Q486-B 9.9v .64V
Q486-C 11.6v 1.34V
Q486-E 9.2v .47V

The "diode" test of Q486 is this:

Q486 (B-E) .022v/.022v
Q486 (B-C) .232v/.232v
Q486 (E-C) .236v/.236v

This is transistor looks bad correct?


--------------------------------------------------
I understand the standby/on part. The power does go "on" for about 3 seconds when I push the power button. It then turns off after 3 seconds, for what I think is a protection circuit.

I tested Q402, here is what I found:
The voltages test ok according to the service manual:
Expected Observed
Q402-B 0v 0V
Q402-C 130v 130V
Q402-E Gnd 0V

The "diode" test of Q402 is this:

Q402 (B-E) 0v/0v
Q402 (B-C) .383v/Open
Q402 (E-C) Open/.383v

So this is good I think?
------------------------------------------------------
I cut the leg of L627, and measured the voltage there, and I get
13.7v which is the expected input to Q627-E (this is the main 12v supply for the circuitboard) So I'm still thinking the problem is near Q627.

The base of Q627 is the collector of Q681, which I believe to be good (it also drives the opto-coupler which turns on the power supply which is working)
JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Sat Aug 28, 2010 4:33 pm    Post subject: Reply with quote

Probably on a wild goose chase. Take Q486 out of circuit and remeasure. While it is out check the 390, 22, 12 ohm resistors and the zener diode.

Q627 is being used as a switch. Does 12 volts appear on its collector during the short time the set tries to fire up?

When you say expected you are saying what should be there with set operating "normally." When you say observed is that during the time: "The power does go "on" for about 3 seconds when I push the power button" OR during the time: "It then turns off after 3 seconds,?"


Last edited by JTS1957 on Sun Aug 29, 2010 7:32 pm; edited 1 time in total
SteveS



Joined: 22 Aug 2010
Posts: 13
Location: North Carolina

PostPosted: Sun Aug 29, 2010 6:59 pm    Post subject: Reply with quote

Q627 is the 12volt switch. I measure 7.4v at the collector when the TV is on for the 3 second period after I press the power button.

I took out Q486, but the power supply went "unstable" when I attempted to power up the set, the relay started clicking every 1/2 second or so, and I never got a good voltage reading out of any of the Q486 C,B,E.


Now I'm really lost. I'm going to re-install Q486, and then test Q490 voltages.
JTS1957



Joined: 21 Jan 2009
Posts: 2347
Location: Far, Far Away

PostPosted: Sun Aug 29, 2010 7:31 pm    Post subject: Reply with quote

What I wanted you to do was (diode check) Q486 out of circuit and while it was out check other four parts (diode check for the diode - regular resistance check for each of the three resistors). These are why you obtained such low (diode check) readings with Q486 IN circuit.

7.4 will be the input voltage that you hope to get 9 volts from to feed Q490's input to obtain 5 volts at its output? How's that gonna work? Diode check Q486 Out of circuit.
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