DSF68NX IDC2 Stuck in stand by

Matt · Guest

Hi Guys

I have a Ferguson DSF68NX IDC2 that is stuck in stand by. Does anyone know of any common problems or can give me any idea's on where to start?

Thanks

Matt

Guest · Posted: Tue Jun 14, 2005 5:24 pm Post subject:

Does anyone know wher thge stand by signal is generated?

Guest · Posted: Thu Jun 16, 2005 6:51 pm Post subject:

Someone Please help. I can't watch a 10" tv any longer!

Guest · Posted: Sun Jun 19, 2005 3:50 pm Post subject:

Does anyone know of any other forums that might be able to help with this problem?

absorbentgnome · Joined: 30 May 2005 Posts: 51 Location: Cardiff, UK

How is the set stuck in standby? Are there any sounds coming from the back of the TV when you switch it on, and when you switch it on from standby? Do any of the LEDs on the front change colour, briefly? Tweeting sounds?

You will need to describe the fault in detail to get a reasonable diagnosis. And be prepared to use circuit diagrams, plenty of desolder wick and a fair bit of patience to get it working.
I do not have experience of this set, however sets that do not come up from standby usually have
a) Shorted HOT (horizontal output transistor) - caused by component failure or poor joints. This component is usually the largest transistor in the set, nearest the LOPT (line output transformer). Failure of the LOPT will usually kill the HOT as well causing similar symptoms. The tweeting occurs when the SMPSU (switch mode power supply) current or voltage protection kicks in.
b) Duff recitifier diodes in the flyback circuit
c) Duff rectifier diodes in the SMPSU secondary circuit
d) Duff capacitors in the SMPSU controller circuit
e) Open resistors in SMPSU.

Since the set comes on in standby then the SMPSU *may* be OK, pointing to shorted HOT, rectifiers or LOPT. Often there are fusible resistors in these circuits that open when when a fault occurs. These will have to be replaced also, with the same type. Be wary of high voltages stored in the mains smoothing capactior in the SMPSU. Give it some time to discharge before handling the circuit board - measure with a voltmeter (up to 350V here!!).

The only way to learn how to do something is to try it - read up Sam's Repair FAQ for TVs and SMPSUs and other guides that are out there. Obtain the circuit diagrams for your TV from here if you can, they will help identify critical components.

I suggest that once you have made yourself aware of the safety issues, remove the HOT for testing, and post back here.

Guest · Posted: Mon Jun 20, 2005 4:06 pm Post subject:

Thanks for the help!

The set deguases when switched on but doesn't make any tweeting noises. The red light on the front of the set stays red all of the time and doesn't change to green as it would when starting up. I checked just about every resistor and diode in the circuit and have so far found no problems. How do i check for a faulty electrolitic Cap? I already have a circuit diagram and although I am an engineer, I have little experience with TV's. I will read the FAQ section you mention and see if that helps with my understanding.

Once again thanks for the help, and any more advice would be great.

Matt

absorbentgnome · Joined: 30 May 2005 Posts: 51 Location: Cardiff, UK

Hmmm. This could be a pain in the arse, but maybe not. It is possible that there is a missing low voltage supply.

Check in the SMPSU circuit diagram which rectifier diodes on the SMPSU transformer secondary circuit give the supply voltages.
Grab a multimeter and clip GND to a grounded piece of metal on the chassis, switch on and measure the voltages at the cathodes of the diodes for the positive supply voltages and at the anodes for the negative supplies. It may help to mark the underside of the PCB with a bit of chalk so you remember where the leads are. Somewhere here you may find a +5V supply or 8V supply or both. Check all the low voltage supplies carefully. Check the output of any voltage regulators as well. In standby, only a few of these supplies will be active, so don't be surprised to see inactive (or very low voltage) ones.

Can you post a link to the circuit diagram? I may then be able to suggest components to remove so we can get the SMPSU up and running.
Also, search for the chassis type on here and post a link to any repair tips that are available.

Guest · Posted: Wed Jun 22, 2005 5:31 pm Post subject:

Hi

And thanks for your help. I have attached the link to the circuit diagram below. I have tested a few of the voltages that should be up in stand by mode and so far it would seem that the U7:5V is OK, but the U3:15V is only reading 12.2V and the 24Volts supply to the op amp is only at 22V. However I still can't find the faulty component. The only other thing that I have found that may be relevant is that U8:5V is ready 0v.

http://www.ddd2002.com/downloadsm/1977/Thomson_IDC2.html

Matt

Guest · Posted: Mon Jun 27, 2005 2:12 pm Post subject:

Hi absorbentgnome

Did you manage to look at the circuit for me? Any idea's?

Matt

absorbentgnome · Joined: 30 May 2005 Posts: 51 Location: Cardiff, UK

Hi Matt,
Had a really busy weekend and lost your thread in the pile of other threads!
From what you have written you are already isolating the problem.
Since U7:5V is ok it is likely the TDA8138S is good. U8:5V will be 0V in standby. Check U7:5V is alive.
Because U3:15V is so low, you may have a problem in this supply.
Remove and replace DP47 and CP47 with known good components. For testing purposes you can use any fast recovery recitifier diode with a PIV and A rating the same or greater than component DP47. Use a similar capacitor.
If you have an oscilloscope available then attach the scope to the cathode of DP47 and observe the supply ripple. Note that the ripple will be high frequency because it is a SMPSU. If the voltage is that low you will probably see a pretty nasty mess. Replace those components and observe again.

If the voltage is still low with nasty ripple then there is a severe load on the 15V supply - remove the TDA8138S to see if the supply returns to normal.