Grundig GU26HDLCD won't power up

[tom66]

Re: Grundig GU26HDLCD won't power up

Ah sorry forgot to mention that! I used the cardboard tube method and discovered it wasn't coming from that inductor. It was coming from the 5vsb transformer. The cap seems to have solved the problem though as the PSU is now totally silent.

Buzzing can be caused by bad capacitors, because it makes the power supply unstable. Glad you fixed it.

[bbjunkie]

Re: Grundig GU26HDLCD won't power up

Fault is still there, I think i've found another part of it. If I shut it down on dvb-t it wont switch back on properly.

If I switch from another input to dvb-t the set stops responding and I have to cycle power to get it operational again (then when I turn it on I have to switch it from dvb-t)

While I have the scope out I decided to check some voltages. There appears to be some ripple on 12v and 24v. difficult to say, probably close to 0.4vdc ripple on 12v and 0.1v on the 24v line

This is way above the acceptable range right? I'm thinking that the ripple might be causing the dvb-t section to lock up or crash.

Excuse pic quality, cant get a good one in this light.

Scope on .2v /div showing the 12v line

[tom66]

Re: Grundig GU26HDLCD won't power up

Fault is still there, I think i've found another part of it. If I shut it down on dvb-t it wont switch back on properly.

If I switch from another input to dvb-t the set stops responding and I have to cycle power to get it operational again (then when I turn it on I have to switch it from dvb-t)

While I have the scope out I decided to check some voltages. There appears to be some ripple on 12v and 24v. difficult to say, probably close to 0.4vdc ripple on 12v and 0.1v on the 24v line

This is way above the acceptable range right? I'm thinking that the ripple might be causing the dvb-t section to lock up or crash.

Excuse pic quality, cant get a good one in this light.

Scope on .2v /div showing the 12v line

Way too much ripple. There's probably a bad cap on one of the outputs which is destabilising the power supply control loop. The buzzing is a characteristic symptom. You will need to use an ESR meter to find the bugger, but you'll have to remove the important ones to test.

(Fun fact: With my LG Plasma I had loads of bulged caps. But in circuit only one of them was bad (1.5 ohm ESR!) so I had one on hand and replaced that. The others measured 0.1 ohms (total) which seemed fine. But later, I removed them all. What did I find? Every cap I removed had more than 1 ohm ESR. Most had 2-3 ohms. I'm guessing a combination of other components, and capacitors in parallel, made the ESR read much lower. And that one cap that measured okay, that was on the output of a buck converter, so it was isolated from everything else.)

[bbjunkie]

Re: Grundig GU26HDLCD won't power up

Ok, thats my work cut out for tomorrow then

I've just got a call from a friend who has got me a 42" lcd with a cracked screen. Perfect for some spare parts

[bbjunkie]

Re: Grundig GU26HDLCD won't power up

Got most caps checked OK - C950 was actually a 1000uf 25v but the schematic says it should be 35v. The silkscreen on the board shows a larger outline than the size of the cap, but it is clear that it has never been touched with a soldering iron. It tested 0.08 ohms and 956uF

Another im unsure about is a 50v 33uF which is testing at 0.45 ohms and 30uF is this ESR acceptable?

After checking the 6.3v caps while I was there, I realised that I have checked every cap on the board and they are all ok.

Plugged the tv back in to check ripple again only to find it has gone, so I switched it to DVB-T mode and sure enough, it came on and is working again.

Perhaps a bad joint? Who knows!

Cheers

Jim

[tom66]

Re: Grundig GU26HDLCD won't power up

Got most caps checked OK - C950 was actually a 1000uf 25v but the schematic says it should be 35v. The silkscreen on the board shows a larger outline than the size of the cap, but it is clear that it has never been touched with a soldering iron. It tested 0.08 ohms and 956uF

That's quite high. And it should be a 35V one because the power supply has a +/-5% tolerance in most cases, so the output could be as high as 25.2V.

Another im unsure about is a 50v 33uF which is testing at 0.45 ohms and 30uF is this ESR acceptable?

0.45 ohms is probably okay for a 33uF. They are generally not ESR critical in such a small capacitance.

Plugged the tv back in to check ripple again only to find it has gone, so I switched it to DVB-T mode and sure enough, it came on and is working again.

Perhaps a bad joint? Who knows!

Cheers

Jim

Possibly. It *is* a Vestel chassis. They are very prone to bad joints.