Mind Cracker

[epi]

Videoseven LTV20H LCD TV with AC adapter model 0218B1260 12V 5A
The AC adapter causes a loud wistle out of the speakers, the fault dissapears with a diferent AC adapter model 0219B1275 12V 6,25A, with original adapter the fault is less audible if you move around the 12V cable that goes to the TV, I removed the coaxel 12V cable and fitted a normal parallel cable, and the whistle now coming out of the speakers is lower.  I have changed all the electrolitics in this ac adapter, checked coils filters, dry joints and no luck. Scope checks showed little difference in ripple and noise comparing with the 0219B1250 under same test condition.
Ripple frequency in power supply aprox 60KHz at 10mV.
Audiable whistle frequency 1.86KHz, checked by beating with an Audio Signal Generator.
If you move the 12V supply lead along the back of the set the audiable frequency drops and sometimes dissapears, pointing to a feedback posible loop?.
What I haven't done is tested the AC adapter with another set as I don't have any in at the moment. When I do I'll update this post.
Any one else had a similar problem, any ideas? I'm Lost.
And of course, the set belongs to a friend.
Frank

[welshblob]

Does the cap at the top on the right look a bit bulgey or is it the way the light is hitting it?

[epi]

Hi Welshblob,
It's the way the light hits it, it's a 35V  and to big for the case, I fitted the correct value so I could pack it in its plastic case. Today I did another experiment with another adapter, checked the noise and ripple levels and they are practically the same. So I wound a coil put it round the noisy adapter took a photo of the scope image, then did the same with the none noisy adapter to see what was radiating, to my surprise both images are the same, same noise coming out, so I've got no idea how that whistle is getting through or were it's coming from. I'll post later tonight images of this last experiment. I can get away (and cheaper) to give my friend a new adapter, but I'd like to find the cause.
Frank
Added the scope test pics I made.

[epi]

Hi, here is a short summary of how I got rid of the molesting whistle, but I am not satisfied with the final result.

The whistle dissapears when I:

1- if I rotate slightly the dc jack in socket
2- wire up a parrallel dc socket with short leads and plug the dc jack here
3- if I fit a short extencion lead (length doesn't make any difference) to the the dc adapter and plug into original dc jack
4- and of course if I use any other ac adapter.

To me the problem is to do with the ac adapter output cable.

With the above in mind I tried the following, and the whistle did NOT dissapear:

1- changing the cable with a parralell one
2- fitting filter coil to output cable
3- fitting a similar cable from another adapter
4- removing the line filter on the cable I just fitted

The problem finally dissapeared when I changed the output cable with another coaxial cable type, as used  in Hi-Fi or microphones (length didn't make any difference).

Well I can't mentally work this out,   and really not satisfied with the result, as there seems to be no real fault just incompatible condition with this particular ac adapter 0128B1260 and the Videoseven LTV20H television, if any one comes across this setup, I like to hear if they have similar or no problem. The whistle is very audiable and molesting at low sound levels.
Frank