No picture only sound when playing MKV

[Gododdin]

Hi all - non techie here and first post. I'm using VideoDownloadHelper to download files on violin playing from Youtube. This is only for the purpose of being able to loop and slow them for further study and VLC has been extremely useful in this respect. I have been working on an excellent series of videos which up to now have been in 'webm' format and everything has worked fine. But now the person creating the videos has changed the format to 'mkv'. So now when I open a downloaded video I get perfect, crystal clear sound but no picture. If I try to convert the file in Handbrake I get an error message 'no valid source found'. Is this perhaps a problem with VideoDownloadHelper rather than VLC, or is this a known issue with VLC and MKV? Thanks

[Gododdin]

Hi again - I managed to figure this out - it is in fact a minor issue with 'VideoDownloadHelper' which I've now sorted. Thanks