Marshall Stanmore repair

Hello,

I have had a defect Marshall Stanmore (first edition) for a couple of years now that I meant to repair. However I haven't been able to find anyone with the similar issue until recently on youtube. However no suggestions to where I should begin troubleshooting.

Have anybody had the similar issue and possibly a fix for it?
Suggestions to where I should start also appreciated.

Video of issue : [URL="https://youtu.be/vft56VFffi0?si=lgFmQAfKRaJ3-lnF"]https://youtu.be/vft56VFffi0?si=lgFmQAfKRaJ3-lnF[/URL]...

Re: New Members - please post your introductions here

I'll introduce myself as a crafts teacher located in Finland. Within the crafts subject (aka slöjd in swedish ) I teach students in high school the basics of wiring and soldering. I thought this forum might come in handy when troubleshooting audio equipment.
ofcourse I'll enjoy reading other threads too on various subjects.

Best regards,
Janne