Hmmm…. this sounds very much like a leaky input capacitor. It’s a known problem on gear of that age.
The early ones used conventional 100uF capacitors, later we used ‘low leakage’ types that were much better.
If you can get it repaired locally, I recommend low leakage 100uF 63V working capacitors. They are easy to see on the board…. they are close to the input.
If you need me to look at it, I will service all old JM gear for a fixed fee of £25 plus the cost of return postage.