Hitachi 3d7 mk2 boombox hum/noise

[IVH]

Hi guys,
I'm currently restaurating a hitachi 3d7 MKII boombox, it is going well, except for one thing: there is a hum hearable while listening at any volume (it gets louder when turning up the volume).
In order to troubleshoot the bad cap(s) I was thinking of temporarily mounting (I mean holding against) a good cap in parallel with another to check if the hum decreases (or changes).
Is this possible without damaging the boombox ? and can this be effective ?
I'm not quite confortable with replacing every cap even though only 1 has probably failed. (I don't have enough in stock and this project is getting quite expensive now).
Thank you in advance !
Idris

[stj]

Re: Hitachi 3d7 mk2 boombox hum/noise

does it do the same on batteries?

[IVH]

Re: Hitachi 3d7 mk2 boombox hum/noise

does it do the same on batteries?

Thank you for the reply !
Sorry, I forgot to mention this:
-it happens on the left and right channel + with headphones
-noticeable when on batteries
-the noise gets louder when turning the volume up
-it can be filtered out using the equalisers
-it is even noticable when nothing is connected (tape deck disconnected from main board, no line-in connected)
-clearly hearable when playing music through the cassette deck or line in.
-the resistance between the ground of the power supply is 0 Ohm to all 'grounds' I measured on the board

The problem might lay in the pre amp, but I don't know where it's located as I don't have a service manual.
That's why I wanted to check it the way I described it in the first post.