Peavey Bass MKIII 400BH hum problem

[Kipnugget]

Hi all! After years of wanting one I finally bought a Peavey Bass MKIII 400BH with a 118 cabinet together for a good price. However, there is a bit of hum on the amp. Not extremely loud, but it is definitely audible when not playing. I thought it would just be a ground issue, but upon further investigation it is also very quietly audible when there is no cab connected, and it also comes through on the line-out. When turning off the amp it also produces a clearly audible low frequency pop (it really sounds more like a "bump"). So I suspect there is something wrong with the power circuit, but my knowledge is limited. I'm hoping someone here knows these amps very well and can shed some light on how to fix this.

TL;DR: audible hum from amp, even without cab connected. Halp?

[Kipnugget]

I made a short video showcasing the problem, I hope that helps.

Edit: it seems uploading files is broken for me, so here is an unlisted youtube link: https://youtu.be/00r4tVkWUqk?si=cjxmSLmyIi_2ejuj

[Kipnugget]

In the mean time someone on Discord told me that this hum is probably caused by the transformer, and that this is probably pretty normal for an amp from this era (~1978).

If someone could confirm this, that would be great and we can close this thread.