SOLVED - Gf-777 loud hum and 'galloping' sound

[Concrete Apple]

I've been watching this thread because I had a similar thing happening with my GF-777. I am using the GF-777 as speakers for my computer. A mechanical "machine gun" sound started coming out of the speakers one day. I thought it was starting to die, at least one or more caps, so I just decided to live with it until I had no choice but to try and fix it.

Well, today the sound wasn't so prevalent, but then I noticed it would get louder then softer. So, I started working switches, knobs, everything. Pushing the left cassette keys made the sound quieter. Odd. I pushed the right cassette keys - no change. Touching the left grille made the sound quieter too. Twilight zone. So, thinking there's a ground issue with the entire left side, I ran my hand down the left side of the GF-777 while touching it. The sound disappeared.

Okay, I'm really flummoxed now. I remove my hand, sound comes back. Then, I had an idea of what the real problem was...

We had recently upgraded our Comcast xfinity modem and the modem was sitting - you guessed it - about six inches to the left of the GF-777. After moving the modem further away (about 18 inches now), there is no noise.

Electronic interference. Who would have thought? Never had an issue before that.

I wish a solution like that could have been found for my box. I took it Armstrong Audio in London in the end to have it repaired. It made the same unholy sound when connected up in their workshops.

Just got it back. Apparently it had 3 hairline cracks on 3 different PCBs. Transit damage it would seem. I have attached a photo of the work on one of the boards with the most noticeable new solder along its tracks (near +c809 and +c825).

It is now fully working which is excellent. A bit beyond my diagnostic and soldering abilities..

Thank you all for you help.