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Bad noise/crackling from GK MB fusion 500, help!

Zebra

Zebra

Jun 26, 2005
417
161
#1
So my amp seems to have developed some bad noise issue suddenly. Plugging in a bass and playing I get a low volume, distorted signal and background hum. I initially suspected a dead battery in my bass, but I have ruled out everything but the head itself now.

The head has a tube preamp with a class D power section. Specs: Link Removed

What I've checked so far:

I've looked at the tubes when turning it on/having it on. There's no obvious sign of one being irregularly bright/quick to light.

The background buzz does not respond to ground lift, tuner mute, or any other switches.

The background buzz gets louder from turning the master up, but is unaffected by the preamp gain knobs.

The background buzz is on both channels.

Based of this and poking around on the internet, I suspect the issues is a tube has a bad contact with the socket, but I would like some more experienced people to weigh in.

Thanks for the help
 
#2
I had a similar problem with my GK Fusion 550. Tried different basses, same problem. Tried a different cabinet, same problem. Checked the tubes (but didn't replace), same problem. Only thing remaining after my trouble shooting was something in the head or the GK 4 pole Speakon cable. Took the cable up to the store and it tested fine so we replaced the head (but used the same cable) - same problem. Took the new head back and got my original one back but with a new GK 4 pole Speakon cable.. problem solved. Odd that the cable tested ok but that was in fact the issue, this was all two weeks ago and have not had a problem since getting the new cable.
 
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#3
96Brigadier said:
I had a similar problem with my GK Fusion 550. Tried different basses, same problem. Tried a different cabinet, same problem. Checked the tubes (but didn't replace), same problem. Only thing remaining after my trouble shooting was something in the head or the GK 4 pole Speakon cable. Took the cable up to the store and it tested fine so we replaced the head (but used the same cable) - same problem. Took the new head back and got my original one back but with a new GK 4 pole Speakon cable.. problem solved. Odd that the cable tested ok but that was in fact the issue, this was all two weeks ago and have not had a problem since getting the new cable.
Click to expand...
Interesting to know, but I doubt that's the issue as noise was coming out both cabs with both outputs used.
 
#5
I had this problem. Took it to my amp tech. Turned out to be a loose wire/cold solder joint somewhere in the amp. He fixed it, it worked fine after that, then I traded it for an Aguilar TH500. I dig the Aggie, but miss the GK and am now looking to trade it for a Fusion 800. Both great amps. It's probably a minor issue...take yours to a good amp tech.
 
#7
A quick easy test to do is putting a 1/4" cable end into and out of the EFFECTS send/return inputs/outputs a few times. Or just leave the cable and retry the amp. Or using some DeOxit into same inputs and following with the cable into these openings. This has cleared up many an amps' noise issue for me and many others. Costs nothing but a little time to try this.

May or may not help. Then see a tech if not.
 
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#8
I had a similar problem with my Fusion 550 just a week ago. (No warranty anymore)
A 115RBH cabinet in Bi-Amp mode connected via original GK 4-pole Speakon cable to my Fusion 550 produces a terrible crackling noise in the tweeter, independent of mute on/off and independent of other settings. Full range mode is okay, which suggests that the 50 Watt power amp for the horn section is defective. Can anyone help me with finding a replacement for this amp section or a schematic for repair?
 
#9
I had similar issue too. My Fusion 550 started to work very strange. There is a lot of white noise from output and some low frequency crackling that sounds like a little storm. When the mute is switch on i can hear some pulsing sound from the flashing mute diode. I checked it on speakon and jack cables on different cabinets. Still the same... The amp is under warranty (fortunately I have 3 years warranty from Thomann) but it will be the second time I have to send it to repair during 2,5 year of use :(
 

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