Some Marshall JMC 900 Help please

[Securb]

A friend of mine has a Marshall JMC 900 2x12 combo he bought on Craigslist. He got it home and it went dead within a week. He contacted the seller and the guy agreed to have it sent to Aztech Electronics in Watertown MA. Aztech is considered one of the best amp repair shops in the Boston area. The problem turned out to be the fuse on the circuit board (not the fuse in the back of the amp). The fuse was changed and the amp was retubed and 6 months later the fuse goes again. He changed the fuse and a month later same problem. A few months later same problem. The amp is not heavily gigged with it leaves the house maybe twice a year. It spends most of its time on the amp-stand being played at house-friendly volumes. Does anyone have any ideas why this keeps happening?

 

[Edgecrusher]

Re: Some Marshall JMC 900 Help please

Does it only blow the fuse? Are there any other issues? Sound or tube life? Has the amp seen a tech again after the initial one?

 

[Securb]

Re: Some Marshall JMC 900 Help please

Does it only blow the fuse? Are there any other issues? Sound or tube life? Has the amp seen a tech again after the initial one?

Only the fuse. The amp sounds amazing and the tubes are in pristine shape. The amp hasn't been back to a tech but that will be the next step if we can't figure it out here.

 

[Chickenwings]

Re: Some Marshall JMC 900 Help please

ive had two different model 900s. My MKIII master dual master 900 had a habit of cooking fuses. I think it was due to one of the bias resistors being burned out even tho it looked ok. The amp always ran really hot. Possibly because it was a combo and the heat from the tubes went up to the circuit board.

 

[Securb]

Re: Some Marshall JMC 900 Help please

My buddy checked out the thread yesterday and was glad to get your input. So if it is a bias resistor it does have to go back to the shop.

 

[glassman]

Re: Some Marshall JMC 900 Help please

There are five internal fuses in that amp (regardless of model); one for the high voltage, two for the low voltage and two for the heaters. Any one of them can shut down the amp. Knowing which one (the value would indicate this) would be really helpful in narrowing down what might have caused the fuse to open.

 

[Securb]

Re: Some Marshall JMC 900 Help please

There are five internal fuses in that amp (regardless of model); one for the high voltage, two for the low voltage and two for the heaters. Any one of them can shut down the amp. Knowing which one (the value would indicate this) would be really helpful in narrowing down what might have caused the fuse to open.

It is the fuse next to the transformer. Not sure if that helps.

 

[Kamanda~SD]

Re: Some Marshall JMC 900 Help please

Bring it to a tech, fuses blow because something is wrong and its protecting itself, and if you wont replace the bias resistors yourself (if that was the problem) then theres probably not much your going to be able to do yourself....not to mention not worth the headache/ risk of shock.

 

[Chickenwings]

Re: Some Marshall JMC 900 Help please

One thing i have learned about amps is this.....glassman knows them better than anyone! Whatever he recommends, just do it!

 

[glassman]

Re: Some Marshall JMC 900 Help please

It is the fuse next to the transformer. Not sure if that helps.

Really need a fuse value...one person's "right next to the transformer" may not match another's "right next to the transformer".