Marshall JCM900 amp hum is fixed :-)

[ido1957]

Got my Marshall JCM900 4100 amp back after only a week from Long and McQuade. (They called the day I took it in and said it could take 1-2 months due to excessive repair workload. Bonus!

I've only had a few minutes to test it since I got it home (because my daughter has to study for a test tonight). There's lots of time to test it though which i will do later....

The hum is pretty much gonzo - noticed the silence right away.....
Amazing difference overall, super quiet, seems louder, more OD, nice tone. It was not 60 Cycle hum which I thought it might be initially.

They did a number of mods to fix it. Here's the breakdown - hopefully this will be of benefit to others in the future. I'd recommend you take your amp to a professional like the guy at L&M for these kinds of mods.....

BTW - the mods included the some of the recommendations I had from guys on this BBS on my initial thread about the hum - you guys obviously know your stuff.

Repair Details:
Replaced second preamp tube.
Replaced low voltage supply zener diodes and master volume switching opamp.
Replaced factory installed (in 1990) 56K bias resistor with 47K bias resistor as per 1994 schematic to lower increase the negative bias voltage from -50V to -38V to properly bias the EL34 tube configuration.
Resoldered misc components and organized wire layout to help with noise further.
Rtv’d misc components.
Test ok

Mesa Boogie 12AX7 preamp tube 13.95
YSL Marshal Part Valvestate IC M5201 5.95
YSL Part 1N5245B 15VO OW5 Z (2) .50
YSL Part 1/2W 47K5 T&R RES .25
Repair Labor (60) 65.40
Subtotal 86.05
GST 5.16
Total 91.21 (CDN)

So a big hats off to Long and McQuade. I'm going to test this amp some more this weekend and I'm hoping this repair is all I needed

 

[Greg_L]

Cool. I want one of those amps so bad.

 

[Gorty]

Got my Marshall JCM900 4100 amp back after only a week from Long and McQuade.
The hum is pretty much gonzo - noticed the silence right away.....
Amazing difference overall, super quiet, seems louder, more OD, nice tone. It was not 60 Cycle hum which I thought it might be initially.
BTW - the mods included the some of the recommendations I had from guys on this BBS on my initial thread about the hum - you guys obviously know your stuff.

Glad it all worked out for you Gerry!

Cool. I want one of those amps so bad.

You just stay away from the amps please Gregory and go play your Mapex kit!

 

[ido1957]

Cool. I want one of those amps so bad.

L and M have a brand new JCM900 4100 Dual Reverb 100/50 in the showroom - saw it when I took mine in for repair. I thought they were discontinued - maybe this is NOS.

Price tag = $1550 Canadian which is (lol) $1612 US.

Here's the Edmonton Store details:

10204 - 107th Ave.
Edmonton, Alberta
T5H 4A5

Tel: (780) 423-4448
Fax: (780)425-6623
E-mail: info@long-mcquade.com

Hours: Mon, Tues, Wed, Sat: 9:30 - 6:00
Thurs, Fri: 9:30 - 8:00

 

[ido1957]

Glad it all worked out for you Gerry!



You just stay away from the amps please Gregory and go play your Mapex kit!

Thanks Gorty - I am feeling much better now that I have the amp back and it's working. It relieves a bit of the pain from a large tree trimming bill from the arborist that I got today

 

[Greg_L]

L and M have a brand new JCM900 4100 Dual Reverb 100/50 in the showroom - saw it when I took mine in for repair. I thought they were discontinued - maybe this is NOS.

Price tag = $1550 Canadian which is (lol) $1612 US.

Here's the Edmonton Store details:

10204 - 107th Ave.
Edmonton, Alberta
T5H 4A5

Tel: (780) 423-4448
Fax: (780)425-6623
E-mail: info@long-mcquade.com

Hours: Mon, Tues, Wed, Sat: 9:30 - 6:00
Thurs, Fri: 9:30 - 8:00

Lol. Do you get a kickback or something?

No, I can find one down here. I see used heads for around 1100 all the time. I don't play guitar in a band or anything, so I can't really justify to the old lady why I'd need a 100w halfstack.