Marshall JCM800 Model 4210 combo

[bsco]

Good morning to everybody...had this come in for repair and it has been in storage for at least 15 years...anyway, went through the usual stuff....had to replace ALL the filter caps...bad connections, corroded jacks, noisy controls, etc.....tested the tubes for shorts and emissions and they tested good...I have the amp working but every so often when powering up, the amp blows one of the heater fuses...there are two 5A slow blow fuses there at present.....the schematic is not all that clear....I will post what I have....any suggestions before i install a complete set of tubes?? it takes 7 in total...five 12AX's and two EL34's
CheersMarshall-JCM800-Splitch-Rev-50W-4210-Schematic.pdf

[blindboybenton]

I would first look at soldering on bases. I would resolder anyway. If that does not cure it. Change the power tubes first. Its more likely to have an issue with them than 12AX7. If you can find a used set from another amp fit and re bias and test

you can also tap each tube with the back of a screwdriver to see if you can induce the fault to happen

good luck
BBB

[Jon Snell]

Does it use 6550 or EL34 output valves.
KT88 or 6550 only draw 3.2A between them. There is no silly conversion to DC like now as then we had valves that weren't leaky heater cathode!
Check the type of 5A fuse. It must be the type with the coil inside, T type tend to blow too soon.
The newer JCM800s use 6.3A Slow Blow. Maybe as the transformer is happy up to 6Amps a 6.3A fuse is acceptable, after all a short will take out a 10A fuse if fitted but don't do that!
I know that from experience, a customer didn't have any 5A fuses only 10A from his light controller, fitted them and they blew with no damage to the transformer. Fitting new EL34s that were not down to air with white getter exhaust inside the valves plus new 5A fuses fixed the amplifier.

[bsco]

Thank you to both of you....I will be going over to the shop shortly...It uses El34's...The heater fuses are not the ones with the coil inside...but they are listed as slow blow....does the newer amps use the same transformer?? maybe I can measure the resistance of the fuse and then the voltage drop across the fuse to see how much current is being drawn when first powered up....it must be spiking up past the 5A mark....I will check it out and post my findings later today....
Cheers

[g1]

When those were current models, the service bulletin came around telling us to update heater fuses to 6.3ASB in cases of nuisance blowing. At least for the Canadian versions with the fuse boards. Can't say for the US versions.
The new ones using 6.3ASB heater fuses as Jon said. 5A just doesn't seem to hold up to the turn on surge.

[bsco]

When those were current models, the service bulletin came around telling us to update heater fuses to 6.3ASB in cases of nuisance blowing. At least for the Canadian versions with the fuse boards. Can't say for the US versions.
The new ones using 6.3ASB heater fuses as Jon said. 5A just doesn't seem to hold up to the turn on surge.

Ok then...This has a separate fuse board so that is what the trouble is.....as long as the fuse does not blow the amp works fine....I will check this out tomorrow...thanks for the heads up...
Cheers

[bsco]

finally got back at this unit last week...I replaced the fuses with 6.3A/SB and it worked out good after.....had to clean a few stubborn pots a few times before they settled down...this thing has been in storage for quite a number of years....even the jack cords that were in the back of the amp had corroded wiring underneath the insulation.....Thanks for that info g1 as I would have been chasing a problem that really did not exist......