JVM Troubleshooting

[greenmosheen]

Looking for input on this:

Yesterday amp was fine, sounded great as usual. Turn it on this morning for a little jam and get nothing... Indicators all come on, but no sound. I did not get any weird pop, fizzle, or anything (either yesterday during playing or when turned off, nor today when turning it on). I checked out the back and no tube glow whatsoever.

Thoughts?

 

[PhloridaMan]

It's a Marshall... Probably just a fuse. My buddies Super Lead just did this recently. Always look at the cheapest possibility first

 

[just42dave]

fuse

 

[raiken]

++ on the fuse. I have a friend who has a JVM. I've fixed it for him a few times.

Since yours lights up, but has no sound, it is most likely the HT fuse, which is silkscreened "FS6" and "T1A" on the board. It is located in the lower corner near the four big filter caps.

If the fuse is blown, it was likely because of a bad output tube.

 

[jerrydyer]

Randalll Aiken is here. Hi

 

[raiken]

Randalll Aiken is here. Hi

Hello, Jerry Dyer!!

This is a great forum. I lurk here a lot, but rarely post, to avoid the appearance of spamming.

 

[greenmosheen]

Thanks guys, I pulled the chassis just now and it is FS3... I guess that is a clear indicator that my amp is pre "week 25 of 2007". Now to find someplace local to grab a new fuse and then to Laura for a refresh on tubeage...

 

[raiken]

Thanks guys, I pulled the chassis just now and it is FS3... I guess that is a clear indicator that my amp is pre "week 25 of 2007". Now to find someplace local to grab a new fuse and then to Laura for a refresh on tubeage...

Oh, of course! I missed the part where you said "no tube glow". FS3 and FS4 are the 10A heater fuses.

Odd, but the official Marshall fix is to remove the fuses and fuseholders altogether:

https://www.google.nl/url?sa=t&rct=j&q= ... 5248,d.Yms

 

[greenmosheen]

Yeah I just read the same.. Any downfall to doing that? I read that it actually improves the sound a bit as it raises the heater voltage a bit... talking with O'Rourke about having him hardwire it...

 

[raiken]

Yeah I just read the same.. Any downfall to doing that?

The only downfall is that if a heater shorts, it will take out a $100 transformer instead of a 50 cent fuse. Having said that, most companies don't fuse the heater, and heater shorts are not very common.

 

[greenmosheen]

Yeah I just read the same.. Any downfall to doing that?

The only downfall is that if a heater shorts, it will take out a $100 transformer instead of a 50 cent fuse. Having said that, most companies don't fuse the heater, and heater shorts are not very common.

Awesome!! Thanks, appreciate the info!!!

 

[greenmosheen]

Well... I put a new fuse in and it seems to be getting no power to the Power tubes.. No tube glow and minimal plate readings using a multimeter... I guess I will need to find someone to repair it.. I am in central ohio if anyone has recommendations on a quality tech.. I could even ship to and from if needed, but was hoping for a local tech.

 

[raiken]

Did you replace both heater fuses (FS3and FS4)?

Do you measure 6.3VAC between the two fuse holders?

 

[greenmosheen]

Now I have replaced both fuses, and FS3 is giving me 6.85VAC, FS4 is showing nothing (hovering around 0.01) - not sure if this is normal though??... I did not originally replace FS4 (DUH! my idiocy there).. I will check the power at PIN3 on the tubes to see what plate voltage I am getting now. Maybe all is not so bad after all..