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Old 01-11-2013, 11:35 PM   #1
Viban
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My amp doesn't start?

So I went ahead and bought myself a JCM 900MKIII and I finally got my combo's cab all wired up for the correct impedance, I went to start it up for the first time and no signs of life whatsoever, the tubes don't glow, don't heat, the switch doesn't light up, the speakers don't pop or make any sound, nothing. I checked the fuse next to the input and it appears to be blown so I'm going to replace it, but I was wondering if a fuse would be the source of my problem or if I should just go ahead and send it back for repairs?
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Old 01-11-2013, 11:38 PM   #2
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Originally Posted by Viban
So I went ahead and bought myself a JCM 900MKIII and I finally got my combo's cab all wired up for the correct impedance, I went to start it up for the first time and no signs of life whatsoever, the tubes don't glow, don't heat, the switch doesn't light up, the speakers don't pop or make any sound, nothing. I checked the fuse next to the input and it appears to be blown so I'm going to replace it, but I was wondering if a fuse would be the source of my problem or if I should just go ahead and send it back for repairs?


Put a fuse in it and retest. Are the tubes seated? Keep the blown fuse...
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Old 01-11-2013, 11:39 PM   #3
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Sounds like you just bought it. Warranty?
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Old 01-11-2013, 11:41 PM   #4
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Quote:
Originally Posted by R45VT
Put a fuse in it and retest. Are the tubes seated? Keep the blown fuse...

I have no spare fuses, I checked the tubes for seating the second it got in my door, then once more before I started it. They were indeed seated, I don't know if they can unseat actually, The pre-amp tubes have the guards over them that don't really let them move and the power tubes have some sort of cap on top with springs attached. I also plan on keeping the blown fuse.
I'm pretty sure it's blown but I can't really tell, There is no filament in the middle of it and there appear to be a couple of tiny burn spots on the glass, I'm assuming that means that it is blown correct?
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Old 01-11-2013, 11:59 PM   #5
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It's toast.

Sounds like a bad tube or something more sinister. Call them and see what they will do for you. You don't want eat a set of tubes to have something else wrong with it.
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Old 01-12-2013, 12:02 AM   #6
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Originally Posted by R45VT
It's toast.

Sounds like a bad tube or something more sinister. Call them and see what they will do for you. You don't want eat a set of tubes to have something else wrong with it.

That's what I'm afraid of, I think it's evident that this fuse is blown, but what I'm worried about is that there are more fuses on the board that are fried, or that there's something more than a stray power surge to blame for the fuse blowing. I'll try a fuse change upon the morrow, and if that fails then I'll call up GC. Luckily I had the right mind to drop the extra money on the repair plan.
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Old 01-12-2013, 01:01 AM   #7
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Get a few fuses. If it blows again remove the power tubes and repeat.
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Old 01-12-2013, 01:14 AM   #8
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Yeah, there's a couple of fuses inside. They're 500mA slow blows. Grab some to have as spares while your buying a main fuse. You've blown the main fuse though so chances are they'll be ok but either way, you should have spares.
You haven't pulled the chassis out to have a look inside yet? I'm dissapointed in you. I thought that would be the first thing you'd do.
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Old 01-12-2013, 01:20 AM   #9
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are you using the right key? some guy at work was trying to start a ford with a dodge key and he called a mechanic in before they figured out why the car wouldn't start.

i would say fuse, but that could indicate another problem.

i would say return the amp if you just bought it... but it does make me wonder if they'd take it back if you 'just wired up the combo properly'
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Old 01-12-2013, 02:02 AM   #10
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Quote:
Originally Posted by Cathbard
Yeah, there's a couple of fuses inside. They're 500mA slow blows. Grab some to have as spares while your buying a main fuse. You've blown the main fuse though so chances are they'll be ok but either way, you should have spares.
You haven't pulled the chassis out to have a look inside yet? I'm dissapointed in you. I thought that would be the first thing you'd do.

I think I can actually change the 500mA slow blows from outside actually, I checked them and they look ok. And I know I know, I should have it all dissected and seen by now, but I'm still trying to figure out how a teardown would work on this thing.




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are you using the right key? some guy at work was trying to start a ford with a dodge key and he called a mechanic in before they figured out why the car wouldn't start.

i would say fuse, but that could indicate another problem.

i would say return the amp if you just bought it... but it does make me wonder if they'd take it back if you 'just wired up the combo properly'

I tried my chevy key a couple times, it opened the door but it wouldn't crank

I double checked with my multimeter, it read out at around 19 ohms when I wired the speakers in series. I am almost 100% sure that I wired it right, the speakers in the combo were wired in parallel to create the 4 ohm load that the combo works on, which would mean that they would have to both be 8 ohms, so it would make sense that in series they would be a 16 ohm load. I even made sure to set the amp to 16 ohms. I'm also rather proud of my solder work on this jack, I tightened up my iron and it let me get a really great solder join on this thing.
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Old 01-12-2013, 02:08 AM   #11
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i never consider a fuse broken visually i also do that when i put in fuses. get a meter on the fuse and see if you have continuity.
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Old 01-12-2013, 02:11 AM   #12
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i never consider a fuse broken visually i also do that when i put in fuses. get a meter on the fuse and see if you have continuity.

no continuity, thanks for making me feel like a dunce though, why did I not think of that . Can you tell the finals are getting to me?
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Old 01-12-2013, 02:23 AM   #13
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Originally Posted by Viban
no continuity, thanks for making me feel like a dunce though, why did I not think of that . Can you tell the finals are getting to me?


lol. not a big deal. everybody forgets once in a while

i am sure you know more about amps than i do, but i could wire up a drag car with no harness in no time flat. i know that side of electronics better than electrical equipment (amps/pedals/etc.)
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Old 01-12-2013, 02:28 AM   #14
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Originally Posted by trashedlostfdup
lol. not a big deal. everybody forgets once in a while

i am sure you know more about amps than i do, but i could wire up a drag car with no harness in no time flat. i know that side of electronics better than electrical equipment (amps/pedals/etc.)

I know jack about amps actually, set me loose on an engine rebuild and I can do it pretty easily, set me on some CAD software and I could make the damned thing for you, but for some reason electrical things just elude me.
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Old 01-12-2013, 02:32 AM   #15
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Running it with the wrong load (or even without a load) generally wouldn't blow a fuse, at least not right away so I would look at something else. Buy some fuses and use your multimeter. Could it have been blown when you bought it or are you sure "you" blew it? (did it work in the store?)
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Old 01-12-2013, 02:34 AM   #16
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Those 500mA fuses are for the power tubes.

http://www.drtube.com/schematics/ma...92-100-iss1.pdf

FS1 and FS2. Notice how they go between the cathode of the power tubes and ground? When the tubes start to fail they draw more current and blow the fuse. Now see the resistor and LED across the fuse? When the fuse is sound it shorts that out and no voltage appears across the LED. When it blows there is voltage across it and the LED lights up. The resistor is just there to limit the current through the LED so it doesn't blow.
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Old 01-12-2013, 02:36 AM   #17
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Quote:
Originally Posted by jthm_guitarist
Running it with the wrong load (or even without a load) generally wouldn't blow a fuse, at least not right away so I would look at something else. Buy some fuses and use your multimeter. Could it have been blown when you bought it or are you sure "you" blew it? (did it work in the store?)

I know I didn't blow it, it never started in the first place, I have a feeling that the standby would have at least worked on first startup if I had blown it. I ordered it to my front door from a GC in Oklahoma so I never got to check if it actually started or not, but now I have a feeling it's a matter of simple troubleshooting to get her screaming to my every touch.
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Old 01-12-2013, 03:34 AM   #18
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I know I didn't blow it, it never started in the first place, I have a feeling that the standby would have at least worked on first startup if I had blown it. I ordered it to my front door from a GC in Oklahoma so I never got to check if it actually started or not, but now I have a feeling it's a matter of simple troubleshooting to get her screaming to my every touch.


You never know what happened in shipping. I would target the power tubes first.
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Old 01-12-2013, 03:42 AM   #19
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Originally Posted by R45VT
You never know what happened in shipping. I would target the power tubes first.

Well there's absolutely non continuity in the main fuse, so that has to get changed before I can even start other troubleshooting procedures, I think I might grab a groovtube El34 from the local pawnshop just for troubleshooting purposes, then once I get a cab I'll get a nice set of JJ's.
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Old 01-12-2013, 03:45 AM   #20
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Well there's absolutely non continuity in the main fuse, so that has to get changed before I can even start other troubleshooting procedures, I think I might grab a groovtube El34 from the local pawnshop just for troubleshooting purposes, then once I get a cab I'll get a nice set of JJ's.


I would buy 3 or 4 fuses:
Install new fuse and turn amp on. Check fuse,
If blown pull power tubes and install new fuse.
Recheck.

If you think the tubes are bad callGC and see what they will do for you.
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