marshall JVM410H tube amp problem

copachino · Post by **copachino** » 20 May 2014, 17:07

Hi, i have a JVM amp, that its a monster i love it, but last saturday during a rig, someone drop the HEAD, and mess up the amp, i have try it, and works fine, except, that, when i turn it on and off, i hear a bump coming from the speaker, i think that its maybe a capacitor that has damage, or maybe a grond pad its lifted, or could also be a broken tube??, amp sound good, except for the bump, id say that everything its normal, any schematic over there, and some tips to trace the cap or the tube or the ground pad??

deltafred · Post by **deltafred** » 20 May 2014, 17:55

Safety Warning - The smoothing capacitors in power amps can hold their charge for long periods so before doing any work or investigation check that they are discharged and if not discharge them. With valve amps the high voltages at best can give you a nasty shock and at worst can kill you.

With any piece of gear that has been dropped it is very likely that transformer and heavy component mountings may have distorted or broken. Also PC boards may have cracked, especially where heavy components are mounted (large capacitors) or where they are mounted to the chassis.

I would take the chassis out (carefully without touching any components), get a lot of light on it, discharge the caps, and then examine it very carefully for any signs of damage.

A large thump at switch on and off sounds like a broken ground connection or something shorting to ground that shouldn't be, but as always I could be wrong. The fact that it is still working is a good sign, I would be very surprised if there is any major damage.

Disclaimer - It is always very difficult to diagnose the fault without having the amp on my bench.

okgb · Post by **okgb** » 20 May 2014, 20:34

Check the JVM newsgroup , there are a couple smart people there and they have sections to deal with problems

DrNomis · Post by **DrNomis** » 21 May 2014, 06:47

My Marshall MA100C thumps when I power it off too, I don't think there's anything major wrong with your Marshall amp since it sounds like it's powering-up normally, but definitely give it a good check out,make sure you do take note of Deltafred's advice since it only takes one little mistake which could turn out to be lethal, if you do any voltage-measurements while the amp is powered-up on the workbench, connect the black probe of your multimeter to the amp-chassis, use one hand to hold the other probe and keep your other hand in your back pocket so that there's no chance of electrical-current going across your chest and causing heart fibrillation if you do happen to get a shock, it's best to avoid getting shocked in the first place though.....

deltafred · Post by **deltafred** » 21 May 2014, 09:00

DrNomis wrote:My Marshall MA100C thumps when I power it off too, I don't think there's anything major wrong with your Marshall amp since it sounds like it's powering-up normally, but definitely give it a good check out,make sure you do take note of Deltafred's advice since it only takes one little mistake which could turn out to be lethal, if you do any voltage-measurements while the amp is powered-up on the workbench, connect the black probe of your multimeter to the amp-chassis, use one hand to hold the other probe and keep your other hand in your back pocket so that there's no chance of electrical-current going across your chest and causing heart fibrillation if you do happen to get a shock, it's best to avoid getting shocked in the first place though.....

Yes, if it didn't thump before and it does now something has changed and needs investigation.

Further to Simon's advice on putting one hand in your pocket I was told as an apprentice by a couple of old engineers (one who worked on radar installations, the other on high voltage power lines) that you put your left hand in your pocket because your heart is to the left of your chest. Putting your hand in your pocket stops it touching anything grounded, like a chassis. The current flows down to your feet which hopefully have well insulated shoes on, not straight across your chest and through your heart.

copachino · Post by **copachino** » 21 May 2014, 22:58

deltafred wrote:
DrNomis wrote:My Marshall MA100C thumps when I power it off too, I don't think there's anything major wrong with your Marshall amp since it sounds like it's powering-up normally, but definitely give it a good check out,make sure you do take note of Deltafred's advice since it only takes one little mistake which could turn out to be lethal, if you do any voltage-measurements while the amp is powered-up on the workbench, connect the black probe of your multimeter to the amp-chassis, use one hand to hold the other probe and keep your other hand in your back pocket so that there's no chance of electrical-current going across your chest and causing heart fibrillation if you do happen to get a shock, it's best to avoid getting shocked in the first place though.....
Yes, if it didn't thump before and it does now something has changed and needs investigation.

Further to Simon's advice on putting one hand in your pocket I was told as an apprentice by a couple of old engineers (one who worked on radar installations, the other on high voltage power lines) that you put your left hand in your pocket because your heart is to the left of your chest. Putting your hand in your pocket stops it touching anything grounded, like a chassis. The current flows down to your feet which hopefully have well insulated shoes on, not straight across your chest and through your heart.

i will check it up, and dont worry im used to work on power, in fact i doit all day long on my job, so i have many proteccion equipment, and a lot of exp wotking on 125KV range power transofrmers and electrical stations... but the bumps wsnt there before

deltafred · Post by **deltafred** » 22 May 2014, 00:09

copachino wrote:.... dont worry im used to work on power, in fact i doit all day long on my job, so i have many proteccion equipment, and a lot of exp wotking on 125KV range power transofrmers and electrical stations...

Pleased to hear this.

On the net you never know whether you are advising someone like yourself who has way more experience of high voltage equipment than most of us on FSB, or some 14 y.o. kid who hasn't got a clue about the dangers, so it always best to assume the worst.

DrNomis · Post by **DrNomis** » 22 May 2014, 04:25

Keep us posted on how it all goes....

copachino · Post by **copachino** » 22 May 2014, 14:41

DrNomis wrote:Keep us posted on how it all goes....

i have check it, and one tube its like damage, also one capacitor on the output its broken, that the sound, then everything goes fine, i will buy that capacitor on ebay, since i dont have high voltages capacitors, and yes, they were charged, its good to have a good way to discharge capacitor, for this jobs, thanks all for you help.