HELP - Marshall JVM 210 100w - resistors burn

[stratboy99]

Dear Guru(s),

I am a newbie in tube amps and newbie here too. I was trying to help my friends Marshall JVM 210 100w
combo. He told me that while playing the amp smoked and after that it went weak. My curioucity pushes me to open the amp to see what was burn. Upon inspection, I saw 2 resistors fried up. The location of these 2 resistors was at V3 and V4 (output), both resistors are 5.6K on pin #5. It is link from here out to I think a  ceramic resistor of 5W 1K Ohm(on each side of the tubes). I am not sure what is going on here, as I read somewhere the pin #5 are negative voltage readings. What I like to know is what causes these 2 resistors to burn. I wish some gurus here can help me out here as I am about to buy the amp off my friend.
Wish to thanks in advance.

cheers
Simon

[stratele52]

Schematic is here at the bottom of the page ;

forum.metroamp.com/viewtopic.php?f=25&t=21932

Probably V3 and V4 are defective. Grid are short  ( ? )

[Cups]

You are answering some of your questions.

The grids are negative and it's created in the bias system. Replace the resistors but you'll have to look at the bias network to see if all good. I'll check the schem to give you an idea as to where the bias is.

[stratele52]

You are answering some of your questions.

The grids are negative and it's created in the bias system. Replace the resistors but you'll have to look at the bias network to see if all good. I'll check the schem to give you an idea as to where the bias is.

The bias negative voltage can't burn these resistor IMO

[Cups]

I couldn't find it in the JVM210 docs but I did find it in the jvm205: It's at the bottom of the page and it says "bias"  I've never worked on these amps so I don't know where it would be on the PCB.

[PRR]

> a newbie in tube amps .... 5.6K on pin #5 .... I read somewhere the pin #5 are negative voltage readings.

Are you sure you are counting the pins properly?

Those resistors (and smoke) are more likely at pin _4_.

Which is where "pin 5" would be if you count-around the wrong direction.

This is a fairly beefy and dangerous amp. Be very sure you know what you are doing.

[eleventeen]

I am going to agree with PRR here.

It takes no particular skill other than good eyes to spot flamed components. As a repairperson, the task is to figure out what circuit conditions caused the scorched parts to scorch. Sure, it is possible that simply replacing scorched parts with new parts fixes the problem. Any decent repairman, if they get away with that, should be watching over their shoulder to wait for the next shoe to drop. Because that rarely fixes the problem. These R's are not like tires that pick up a nail and go flat and you just have to buy and stick on a new tire. To burn out 5 watt resistors requires a fair amount of power...that's why the mfr chose to specify power resistors in the first place. Yeah, maybe the amp was just left on for a few days and just got too hot. But maybe, indeed probably not.

To be frank, you don't sound like you have the prerequisite skills to troubleshoot this amp and there are some lethal voltages running around in it. That means you may not have the descriptive tools to properly describe what's going on, even if everyone on this forum joined in and worked with you to attack the issue. I don't think the repair is a big deal nor difficult, but really, and please take this the right way, this is not anyone's idea of a first project.

So I would urge you to proceed more slowly, and see if you can get the repair done by a qualified tech. At the very least, shouldn't this be good cause to ask the seller to lower his price? Alternatively, you could insist the seller have the amp repaired at his expense (which he'd be justified in passing on to you) but then you would be buying a "gone-through" amp instead of a pig in a poke.

[stratele52]

I couldn't find it in the JVM210 docs but I did find it in the jvm205: It's at the bottom of the page and it says "bias"  I've never worked on these amps so I don't know where it would be on the PCB.

Sorry , it was the 205 at the bottom of the page