Marshall Valvestate vs100 Problem

[fallingdown]

I use this head to play at home. It's a solid state, 4 ohm, 100 watt head. I was playing on it and it just lost power and stopped working. It will not turn on at all. I haven't opened it up and looked at it yet, but I was just wondering if anyone has any suggestions or advice?

This head is cheap and not very nice, so if the repair is troublesome or expensive I'm just going to forget about it. Just thought someone who knows more than me about the subject could point me in the right direction. Thanks.

[slacker190]

If it doesnt power up at all, i.e. no power light, etc, I'd check the fuses.

[fallingdown]

how do I check the fuses, and how much do they cost to repair (buy)? thanks.

[Amp Kat]

Fuses are about $1 a piece and you can get them at home depot usually or an Auto part's store locally. Chances are if it blew that one it will blow again but I guess it's worth the try. There are two dropping resistors in that amp going to the +/- 15 volt rails that get pretty hot which is a design flaw. Most of the problems or around there but I have also seen the output transistors go bad and cause fuse blows also so good luck with it either way.

[quazcha]

see if there is an output IC mounted to a heatsink (probably a TDA7293) very common for marshall solid state amps. unplug any connectors going to the small PC board on the heatsink, replace the fuse, if the fuse does not blow, replace the connector(s) power on.. if the fuse blows the problem in in the output IC you can get from www.digikey.com as far as not wanting to get "too involved" it is worth a little time and trouble to replace the IC (if not contact me and maybe we could work something out? I live in stratford NJ USA) also checkout posts on Marshall AVT150, Marshall MG100, AVT275 etc for more info

[Enzo]

Nah, this amp is 10-12 years old or so, the power stage is discrete. Used the BDV transistors, which are like the preferable TIP142/TIP147 types.

Look on the heat sink for the larger transistors and see if they are shorted. That is by far the most likely reason for a mains fuse blown. And a mains fuse is the most likely thing to cause the amp to be dead.

[fallingdown]

thanks very much for all the advice. this website is really amazing. I have two questions about what people have been saying. First, is it true that if I simply replace the blown fuse(s), the head will work, but is likely to blow again?

And second, how can i identify the heat sink? Thanks alot.

[Enzo]

No, that is not the likely scenario. SOmetimes fuses just blow from age, but mostly they blow for a reason. And in those cases, if you slap a new fuse in there it will blow right now, as opposed to three days later. Even if there is some intermittent problem, like a shorting tube, it might work briefly, but the bad tube will take out the fuse soon enough. That is because the underlying problem was not solved.

A heat sink is a general term for a passive cooling device. SOmething that helps drain heat away from some parts. The cooling fins on a motorcycle engine or the motor of a lawn mower would be a heat sink of sorts. In amps, heat sinks are generally aluminum things with fins on them. In your amp they would be black things on the main circuit board. Each one would have a transistor or two clamped to it, either by a metal clip or by a screw through a mounting hole.

Yours looks different, but here is a heat sink

http://www.alliedelec.com/Images/Products/Small/619-0038.jpg

[fallingdown]

ok, thanks for the advice. I opened up the amp to take a look. there are 4 heat sinks. each has one transistor clamped to it, except for the one second from left -- it has two transistors clamped to it. They are clamped to either side. Because of the layout, it's really hard to look very closely at all but one of the transistors.

How will I be able to tell if one or more of the transistors are shorted? Sorry for all the questions, I'm pretty ignorant about this stuff but I want to learn and can't afford to pay someone else to fix it for me anyways. I really appreciate your help.

[Amp Kat]

You need to get a meter and measure across the leads and see if you get continuity or a beep in beep mode. These also have very low ohm emitter resistors and should be checked to make sure they are not open. What happened when you replaced the fuse ?

[fallingdown]

I didn't replace the fuse yet because I assumed it would just blow again. Can I get a meter at a radio shack/home depot or someplace like that?

[Amp Kat]

You can get one at either or and on the internet too pretty cheap for what your doing.

[fallingdown]

cool. i'll go pick one up and try that. also, you said in an earlier post: "There are two dropping resistors in that amp going to the +/- 15 volt rails that get pretty hot which is a design flaw." ---- I tried to identify these 2 dropping resistors you refer to, but I couldn't. Any advice? thanks.

[The Kid]

I've got a Marshall MG100HDFX head that blew a fuse too. this was after i had taken it apart to change the fan. if i seperate the heat sink from the little metal plate that screws onto it, the fuse won't blow but when i screw it back in it blows the fuse. could this be a result of too little thermal compound on the heat sink? sorry to just jump in the thread with my question but your problem sounds a lot like mine and i can't find anybody with a similar problem. thanks.

[Enzo]

Not a lack of thermal grease, a lack of insulating mica washer. Was there not a square of something between the power IC and the heatsink? Is there a small plastic washer on the mounting screw? The washer keeps the screw centered in the hole so it won't actually touch the metal.

[Mars Amp Repair]

Hey Kid,
the heatsink MUST be isolated from the chassis as it is hot. Meaning, there are as I recall 4 insulating washers that keep the heatsink from electrically connecting with the chassis. you have put them in the wrong position thereby letting the heatsink become connected with the chassis.
the version of the amp in the prior posts is different from yours. His has a discrete power stage where yours is a single IC....glen
glen

[quazcha]

I have 2 MG100HDFX's one circa 2005 and the other 2006. the 2005 had the earlier fan style. The earlier marshall MG100HDFX fans were mounted to the heatsink by 4 screws insulated (from the chassis) at the head end by the same type of TO220 "transistor mounting screw" insulators. The later ones have a plastic fan shroud that mounts to the chassis and the heatsink mounts to the other side of the shroud the screws are offset as not to touch each other to provide insulation. The heatsink is electrically live (i believe +48vdc) and must be insulated.
I replaced it with a thicker computer fan/heatsink (40mm x 40mm x20mm thick) and was able to find a heatsink with the fan mount locked onto the heatsink and then used short sheet metal screws to mount the fan to the chassis. before you power up, use an ohm meter and measure the resistance from the chassis to the heatsink it should be in the megohms or you will bow to the TDA7293 replacement gremlins once again...