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I took my backup amp (Marshall Mosfet 100) to the studio tonight to put it through its paces. Its the first chance iv had to really try it out. All was fine for about an hour then it started to get a bit crackly and the volume dropped off. At this point I switched it off as I could smell burning. I got it home and opened it up to find the mess below, R46 & R47 fried. Is it worth just replacing the fried resistors or should I get it checked out by a tech. Bear in its a £100 amp so I don't want to chuck too much at it
http://i879.photobucket.com/albums/ab355/hhwarner/mosfet1.jpg
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Comments
"I know I'm a little late in answering this query, but thought this might be of benefit to others with the problem of fried resistors in the Lead 100 Mosfet. The resistors R46 and R47, both of which are 270 Ohms 2 Watt, overheat because of a fault in the reverb driver circuit based around TR5 and TR6. Remove both transistors and test the small diodes D9 and D8, both of which should show infinite resistance in one direction. These diodes are prone to failure and can cause the failure of either TR5 or TR6 or both. In addition, failure of these transistors will almost inevitably burn out the input (send) coil in the reverb chamber, which will also need to be replaced. TR5 is an MPSA06 or BC538; TR6 is MPSA56 or BC528. Both are 80V transistors, so don't be tempted to use any lesser device. They should both measure infinite resistance between collector and emitter when removed. The diodes D9 and D8 (originally 1N914) can be replaced with 1N4148.
Best wishes, Ray"
:- /
Thanks for that, it makes perfect sense. The reverb has been intermittently working. I wasn't overly worried as I don't use the reverb
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Cheers ICBM, I have had problems with the reverb working intermittently, the tank is ok so reverb driver is the likely culprit
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Ill give that a go, many thanks once again
Looking at the schematic for mine I think Im correct in saying its TR4 & TR5, is that correct
Schematic
The proof of the pudding is that the amp works OK without them. It's possible that voltage spikes from a bad connection to the reverb tank 'send' could have taken out TR4 - worth checking should you ever decide to replace it. (I'd leave TR5 out until you do).
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
The reverb pot could have some audible effect, but I would expect it to be pretty subtle. If it's very noticeable, try shorting out the phono plug on the end of the lead that plugs into the output of the tank in case there is some weird feedback / oscillation thing going on.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein