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If the power valves and the phase inverter are lit but the other three aren't, it's the filament bridge rectifier - this is the 'DSL401 fault' and is caused by it not being heatsinked, so over time it overheats and desolders its own pin connections. It can be fixed with a bolt-on heatsink so it won't happen again. This fault usually appears when the amp is hot, and can be intermittent at first.
"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
It's easiest with the amp lying face-down on a table with the handle facing you. When the chassis is loose you can lift it out holding the transformers - be careful it doesn't snag and tear the vinyl at the sides, it may be best to cut some cornflake packet into strips to slide down between the sides of the chassis and the cabinet first.
Lift the chassis out and lie it across the back of the cabinet with the valves in the space above the speaker. The fuse is near the third power valve socket from the right-hand end of the main PCB as you look at it with the knobs towards you.
You can get the fuses from Maplins if you can't find them online easily, but try Ebay first. You need a T10A 20mm fuse.
But check it first before ordering - if the fuse is *not* blown, it's likely to be either something trivial (a loose connector, probably) or more serious (blown power transformer winding - unlikely).
"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
I want to make sure there's no current/voltage left to get a shock from - I know that can be kept there for some time - how do I make sure capacitors etc are drained? I also read that, whilst you don't turn the power on obviously, you should keep the mains lead connected and plugged in to keep an earth and protect components against static charges etc...is that sensible advice or rubbish?
Generally the components which are at risk from a static discharge are digital logic chips, and there are none of those in this amp.
There is also no risk of a shock from the caps in this amp if it's off and disconnected from the mains and has been for more than a minute or two - there is a built-in permanent drain resistor in the power supply. All amps should have this. It's especially necessary in amps with a filament fuse, since if it blows there is no way of discharging the caps via the valves.
"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
He fully concurred with ICBM that its a nuisance fuse & put in a T10A so theoretically it shouldn't go again. He added that its totally daft Marshall didn't give easy access to this fuse, and you really shouldn't have to unscrew umpteen screws and drop the chassis just to get to the damn thing! Why do amp designers make what should be simple, so awkward!?
Anyway, all done & fingers crossed I don't get any further problems.
Better than the TSL122 I have here anyway, which I had acquired last year for the parts value - over the holiday I thought I would have a go at actually finding out what was wrong with it, since it seemed a bit of a waste to scrap it, but I didn't want to spend the money on a new main board from Marshall. I thought I'd cracked it, but yesterday - luckily before I put it up for sale - it developed a different and completely unrelated fault.
So back to Plan A. At least the DSL401 is better than these...
"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
I had actually dismantled it last year, but I don't like to give up on things so when I had a couple of days free over the holiday I thought I would have a go at fixing it instead. It's still worthwhile finding out that your original instinct was correct though, it will save time in future .
"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