It looks like you're new here. If you want to get involved, click one of these buttons!
Subscribe to our Patreon, and get image uploads with no ads on the site!
Base theme by DesignModo & ported to Powered by Vanilla by Chris Ireland, modified by the "theFB" team.
Comments
If they are the originals I would change them as they can make a huge mess inside the amp.
All rather depends upon the quality of the original capacitors (ICBM will surely know) and the life the amp has had but you can do a visual inspection. If the contact ends of the caps are clean, free of any powdery deposit and not bulged in any way I would leave them.
Dave.
I always change the bias filter caps in Marshalls of this age.
If these caps go leaky the amp can go into thermal runaway.
This is more likely in 50W Marshalls as this bias supply is fed from a high value resistor so even a small amount of leakage causes the bias to drift towards to ground, which biases the power valves hotter.
100W Marshalls are less prone to this problems but I've seen bias failure in these too.
Bias failure can and does cause damage to output and mains transformers, so can be expensive.
It's rare to find filter caps that need changing in Marshall amps of this age, and I would not routinely change the caps in an amp this age.
https://jpfamps.com/marshall-jmp-amplifier-repair/
Visually all the caps look ok.
"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
Ah! Bias caps, yes I would change those anyway. There is a cheap way to derive a bias supply which involves a foil capacitor and a high value resistor, around 100k. Both components stand changing in an old amplifier in my limited experience.
Dave.
"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
Not too difficult a job luckily.
The correct (and fiddly) way is to remove the board - have fun with that - flip it over and unsolder the old caps from the underside of the board where the PCB traces are. Fit new caps, check orientation, and resolder, reassemble.
The bodge way is to snip the cap legs above the board and attach the new caps to the old legs - not recommended unless you're in a pinch and can do the correct repair later.
Rift Amplification
Brackley, Northamptonshire
www.riftamps.co.uk
....or you can just wait until they really do need changed.
I do admit to that if I'm just replacing the bias caps and not the main filters - I don't think it's too much of a bodge if you do it neatly, and there's enough length of wire from the old caps that there's very little danger of overheating the joints under the board. The logic is that if you later replace the main filters you can re-do the bias caps properly at the same time, if you want.
Everyone seems to worry about it 'ruining the tone' - not helped by a lot of what you read online - but I have never, not once, heard an old Marshall that sounded better before a recap than afterwards or had any complaint about it from the owner.
"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
Are you supposed to "bring up the voltage slowly" when you change filter caps? I didn't, just turned it on, and there were no problems.
It doesn't matter with new caps.
The only time it's really a good idea is when powering up an old amp that hasn't been used for several years or more.
"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