Changing caps

Jethro Rocker

Jethro Rocker

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My 1982 Marshall 4010 combo needs a cap job, hums like a bugger. Originals still in. I ordered a set o FnT.
That said, it hasn't been fired up in over 2 years. Existing caps likely discharged by now? Or, best easiest way to discharge?
Thanks.
 
Jethro Rocker said:
Existing caps likely discharged by now?
Click to expand...
In this circuit, YES. But one should be in the habit of verifying, so go ahead and connect a meter from any of the filter cap's positive lugs to a chassis ground.
Jethro Rocker said:
Or, best easiest way to discharge?
Click to expand...
Jumper wire with clips method: Connect one end of the jumper wire to any of the two plate/anode resistors of V1. Other end of jumper wire goes to a chassis ground. The mounting holes of the chassis is a good and easy ground to connect to, or the pcb's mounting hardware is just as good. If there are any stored voltages in those filter caps, all the voltages will positively be drained through this connection.

Your amp should look like this (with jumper wire illustration):
50864393796_4d412b2a9c_b.jpg
 
Jethro Rocker said:
My 1982 Marshall 4010 combo needs a cap job, hums like a bugger. Originals still in. I ordered a set o FnT.
That said, it hasn't been fired up in over 2 years. Existing caps likely discharged by now? Or, best easiest way to discharge?
Thanks.
Click to expand...
First
take out the 3 smaller preamp tubes.
Turn the amp on...
Is the hum still there?

Install the small preamp tube closest to the power tubes. (phase inverter)
Leave the other 2 preamp tubes out.
Turn the master volume to zero.
Turn the amp on.
Is there hum now?
 
Amp Mad Scientist said:
First
take out the 3 smaller preamp tubes.
Turn the amp on...
Is the hum still there?

Install the small preamp tube closest to the power tubes. (phase inverter)
Leave the other 2 preamp tubes out.
Turn the master volume to zero.
Turn the amp on.
Is there hum now?
Click to expand...
While I haven't tried this, The hum is from within the power section. There is no way to make it stop and gets louder as volume is increased. Probably a good idea to change caps out as they are from 1982.
What would the above method show me? It is NOT from a preamp tube.
 
syscokid said:
In this circuit, YES. But one should be in the habit of verifying, so go ahead and connect a meter from any of the filter cap's positive lugs to a chassis ground.

Jumper wire with clips method: Connect one end of the jumper wire to any of the two plate/anode resistors of V1. Other end of jumper wire goes to a chassis ground. The mounting holes of the chassis is a good and easy ground to connect to, or the pcb's mounting hardware is just as good. If there are any stored voltages in those filter caps, all the voltages will positively be drained through this connection.

Your amp should look like this (with jumper wire illustration):
View attachment 86205
Click to expand...
Thanks for that. I knew there was something but couldn't recall.
 
syscokid said:
In this circuit, YES. But one should be in the habit of verifying, so go ahead and connect a meter from any of the filter cap's positive lugs to a chassis ground.

Jumper wire with clips method: Connect one end of the jumper wire to any of the two plate/anode resistors of V1. Other end of jumper wire goes to a chassis ground. The mounting holes of the chassis is a good and easy ground to connect to, or the pcb's mounting hardware is just as good. If there are any stored voltages in those filter caps, all the voltages will positively be drained through this connection.

Your amp should look like this (with jumper wire illustration):
View attachment 86205
Click to expand...
BTW if I attach a meter, what am I reading? Forgive my ignorance.
 
Jethro Rocker said:
My 1982 Marshall 4010 combo needs a cap job, hums like a bugger. Originals still in. I ordered a set o FnT.
That said, it hasn't been fired up in over 2 years. Existing caps likely discharged by now? Or, best easiest way to discharge?
Thanks.
Click to expand...
First
take out the 3 smaller preamp tubes.
Turn the amp on...
Is the hum still there?

Install the small preamp tube closest to the power tubes. (phase inverter)
Leave the other 2 preamp tubes out.
Turn the master volume to zero.
Turn the amp on.
Is there hum now?
Jethro Rocker said:
While I haven't tried this, The hum is from within the power section. There is no way to make it stop and gets louder as volume is increased. Probably a good idea to change caps out as they are from 1982.
What would the above method show me? It is NOT from a preamp tube.
Click to expand...
If it gets louder as the volume increases, it's probably not the filter caps.

Although old caps might need changing...
 
Amp Mad Scientist said:
First
take out the 3 smaller preamp tubes.
Turn the amp on...
Is the hum still there?

Install the small preamp tube closest to the power tubes. (phase inverter)
Leave the other 2 preamp tubes out.
Turn the master volume to zero.
Turn the amp on.
Is there hum now?

If it gets louder as the volume increases, it's probably not the filter caps.

Although old caps might need changing...
Click to expand...
It gets louder even with NO input at all.
 
Ok i lied.
Lied a lot.
:cautious:
I recall last using the 800 at the dude's studio 2 yrs ago, it hummed like hell, no way to get it out. Started thinking today other amps have hummed at his too, maybe a potential ground issue or even his LED lites.
So...
I tried again here. It hummed really loudly at like 6 plus on volume. I switched out V2 for V1.
It got quieter.
I replaced the now V1 with a different tube from the stash.
It got quieter.
Hum? A little, sure, at like volumes loud enough to shake the windows.

Now, add the Lynch Mod into V2 which adds another tube gain stage. More hum?
Yep but that's expected. Monkeyed with a few combinations of tubes in the Mod and found 1 microphonic one. Works great as PI etc but not a gain stage.

So now, I can put up with what hum one gets using LM and without it is surprisingly quiet.
So I lied.

Won't hurt to change caps though as it is officially 40 years old.
Thanks for the help lads.
@Amp Mad Scientist I did indeed remove V1 and V2 and had no hum. This precipitated my venture.
After I said it wasn't preamp...:rolleyes::cry:
 
Jethro Rocker said:
It gets louder even with NO input at all.
Click to expand...
OK I think I know what it is now...(brain is full of cob webs)

Plug into the LOW input jack with a guitar / cable.
The hum is gone now isn't it?

To fix this problem, you have to replace the LOW input jack.

The LOW input has a switch in the jack.
When the switch contact gets oxidized, it breaks the signal path, causes the hum.

Cleaning the LOW jack contacts is temporary.
The problem always comes back later.
Therefore replacing the LOW input jack is really the only reliable solution.

Warning- fake Cliff Jacks:
Be sure to use a REAL CLIFF UK jack.
If the jack is not stamped: "CLIFF UK," it is a FAKE knock off jack- do not install the fake jack!
Look for the "UK." Make sure it's a real UK jack.
The fake jacks will fail very quickly.

Behold: The Fake Cliff Jacks! How To Spot Them...

Surprise The Jacks I bought from a very well-known vendor turned out to be FAKE knock-offs. The REAL Cliff Jack says: "Cliff UK." The FAKE Jack does not say "U.K!" That's how you can spot it. This is a REAL Cliff Jack. Says "U.K." This is the FAKE Cliff Jack. The REAL Cliff jack is made...
www.marshallforum.com www.marshallforum.com
 
Amp Mad Scientist said:
OK I think I know what it is now...(brain is full of cob webs)

Plug into the LOW input jack with a guitar / cable.
The hum is gone now isn't it?

To fix this problem, you have to replace the LOW input jack.

The LOW input has a switch in the jack.
When the switch contact gets oxidized, it breaks the signal path, causes the hum.

Cleaning the LOW jack contacts is temporary.
The problem always comes back later.
Therefore replacing the LOW input jack is really the only reliable solution.

Warning- fake Cliff Jacks:
Be sure to use a REAL CLIFF UK jack.
If the jack is not stamped: "CLIFF UK," it is a FAKE knock off jack- do not install the fake jack!
Look for the "UK." Make sure it's a real UK jack.
The fake jacks will fail very quickly.

Behold: The Fake Cliff Jacks! How To Spot Them...

Surprise The Jacks I bought from a very well-known vendor turned out to be FAKE knock-offs. The REAL Cliff Jack says: "Cliff UK." The FAKE Jack does not say "U.K!" That's how you can spot it. This is a REAL Cliff Jack. Says "U.K." This is the FAKE Cliff Jack. The REAL Cliff jack is made...
www.marshallforum.com www.marshallforum.com
Click to expand...
Uh, no, as i said I solved it with preamp tubes...
 
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