Marshall Mosfet LEAD 3210 blowing power fuses - help locating capacitor(s) - diyAudio
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Old 10th August 2009, 05:18 PM   #1
jmilch is offline jmilch  United States
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Default Marshall Mosfet LEAD 3210 blowing power fuses - help locating capacitor(s)

Greetings-

I inherited a 1985 Marshall Mosfet 100W LEAD solid state head. It was immediately blowing fuses when powered on, so I did some reading.

I tested the (2) output transistors to the best of my abilities and found one to be weak and or faulty. So I replaced them both as a matched set, as I was instructed to.

It still continued to blow fuses. I rechecked my installation and verified the thermal grease was applied, and re checked the functionality of the mosfets, they were installed and working properly.

My attention was then turned to the large power supply capacitors next to the transformer. I was told by the previous owner that one of them may have been faulty. I tested one for capacitance and it held no charge, so I am assuming they both should be replaced.

My problem is, I cannot seem to find the equivalent capacitor, or something sufficient to use in its place.

The rating on the capacitor reads:

"Capacitor Technology"
FA0 45002 85*c
20000 mfd 60 WVDC
FA0203G060RF1H
8927 460 4021

The way I understood, I could replace the capacitor with one of a higher voltage value, and a slightly higher capacitance value without effecting the power supply. Is this true?

Also, can anyone point me in the direction of where to buy these capacitors? Ebay would be fine if need be, or a parts store online.

They are relatively large, but I dont necessarily have to have them fit back into the chassis, they can be relocated externally in the head if need be, since I cannot seem to find the exact same capacitor anywhere online.

Any and all help will be greatly appreciated.
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Old 10th August 2009, 08:45 PM   #2
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Default Re: Marshall Mosfet LEAD 3210 blowing power fuses - help locating capacitor(s)

Quote:
Originally posted by jmilch
Greetings-

I inherited a 1985 Marshall Mosfet 100W LEAD solid state head. It was immediately blowing fuses when powered on, so I did some reading.

I tested the (2) output transistors to the best of my abilities and found one to be weak and or faulty. So I replaced them both as a matched set, as I was instructed to.

It still continued to blow fuses. I rechecked my installation and verified the thermal grease was applied, and re checked the functionality of the mosfets, they were installed and working properly.

My attention was then turned to the large power supply capacitors next to the transformer. I was told by the previous owner that one of them may have been faulty. I tested one for capacitance and it held no charge, so I am assuming they both should be replaced.

My problem is, I cannot seem to find the equivalent capacitor, or something sufficient to use in its place.

The rating on the capacitor reads:

"Capacitor Technology"
FA0 45002 85*c
20000 mfd 60 WVDC
FA0203G060RF1H
8927 460 4021

The way I understood, I could replace the capacitor with one of a higher voltage value, and a slightly higher capacitance value without effecting the power supply. Is this true?

Also, can anyone point me in the direction of where to buy these capacitors? Ebay would be fine if need be, or a parts store online.

They are relatively large, but I dont necessarily have to have them fit back into the chassis, they can be relocated externally in the head if need be, since I cannot seem to find the exact same capacitor anywhere online.

Any and all help will be greatly appreciated.

If the fuses are being blown then its likely to be a short circuit somewhere.
Usual places are transformer, bridge rectifier, smoothing caps and output transistors.
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Old 10th August 2009, 11:57 PM   #3
Enzo is offline Enzo  United States
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Remove the suspect cap, and test it out of circuit. While still in circuit, a low resistance path across it would prevent it from holding a charge.

Check your main rectifiers before chasing down caps.

Digikey shows a 22000uf 63v cap stock # P10029-ND

When searching, I think 22000 would be a more common value than 20000, and 63v is more common these days than 60v if you make your search too specific, you miss possibilities.
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Old 11th August 2009, 01:06 AM   #4
jmilch is offline jmilch  United States
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Thanks for the responses guys.

I removed the cap in question, and for poops and giggles I simply connected the leads together with the other cap. Now I know this isnt going to work for functionality, but I thought I could flip the switch and see if it elimiated the blowing of the fuses. It did.

With the cap removed and the leads left open, the fuse blew. With the leads attached to the other leads on the remaining identical cap next to it, the fuse does not blow, but the transformer gets very warm in about 5 seconds. I know this isnt good for the amp, but I think that narrows down the problem to either the capacitors correct?

If the transformer was bad, it would still blow the fuse regardless. So I am guessing without having too much more testing equipment, that the previous owner was correct and the caps need to be replaced.

What do these caps do for the amp? Sorry for the newbie question.

Also, can someone send me a link or point me out directions on how to test the transformer for functionality? I think the issue is isolated to one or both or the caps, I hope atleast.

Thanks for any and all help, I appreciate it very much!
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Old 11th August 2009, 05:14 AM   #5
jmilch is offline jmilch  United States
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I dont know if I specified the correct capacitance earlier, but the cap is quite large and reads 20000 MFD,

Its along the lines of this

http://cgi.ebay.com/Mallory-Capacito...3286.m20.l1116

Only that one is rated 40VDC. So its about the size of a can of soda.
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Old 11th August 2009, 05:07 PM   #6
jmilch is offline jmilch  United States
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Ok, can anyone assist me in finding a decent replacement capacitor? The one linked was the wrong capacitance (my fault the way I wrote it)

So I need to find the equivalent or better for a 20000 MFD 60WVDC capacitor. It has the two top mounted screw terminals similiar to this style

Click the image to open in full size.

Any and all help would be greatly appreciated, thanks for all the responses so far
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Old 11th August 2009, 08:58 PM   #7
jaycee is offline jaycee  United Kingdom
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Any such type of cap rated for 63V operation will work. 2200uF will work, but you could also put larger such as 4700uF without trouble. Replace both of them. The screw terminal caps are often called "computer grade"
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Old 11th August 2009, 11:13 PM   #8
jmilch is offline jmilch  United States
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thanks a ton for the response, I am looking for a replacement now. I was fooled by the size, some of the caps I am looking at are a bit smaller, so it seemed weird they had a larger capacitance.

But I will trust what the cap says, and order some.

I am hoping thats the issue, considering what I described earlier.

Does anyone else have any suggestions or ideas?
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Old 12th August 2009, 12:59 AM   #9
jmilch is offline jmilch  United States
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How do these capacitors look?

23000uf, 75VDC, computer grade.

I just want to doulbe check before I order a couple. Thanks again
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Old 12th August 2009, 08:45 AM   #10
Tarzan is offline Tarzan  Belgium
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Hi JMilch,

First of all; Wich Marschall is it?
3005, 3310, 3505, 3510, 3520, 3530, 3540, 3550, 3560?????
There is a model number on it to identify it.
As Marschall made severall Fet Amps.

Then; Why would you suspect the caps?
First there is the transformer, then the bridge rectifier, then the caps, then the output fets (as stated by nigelwright)

Start all over again.

1. Label and disconnect the secundary wires from the transformer and isolate them.
Switch on the power. Is the fuse blowing? Then there is a problem around the transformer; wiring or the tranny it self.
All went OK?

2. Remove the rectifier bridge or diodes. Make notes of the direction they were placed on the pcb.
Measure them with your ohm meter. Info on the net -> google
All OK?

3. Remove the caps from the board.
Here again take notes and see how they have been mounted and/or wired.
Test them with an ohm meter.
No shorts?

4. Leave them out of the circuit and measure for shorts on the plus and minus supply of the pcb (amp) versus ground.
There must be a short...

Search further to begin with the output Fets.

Leave feedback of your progress.
Good luck,
Tarzan
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