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Author Topic: Fixing Marshall Lead 20 Solid State (5002) Combo  (Read 1314 times)

Mashallmatter

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Fixing Marshall Lead 20 Solid State (5002) Combo
« on: January 27, 2018, 10:49:38 AM »
Hi,

Could someone please help :) I have fully functional Lead 20 with small hum issue.

This is only appears on when volume is between 0 and a bit over 1. Opening volume more, hum is gone. With full gain comes op amp hiss but this is not the case. Gain between 0-9 does not change the hum character. I do not have oscilloscope but I guess that the hum is 50Hz AC hum. It is solid hum without any fequency change. Bass, Mid or Treble does not affect to the hum character.

I like to also know is it normal that R7 is missing and also C7. If this is mod, can you say what kind of?

Already changed or tested:

- New power caps 2200uF/50V
- Tested also with additional 2200uF in paraller with both power caps, the hum does not change
- All small electrolytic capacitors changed
- Bad pots changed
- W005 silicon bridge rectifier changed
- Speaker wiring moved away from AC section
- All components touched multiple times, no changes on humming
- Same situation using low input, hi input or no guitar on input
- Opening or closing guitar volume does not affect to the hum amount or frequency

So what next? Op amp? Those small transistors? Power transistors? Ceramic caps? And could you please comment missing R7 and C7.

What about 50Hz vs 60Hz. This is USA model, there is 60Hz AC? In my country 230V/50Hz.

Thanks!

blackcorvo

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #1 on: January 27, 2018, 11:48:04 PM »
R7 and C7 aren't mentioned anywhere in the schematic, my guess is that they used the same board for some other amp model, and added/removed components as needed for each model.

You mentioned having changed the bad pots, have you changed the volume one as well? If not, it might be also bad. If you did change it, check for possible DC leakage on C8 (470nF). Check DC voltage between the center lug of volume pot and ground when it's at the humming position. It should be under 1v, ideally 0v. If it's above that, change C8 for a new one and try again.
Since the issue is at the volume pot and nothing before it affects how it behaves, it's either AT the volume pot or RIGTH AFTER it.

phatt

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #2 on: January 28, 2018, 03:18:09 AM »
Next you stop! The transformer is usually the source of hum as it's inside the chassis  the magnetic field radiates and your circuit picks it up.
It's unlikely you will get rid of all the hum. :'(
You could move the transformer further away or outside the case but it still may hum and could be dangerous if the mains are left exposed.
You still have the Mains Fuse on the PCB which could also leak hum into the audio.
I actually hate that idea as it puts mains on the same PCB as the audio.
They do it to save the cost of a back panel fuse holder. :trouble

Randomly replacing components in the hope you will fix it is a mugs game.  :duh
If the Amp only hums below 1 then quit before you break something. 8|

If it's a bedroom amp then hum can be annoying as you are in a confined space but if you practice with others you would never notice the hum.
Phil.

Jazz P Bass

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #3 on: January 28, 2018, 10:12:20 AM »
Have you verified that the opamp supplies are stable & that they have zero volts ac ripple?

If the volume control affects the hum then the problem lies Before the control.

Mashallmatter

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #4 on: January 29, 2018, 05:04:34 PM »
- Volume pot change did not help
- Every soldering dots, heated once, not help
- C8 ok
- C9 ok
- What about those BC184 transistors?
- I think I will test BC184’s and opamp next
- Any Lead 20 owners with inside picture? It would be nice to compare is there same missing ones..

Enzo

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #5 on: January 30, 2018, 12:19:31 AM »
Find the schematic for the Bass version of this amp, and see if those "missing" parts are on that.   I bet you missing cap is a treble rolloff, which the guitar amp version does not need.

Parts "missing" in this fashion are quite common, especially on entry level gear like this.   Probably most common is when they offer similar amps and one has reverb and the other no reverb.  They use the same board, but on the one version they simply leave out the reverb parts.

Mashallmatter

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #6 on: January 31, 2018, 10:45:25 AM »
Missing components ok, checked pictures from the Internet and compared to Bass 20.

Could someone comment BC184 transistors. There is multiple different variants available mostly with C or B etc. It's harder to get original BC184. What alternatives I can use? Is BC184L identical enough with different pin order?

http://www.mouser.com/ds/2/149/BC184-310135.pdf
http://www.mouser.com/ds/2/149/bc184l-291988.pdf
« Last Edit: January 31, 2018, 10:54:18 AM by Mashallmatter »

blackcorvo

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Re: Fixing Marshall Lead 20 Solid State (5002) Combo
« Reply #7 on: February 01, 2018, 06:45:52 PM »
Have you checked that it is in fact the BC transistors? What are the voltages on them? Do that first if you haven't checked.
If you HAVE checked and you think they do need replacing, any General Purpose NPN with similar specs will probably work fine.

 

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