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Author Topic: Marshall 5210 problem. Loud hum.  (Read 6769 times)

custimguitarman

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Marshall 5210 problem. Loud hum.
« on: January 06, 2015, 06:12:01 AM »
Hi all, new to the forum.
Just picked up a 5210 from 88 and it plays fine for a few minutes then goes into this really loud 60 cycle hum. Its like you have everything cranked all the way up with a single coil plugged in. This can be turned down with the master v. But not the preamp volume. Also cord plugged in or not makes no difference.
To me it seems like the preamp is taking off on its own generating signal or a transistor has latched full on.
Any ideas?

J M Fahey

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Re: Marshall 5210 problem. Loud hum.
« Reply #1 on: January 06, 2015, 10:13:50 AM »
Somebody please post the schematic or a link :)

g1

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Re: Marshall 5210 problem. Loud hum.
« Reply #2 on: January 06, 2015, 11:24:32 AM »

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #3 on: January 07, 2015, 02:47:40 AM »
Hi,
Yes I got the schematics before I posted here. I need a little help deciphering.

Roly

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Re: Marshall 5210 problem. Loud hum.
« Reply #4 on: January 07, 2015, 03:53:32 AM »
Welcome custimguitarman.
 

Quote from: custimguitarman
This can be turned down with the master v. But not the preamp volume.

Which places the problem in the preamp before the Master, but after the Gain control.


{looks at cct, iss5 is effectively unreadable, so I'll go with iss3}

Oh yeah, this is the one with the oddball grounding contact on the input that goes deep into the preamp, and has previously been the cause of strange noises.  This grounds the preamp output when the input lead is unplugged, rather then the more conventional arrangement of grounding the preamp input.

First thing to always check is the power supply - are the +/-15V rails correct and stable when the fault is apparent?

Are any of the IC's in the preamp getting warmer/hotter than others?

Does the Channel indicator LED do anything odd?

Do you have the footswitch plugged in?
(the only thing that has a direct connection to outside, and could potentially pick up and inject hum between the preamp Gain and Master controls, is the channel switching.)


If the IC's are in sockets, try pressing them down/home into their sockets.

Assume you have a multimeter, but do you have access to a CRO?

But first, confirm the preamp supply rails under fault conditions.

If you say theory and practice don't agree you haven't applied enough theory.

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #5 on: January 07, 2015, 07:35:43 AM »
I do have a good fluke meter but no access to a cro. I will play with it tonight. Looks like some one recently replaced the power transformer. Possibly a misdiag.

Roly

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Re: Marshall 5210 problem. Loud hum.
« Reply #6 on: January 08, 2015, 11:22:09 PM »
Quote from: custimguitarman
it plays fine for a few minutes then goes ...

This generally suggests some sort of thermal association, something heating up and malfunctioning, but do the preamp supply tests and we'll see what shows up.

Quote from: custimguitarman
Looks like some one recently replaced the power transformer. Possibly a misdiag.

That's not good because trannies rarely play up, but if the fault was misdiagnosed it doesn't make us confident that a suitable replacement was fitted.  If the supply voltage for the preamp was low, for example, it could mean that the zener regulators don't have enough headroom and are dropping out of regulation and allowing hum/ripple to get though to the preamp.  It's a bit of a long shot because op-amps generally have a very high rejection of supply noise, but if the +/-15V supplies are nice and steady (say within 0.5V of nominal) and not sagging when the fault appears I think we can eliminate that possibility, however if one or both is sagging...

I normally disparage "replace all the caps", but this is a fault were it is possible that the supply electrolytics are on the way out.  Do you have any idea how old this amp is?

Anyway, note my other questions...
Quote from: Roly
Are any of the IC's in the preamp getting warmer/hotter than others?

Does the Channel indicator LED do anything odd?

Do you have the footswitch plugged in?

Post results and we'll see how we go from there.  :tu:


A CRO is very helpful because you can see what is going on (such as hum on the supply) which is a bit more complicated otherwise.  Luv my CRO's.   <3)

To measure hum on a DC rail you will need to use a cap in series with your meter probe, say around 0.1uF but it's not at all critical, while set on a low AC Volts range.  A Fluke may not be worried by DC with AC, but most multimeters are and will give wildly high AC readings.

You can use a powered computer speaker as a signal tracer in the same manner, a cap in series with the signal probe, then you can at least hear what is going on.  {In Ye Olde Days when CRO's were expensive,  rare, and not very capable audio signal tracers were in every workshop, a poor mans substitute.  A front-loader cassette deck can also make an improvised signal tracer, input to the Rec In, set to Record, headphones to monitor (and you normally have meters as well).  Again, a DC blocking cap is a good idea.}
If you say theory and practice don't agree you haven't applied enough theory.

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #7 on: January 09, 2015, 08:11:36 PM »
Ok.
Last night I had the amp unscrewed from the cabinet and flip upside down so i could see all the piecey parts. I am reading 20v ac out on the transformer. My initial guess is that's too high. No markings to identify. I then put the meter on dc and touched my leads across the supply caps and got 27v?!
 The amp actually behaved well and I played through it in this configuration for maybe half an hour. I had won a footswitch on ebay which was why I delayed the test. The hi gain channel switches in and out fine. The reverb was a little less exciting than I had anticipated considering it is a spring type. Was barely noticable.

I do seem to be getting some distortion very easily even on the clean channel. I now wonder if that is due to the high supply voltage?

JHow

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Re: Marshall 5210 problem. Loud hum.
« Reply #8 on: January 09, 2015, 10:59:18 PM »
When you say supply caps, do you mean c46 and c47?  The ones next to the zeners?

Roly

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Re: Marshall 5210 problem. Loud hum.
« Reply #9 on: January 10, 2015, 02:43:21 AM »
Quote from: JHow
When you say supply caps, do you mean c46 and c47?  The ones next to the zeners?

No.  Those are to bypass the +/-ve supply rails to ground for AC, not for hum filtering.  When working correctly the voltage across them should be the zener voltage only and have little or no AC component.

The caps that would have an effect on the ripple voltage going into the zener regulators are the main filter caps C26 and C27 (4700uF).  If these have lost capacity or got high ESR due to age, or one or more of their solder connections has fatigued and cracked, that could cause excessive hum.



Quote from: custimguitarman
I am reading 20v ac out on the transformer.
...
across the supply caps and got 27v?!


20VACRMS x root(2) = peak voltage

20 * 1.414 = 28.28 volts

So 27VDC under idle load is quite reasonable. 

A zener is like a dam spillway (where the voltages on C46 and C47 are the dam levels for the +ve and -ve preamp supplies).  Up to 15 volts these zeners won't conduct at all, but as the voltage comes above 15 volts they will turn on and try to pass as much current as is required to pull the voltage down to 15V, which is why they have to have supply side resistors to "work against" or they would fry themselves trying to crowbar the 27V supply down to 15V.  The difference is dropped across the power resistors R59 and R60 (220 ohm, power), which will normally get quite warm/hot as a result.

The raw DC supply coming in from C26 and C27 is pretty basic and so has a fair bit of hum ripple on it, but zeners are pretty good regulators and do a fair job of eliminating this hum ripple on the preamp supplies.

Quote from: custimguitarman
I do seem to be getting some distortion very easily even on the clean channel. I now wonder if that is due to the high supply voltage?

It isn't high, so no.


Now if you want to make linear progress rather than randomly jumping about it goes like this; we ask questions, and you answer them specifically...

Quote from: Roly
First thing to always check is the power supply - are the +/-15V rails correct and stable when the fault is apparent?

Are any of the IC's in the preamp getting warmer/hotter than others?

Does the Channel indicator LED do anything odd?

To which I will add, carefully check that the solder joints for the main filter electros, C26/7 (4700uF), are intact and not cracked (bright light and lens).

Post these findings.
If you say theory and practice don't agree you haven't applied enough theory.

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #10 on: January 26, 2019, 06:25:09 PM »
Very sorry to the group for not ever responding.
I indeed found a bad solder joint at the c27 cap. soldered and amp worked fine.
I stored the amp and never used it until 2 weeks ago. now I have a loud hum when I turn up the treble on the normal channeThe schematic is hard for me to fallow but I am working at it.

otherwise everything works.

solderer25

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Re: Marshall 5210 problem. Loud hum.
« Reply #11 on: January 28, 2019, 12:03:09 PM »
If an amp has been left in storage unused for a long time and gives problems when fired up again I always spray all moving parts with contact cleaner spray. So that means all jack sockets as well as switches and the control pots (e.g. gain, volume, tone controls). Damp air gets in these parts and oxidises the contacts - the resulting pops and crackles (and also hum) are a sure sign of this. If this does not cure the problem then we have to delve deeper.

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #12 on: February 23, 2019, 12:34:35 PM »
Thank you for the help.
I sprayed some deoxit in the pots and Jack's. Seemed to help some but what I notice most is the constant hum goes away when I touch any of the pots. Sounds to me like a ground issue? Bad filter cap?

phatt

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Re: Marshall 5210 problem. Loud hum.
« Reply #13 on: February 23, 2019, 11:55:57 PM »
If the pot covers are not grounded to chassis then yes you will get hum.
The tone controls are high Z type and as such are highly prone to pickup hum.

You may find that there is corrosion between the chassis and pot mounting holes and that can cause the pot covers to loose contact with chassis.

You can solder a jumper wire across the back of each pot case and then ground it back to chassis. Don't ground the pot cases back to *Circuit Ground* as that may cause other problems, you want to ground those pot cases to *Chassis*
Just by undoing the pot nuts and re-tightening them will likely scrape off any corrosion and return the pot case to chassis ground.
Phil.

custimguitarman

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Re: Marshall 5210 problem. Loud hum.
« Reply #14 on: February 24, 2019, 03:17:05 PM »
Well, I feel pretty stupid.
Turns out when I had the board out to repair the bad solder joint at the one filter cap I forgot to reinstall the screw for the ground on the board.
Forehead slap.
Works now

 

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