Marshall JCM 900 4102 ground problem

[sporkrazm]

I am not a service tech, but a have a basic understanding of my amplifier. I am hoping someone can give me good advice so I can be sure I get proper service. My Marshall has always had what I believe to be a ground issue. Whether I use it with single coil or humbucking pickups, there is always a fair amount of noise present that subsides somewhat when I touch the strings. I use it primarily in a studio where other amps don't exhibit the problem. This will occur with the guitar plugged straight into the amp with a quality cable. The problem is worse when I use most stomp boxes. Recently, I connected my guitar to a Rapco AB-100 A/B switch and connected both the Marshall and a Polytone Mini Brute II to the switcher to make some comparisons. I noticed that the noise problem with the Marshall disappeared completely. It doesn't matter whether the Polytone is on or off as long as it is plugged into an AC outlet and both are plugged into the A/B switch. I am sure this is a very basic problem that someone can answer and help point me in the right direction. I'd attempt the repair if it is simple enough, but just want to be sure that if I do take it somewhere I will be treated properly. Thanks.

[kg]

what happens to the noise if you use a piece of wire to connect the shield of the instrument cable to some metal part of the amp's chassis?

[Steve Conner]

Does the Marshall have a three-prong power cord?

[kg]

jcm900, yes... IEC connector.

edit: oh you're asking if HIS has a three prong... lol *oops*

[Steve Conner]

I didn't realise it had an IEC connector with a detachable power cord. In that case, maybe try another cord? The ground pin or ground wire in the cord could be broken.

Another possibility, the outlet could have a faulty ground. Try plugging the Marshall into the one that the Polytone is plugged into when it magically stops the whole setup from humming.

[52 Bill]

Try plugging the Marshall into the one that the Polytone is plugged into when it magically stops the whole setup from humming.

The Polytone has a ground cap and polarity switch installed. If the A/B box connects the ground of both chassis' to one side of the line with the cap it could help the Marshall stop humming as well.

[jmaf]

My 2 cents: JCM 900's use separate circuit and chassis grounds. The Marshall input ground is circuit ground, which is at some small DC potential above chassis(and Earth). Hooking the entire thing up to other amps via A/B boxes and pedalboards can be troublesome, because everyone else is likely to have circuit ground = earth(in a proper installation).





You may want to try simple tricks like this: YouTube - ‪Understanding Ground Loops‬‏

Remember if you lift any of the grounds, at least one of your equipments must be grounded for your safety.

Now question for the experts: The guitar strings must go through a 22 R resistor to reach safety Earth on JCM 900's, isn't that an improper safety practice?

[Steve Conner]

Yes, it is a safety hazard! Douglas Self mentions it in one of his books. I never liked it for that reason.

So there's another possibility: the amp is humming because the ground lift network is burnt out. To be fair, the 1N5408 diodes shown mitigate the risk, they are a lot harder to blow open than a 22 ohm resistor. If they do go they are likely to fail short, i.e. safe. That's why they are used.

Or (my money's on this) none of the outlets in the studio is grounded, and when the Polytone is used, it "grounds" out the whole setup to neutral in the bad old way, via its death cap.

[jmaf]

Good to know. I always looked for the gotcha where the Marshall compensated for the obvious security flaw...turns out there is no gotcha.

Most common scenario in Brazil is to find the safety earth connected to mains neutral at the distribution box. Since they're grounded together at the stepdown transformer, they figure it's really smart to do the same 500 meters away, 6 stories high.

[sporkrazm]

Thank you all for responding so quickly - you have made a new guy to this forum feel good. First off, the outlets in my studio are all properly grounded - I just double checked all the outlets with a GFI to confirm that nothing has mysteriously changed there. This particular Marshall is the only amp I use where I have ever had significant noise problems. It does have a detachable three-pronged power cord and I have tried swapping out the power cord with no resolve. I also have a JCM 2000 TSL that is very quiet when plugged into the same outlet. Both amps (JCM 900 and Polytone) are plugged into the same Furman power strip/conditioner. I don't usually use the Marshall and the Polytone together, but was experimenting with the A/B switcher to try to get similar tonalities so I could use the Polytone in smaller gigging situations. I was pleasantly surprised when the Marshall was relatively noise free for the first time in nearly 20 years. I have also (reluctantly) tried ground lifts and they have failed to resolve the problem. I would just get rid of this thing, but it's one of those amps that just sounds so darned good despite the noise that I just tolerate it. For what it's worth, for portabilty purposes when gigging, I swapped out the Polytone for a small Crate XT 15R and it provides the same "noise cancelling" effect as the Polytone. The Crate also has a three-pronged, hard wired power cord similar to the Polytone. A wire from the guitar cable's shield to the amp chassis did nothing. The noise sounds like very loud 60 cycle hum, it is reduced slightly when I touch the strings or the bridge of any guitar plugged into it. It is greatly reduced (almost non-existent) when connected to the A/B-switcher/second amp situation as described earlier.

[jmaf]

Wild guess here: but are your power tubes matched? Have you tried another quartet?

[Steve Conner]

Hmm. I don't think it is the tubes. How would plugging it into a Polytone suddenly make them matched?

I think we've covered all the obvious bases, it is now time to take the amp to a tech, explain the symptoms, and ask him to check that it is properly grounded. If you have or can borrow a multimeter, you can do this yourself. Plug a guitar cord into the amp, set the meter to ohms, and check the resistance between the body of the guitar plug, and the ground prong of the mains plug. According to the schematics from jmaf you should get about 20 ohms. If you get a very high resistance, there's yer problem, the ground has somehow got disconnected inside the amp, and needs fixed.

[jmaf]

You're right, I forgot that. It seems to be the case that the Marshall ground has an issue as you say, and hooking up the Polytone completes the missing ground.

The Marshall virtual ground has a capacitor (and in the grounds further ahead, diodes too) in shunt with the resistor. It's a bitch to measure resistance without unsoldering it.

[sporkrazm]

Thanks again guys. My power tubes are relatively new and all matched. I will try the multimeter test between the guitar and the amp's mains ground prong. I will also look inside to see if anything obvious has come loose. I know to be careful around the caps, even when disconnected from AC. I truly appreciate all the perspectives and advice. Once it's repaired I'll let you know what happened. It might be a few weeks. Thank you!