Marshall JCM800 Lead (2205) VERY HI HUM---HELP!

[drewl]

Ohh, the green equipment, is that Bruell & Kjarr?
And the Tek scope with plug in modules! haven't seen one of those in a while!
That B&K audi stuff was awesome! Always loved working on that.

I'm having a similar hum related problem on an 18W/1987 type Marshall clone I built. I believe mine is due to noise being radiated through the chassis.

Have you tried changing the grounds on this amp? Maybe Star ground it?

[nevetslab]

Yup...the Green equipment IS Bruel & Kjaer.....1027 Sine-Random Generator, 1617 1/3-1/1 Oct Filter (patched into the 1027 so it will generate narrow band pink), 1023 Sine Generator (with Warble modulation....drives speakers directly). 2010 Hetrodyne Analyzer & 1902 Distortion Filter for the 2010 (2010 needs work). The generators in that stack are totally awesome! Tektronix 7633 Storage Scope Mainframe, 7B53A dual time base & pair of 7A18 vertical amp plug-ins. Been using the 7000 series for decades, with a collection of plug-ins and other mainframes tucked away, as well as at another shop.....loaded up onto one of their scope carts with shelves below to hold the TM-500 series gear.

When I couldn't get anywhere by moving the low noise ground wire to the chassis, it made me think the hum source was from eddy currents from one of the three transformers, but even after systemically eliminating each and still had it, it left me speechless. Yes, I did try Star Grounding, though doing it where they tied the main charging current from the HT winding's C/T into the Buss Cap's ground, added with the heater C/T and LV Gnd for running the IC1 channel switching, it was actually worse. I didn't try a variation of star ground, where you run the buss caps' ground together, with the HT C/T, Heater C/T & LV Gnds to the buss, then run a wire from that node to the chassis where the low noise preamp ground attaches to chassis. That is technically a bit different, as you're not dumping charging current into the chassis doing that...you've kept the charging current at the buss, and merely taking a reference line to chassis where the low noise ground from the preamp attaches to chassis.

Nor did I try elevating the preamp ground as Marshall and others have done with a 10 ohm resistor, diodes and caps. Reason for not doing those? The amp went out working normally, and came back with the hum problem. That is what kept me chasing my tail, with no resolve. It is one thing to revise their 'stock' grounding scheme to lower the hum, and another to find what changed overnight.

When I put it all back together and listened to it, it was still obnoxiously loud with the Boost Ch turned up all the way. I did go back in and clip out D1...that cathode resistor diode at V1B...which lowered the hum 11dB. Later that day, after writing up the invoice and sending it off to the client, and read thru some of my notes, I discovered I may have forgotten to restore a 22uF cap between the LV supply ground and the low noise ground...C10. I had to open it back up to check...sure enough forgot to return it, so added a new one, and it seemed like the hum level dropped a bit lower after doing it. I didn't check it with instrumentation.....just what my ears told me. It made no difference when I removed it, as far as the hum went. It still sounded obnoxious, so all I can hope for is the client will live with it. After 7 days of trauma from it, I'm not going back in to pursue that any further.

[drewl]

Even though those low voltage poer supply caps may seem "okay" they can often be a source of hum.
I see that alot on Carvin X100's.

You can even try raising the value of those caps.
Check out the later Marshall's with more solid state stuff inside.
They filter the hell out of those supplies.

[nevetslab]

That supply running off the heater windings was my first thought, but it wasn't the source. I had taken it completely out, after first removing IC1, finding it was ok. Had substituted the 3300uF cap with no change, added more capactance with no change, removed it all together with no change. It would have made life a lot simpler if it was the cause, for sure. Supply ripple on it was the same as on our 'quiet' JCM 800 from inventory.

[TimmyP1955]

Did you look at the grids/plates/cathodes of individual tube sections? That's how I got to the bottom of an oscillation a few days ago.

[nevetslab]

At first, no....I was looking for the obvious things, but as those avenues got dark, I went probing. Bit by bit, I removed tubes and isolated it to the first two tube stages of the Boost Channel...being V1 & V3. That eventually evolved into powering those two stages from a different power supply, and still without resolve. I only had the 'normally quiet' JCM 800 from our rental inventory over here for a couple days to make comparisons. It's all back together and now back in the hands of the client. I'll wait to hear back from them on their findings. It's better than it was, but, from my seat in the house.....a horrible excuse for a Marshall JCM 800 as it presently stands. It beat me this time.

[Jazz P Bass]

Hey, it's not like you didnt give it your all.

A kind word of advise: "Let it go".

Someday, somewhere, some amp, you will stumble onto a similar occurrance & most probably nail it.

[TimmyP1955]

Does it have a hum balance control like the 2203? I had one crap out - mighty hummy that amp was - as was one I had open today that was set at 12:00 and needed to be about 9:00 for minimum noise - a many many dB difference.

[nevetslab]

No hum balance control on it, but as I stated a few posts ago, adding one did nothing but balance the 60Hz & 120Hz components......and made NO reduction of the induced hum present. Sigh.................

[Alex R]

No hum balance control on it, but as I stated a few posts ago, adding one did nothing but balance the 60Hz & 120Hz components......and made NO reduction of the induced hum present. Sigh.................

Separating stage one grounds from stage two and later once removed such a hum for me. Star grounding is good in one way, but what you might call 'river' grounding is good in another. Mains ground is the estuary, input jack the source and the grounds follow in order between. PSU grounds at the estuary mouth.

[nevetslab]

On the JCM800 Channel Switching amp's PCB, they did NOT separate the circuit grounds of the input stage from the pots of both channels nor the succeeding stage grounds down thru V4. I'd have to look again to see if V5 stage ground was also on that long ground path. They took the series ground line to chassis by way of the shield of the Effects Loop rear panel jacks, with a wire at that those jacks grounds and tied it to the ground lug attached to one of the Reverb Drive Xfmr. V1's cathode resistors (both A & B) ran up to the input jack ground, and then flowed along the front edge of the PCB to pick up all the front panel pots that go to ground, and then near the right end of the PCB, turned around and picked up the rest of the preamp stage grounds, heading back towards V2.

It worked on our JCM 800 in our rental inventory, having made comparison to it. That one was reasonably quiet I thought it was an odd way of doing the ground path. Grounding changes on this amp didn't cure it. I haven't heard anything from the client so far. Actually hope I never see that one again. Whatever changed on it is very well hidden.....or in such plain sight and obvious I'd never find it.

[Chuck H]

I've been following just because it's such good drama. I wish I had something new to add. I don't at this time. It's starting to look like a curse (cue pipe organ). I just wanted to say that by the time this get's figured out you will be able to write a paper on the grounding specifics on this model. Probably similar to other models too. That's not a bad thing.

[marshallquest]

Hi do you check the circuit board to see if it had any open's or short's across any of the traces on it, For the Marshall JCM-800 2205 Amplifier with HI Hum. I worked with a band years ago in a night club setting were different AC outlet's were being used on stage to power different devices the guitarist keep't getting shocked badly while holding there guitars and coming into contact with the microphones. Using a volt ohm meter there was a difference of over 240 volt. So if some other devices were plugged into this amplifier and the polarity was great enough then it could have damaged the circuit board leaving some trace open somewhere allowing the signal to take a different path and causing this (Hi Hum). It's just a though because you have tried everything else with absolutely no luck or very little luck. I admire your persistent most people would have gave up after a day or so.

[Lshreding]

Not sure if you still have the amp . But did you Try snipping out C18 . There is a ton of Hum coming out of V3A and if you snip the cathode cap it drops the gain and lowering the 60hz hum in my case was lots of 60hz hum from V1B and V3A. The D1 diode makes lots of hum that should be thrown in trash and replace R3 with 1.5K to 1.7K . What to replace C18 I have not decided ,have to get my cap decade box to see if I could get some gain back whit out so much 60hz hum.

[gbono]

Sorry if I missed this but did you replace V1? heater to cathode leakage?