Marshall JCM800 earthing arrangement

[joebog1]

I was recently asked to "cure" hum in a JCM800 about 1985 vintage.
The man runs a small digital studio, and was concerned about the "bark" that this amp had.
Bark? It sounded like any other Marshall amp to me ( modern issue, anyway)
But it did forget the music !!!! HUM like mad. Some other person had replaced the main filters with Yugoslavian ( super-fi) caps. The job was OK, at least Ive seen very much worse. I looked ( for the first time) at Marshall's earthing system. Its wonderful, and one could obtain a PhD in explaining it.

There are 13 earth points scattered around the chassis. This does not include VERY long and thin PCB tracks !!. All are achieved by using plain brass screws and nuts with a small black ITL washer. This is screwed to the ( cadmium, at that date? or maybe passivated zinc chromate) plated chassis. Mains earth has three junctions before it gets to the real earth of the chassis.

I decided to make a star earth system and check results.
When I removed the chassis earth screw/nut/washer I found a delightfully green brass screw and nut, the washer being quite rusty. The rust was easily removed from the washer, but the screw and nut were a different matter.

I replaced the "main earth" screw with a modern plated steel screw, two ITL washer either side of the copper solder tag.
I then added 13 new wires from various points around the chassis, removed the origional tinned wire and connections and made a solid copper earth point, that is grounded at a single point only.

The amp now only forgets a few lyrics and rarely hums!! For musos that have noisy "modern" Marshall’s its well worth the effort.

The pics are self explanatory

best regards
Joe

[Skywave]

In photo 1, there are two capacitors on the left, connected in series, with voltage-equalizing resistors fitted. On the right, there is another pair of series-connected capacitors, but the voltage-equalizing resistors seem to be absent.

Were those resistors originally fitted but during 'maintenance' were they removed and not replaced? Is that the way this assembly was originally put together?
If the answer to either of those two questions is "yes", surely those resistors need to be added? Or have my eyes deceived me - and that they are actually present - but not clearly visible?

Originally Posted by joebog1

The amp now only forgets a few lyrics . . . .

I don't understand that remark.

Al.

Whats an ITL Washer?

[turretslug]

Could it be that the yellow wire going to the right-hand duo of capacitors is the transformer HT centre tap? (I'm assuming a solid-state bridge with HT winding centre tap, not unusual with high-power amps and high-ish HT).

It's puzzling that there was such a seemingly cavalier attitude to commoning- perhaps they got away with it when everything was new and shiny.... You'd think Marshall had been in the business long enough!

[turretslug]

Internally Toothed Lock.

[ms660]

According to one online schematic the the junction of the two reservoir capacitors on the right hand side do indeed connect to the center tap of the HT winding, the two filter capacitors on the left with the resistors across them are connected to the output side of the HT choke:

http://www.drtube.com/schematics/marshall/jcm800pw.gif

Lawrence.

[ms660]

The bias circuit in the 100 watt version is confusing me..

Lawrence.

[Skywave]

Re: post 6: thank you, Lawrence.

Al.

[Skywave]

Originally Posted by pwdrive

The bias circuit in the 100-watt version is confusing me..
Lawrence.

Indeed so: the bias supply is delivering a +ve voltage to the control grids!
Compare that to the 50-watt version: it's then quite clear that the half-wave rectifier diode in the bias supply for the 100-w. version is drawn back-to -front!

Al.

[Half a Mullard]

Originally Posted by Skywave

I don't understand that remark.

Without stepping on the OP's toes, I think it's just a touch of humour, singers tend to hum if they forget or don't know the lyrics

[vidjoman]

I've seen many wrong drawings relating to music amplifiers. Wrong way round diodes are common, wrongly connected/drawn transistors were very common as with capacitors. I suspect that the firm didn't employ a technically qualified draughtsman - why should they as he/she would only produce a few drawings a year. I recall a VOX mixer where some capacitors were wrongly connected in the units which led to high noise and low gain. They had copied the basic circuit from a poorly drawn Japanese unit back in the early 60's.

[Skywave]

Originally Posted by vidjoman

I've seen many wrong drawings relating to music amplifiers. I suspect that the firm didn't employ a technically-qualified draughtsman - why should they as he/she would only produce a few drawings a year?

Yes, why should they indeed? However, the drawing was produced by someone - and in the absence of a draughtsperson, the most likely candidate would have been an electronics technician. In many years of employment, in many different organisations, I held that type of post and often I was required to produce hand-drawn circuit diagrams. Before they were released into the 'wild', they were always checked by a competent colleague - who sometimes was my technically-qualified manager, so I had to be quite sure that I hadn't made any silly drawing errors.

Looking at the sheet with the cct. diagram thereon, I notice that in the right-hand lower corner there is the usual 'designed by, approved by, checked by' box - but only one signature appears - presumably the person who produced it. Hence, the drawing was never approved nor checked. Now refer to the above remarks about the 'earthing arrangements' and you are led to the conclusion that QC was not this firm's strong point.

Al.

[Skywave]

Originally Posted by joebog1

The amp now only forgets a few lyrics . . . .

Originally Posted by Skywave

I don't understand that remark.

Originally Posted by Half a Mullard

Without stepping on the OP's toes, I think it's just a touch of humour, singers tend to hum if they forget or don't know the lyrics

O.K.; thanks. I'll put my puzzlement down to a 'cultural thing'.
But I still don't understand what 'fault' symptom (if, indeed, there is one) that that remark is meant to describe.

Al.

[joebog1]

My apologies to Skywave!
It is my poor sense of humour

I have always believed that laughter is the best medicine

Joe

[McMurdo]

Corroded or loose earth tags and their screws is a common reason for vintage marshalls to hum...usually intermittently. Sometimes the tags are mounted along with other chassis parts eg capacitor clips and valve holders. A good tighten and a dab of varnish usually holds them and is a good thing to check on any repair even if it's not humming just yet.

[turretslug]

As I mentioned above, it's odd that this should be a significant issue- I wouldn't expect a stage PA amp to have the near-immaculately hum-free performance that some of the "brave new" (and very expensive) valve hifi power amps of the last couple of decades or so achieve but it really ought to be pretty good out of the box- and stay that way. It would only take probably several additional inches, at the most a couple of feet of sensibly-gauged internal wiring to make earthing detail closer to that which has been recognised as good practice for many decades, and prudent assembly planning and practice would make for pretty much negligible additional production time and effort.

In a simple table radio or similar of a few watts output and modest gain involving a lowish HT current, one might be able to get away with circuit 0V returns at the nearest chassis tag or valveholder screw (regarding the chassis and its connections as a perfect and undegrading sea of zero resistance and impedance) but doing this with a high-power, high HT current PA is asking for trouble, or at least disappointment. Perhaps there's a certain reassurance in a gentle audible quiescent hum (something that I've been aware of in band rigs over the years!) and, if it really is gentle, it'll certainly be lost when the wick goes towards 11.

Still puzzlingly slack practice, though, and capable of easy resolution if applied at the outset.

[Skywave]

Originally Posted by joebog1

My apologies to Skywave!

Apology not needed, Joe: just a simple misunderstanding, that's all; nothing wrong with your S-O-H. But I do agree with the sentiment of your last line above, though.

Al.

[joebog1]

hhmmmm it wasnt "gentle audible quiescent hum " more like several watts, and it didnt disappear!!! At moderate levels it worked as a modulator!!! at full tilt it couldnt be discerned due to blood pouring from the ears.
The "style" of playing used by the owner however could have been used the hum as the latest stomp box!!! Would fall into the "offtune death thrash metal" category !!

Joe