Marshall fixed by Hum problem, sort of, not perfect but much, much better than it was was, however not over impressed with the repair or the fix. The shop where I purchased the amp arranged for the repair, the information that came back was that some hum and some do not, it seems to be somewhat random......! Guess I had one that was far worse than others, no more information about the fix than that. I therefore had to take it apart to see what was done. The first thing I found was that the main EL84 tube was not installed correctly, the person doing the repair forgot to put the screw back in the Tube retainer and knocked it putting it back together, the journey home probably rattled it loose in the car. Then I found Pin "A" had fallen off the board with the purple wire still connected, I suspect this happened because they had been soldering around this area, it was sort of in place by the fact that it is a bit cramped where it's located, so it was being pushed on to the board, but fell off when I had a look, not good.
So what were the changes? Well it looks a bit odd to me, R2 and R4 have been removed and re-laid out under the PCB, as far as I can see the component values have not changed, just how they get connected has been changed i.e instead of the following the PCB traces they are now soldered under the PCB with Cable and Heat Shrink. Since on the circuit diagram they are marked as 'Flame Proof', I assume this is because they might get hot, being 2w and 1w resistors dropping power to the pre amp tubes, I'm a little concerned about the quality and reliability of the changes; they were flame proof and mounted off the main PC for a reason in the original design, now on my amp they are mounted on and under the PCB. We'll see how this works out long term, but a major failure could burn a hole in the PCB, so possibly not good.
Did it fix the problem? Yes it does seem to be much better now. there is still some hum, but nothing like it used to be. Interestingly, another unit in the shop that was not modified, but newer, had about the same amount of hum.
I wonder if Marshall have changed/improved the PCB layout, or made changes to the amp layout in general since my version of the MK1? Newer MK1 units seem to not have the hum issue as far as I can see, some older ones too, not listened to a MK2, but no one commented they had a problem so I assume MK2's do not suffer this problem.
On a forum that is not so popular on this forum (relating to certain Gibson Guitars), (Sorry SteveD) a poster who posted some interesting mods, but with less than Marshall friendly comments also posted a suggestion as to how to resolve the Class 5 hum - has anyone tired this or have any comments? If no replies I might give this a go and post back - here is the original persons post:-
OK, here's a photo showing how to get rid of the constant hum in the Class 5:
Rotate the OT 90 degrees. You can use one existing screw and mounting hole in the chassis. I drilled out an additional hole and used an 8x32 stainless steel machine screw and an 8x32 stainless steel nylock nut (my standard practice).