Postby 6B4G » 20 Sep 2011, 06:59
Hi djw171 - Yes I agree, after having the thing in bits today and checking all the grounds, PSU, cabling, ran the soldering iron over it in case of any dry joints etc. again changed Tubes for a known good set, my 'Little Beastie' still hasn't learnt the words and hums along. I changed the route of the cables from the output transformer slighly, cables are short so not many options, it seemed to be a little better, but subjective hearing is difficult with these sorts of things, it most certainly is nowhere near as silent as the one I listened to in the shop that is of a similar period today. I'll contact Marshall via the dealer where it was bought and follow it up. Even if I have to pay to get it sorted it will be worth it as I like this little amp, and especially after the mods (very minor) that I have done to it to tweak it to my personal tastes.
Thanks for your help.
-- Wed Oct 19, 2011 8:02 am --
Well 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 it 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. 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 since my version? New units seem to not have the hum issue as far as I can see, some older ones too. Still love this little amp, and it is much better now, lets hope it has no more problems.
This is not a modification that I would recommend doing if you have a hum problem as it's difficult to document, best thing would be to get Marshall to have a look if you are concerned.
I still highly reccomend this amp, and I'm sure the new versions in the shops now are greatly improved over my very early MK1, so don't let this put you off if you are considering one, it's a great amp.
Cheers all
-- Thu Oct 27, 2011 11:41 am --
On the subject of my Class 5 Hum and the mods performed recently, on the forum which some do not like is a post as follows that suggests a method to remove the Class 5 hum - has anyone tired this or 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).
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