bass amp hum - cheap to repair?

Paul_C

I've been given an Eden Nemesis NC250 bass combo, partly because the owner wanted rid of it and partly because they feel it is currently un-sellable due to a buzz, which I've now had a listen to and will need to get fixed if I'm going to be able to do anything with it.

The buzz is there as soon as you switch on and appears unaffected by any of the controls - even with the gain and master turned up full it still makes the same noise as when they're both all the way off. Every control does something when a bass is plugged in (though the compressor seems a little weak (and its LED doesn't work) and again the buzz is there in the background but doesn't change whatever is adjusted

I ran a patch lead across the send - return of the FX loop and plugged in some headphones - no change.

As I know nothing about repairing amplifiers I'm going to have to get someone who does to look at it, what I need to know now is probably a question that's impossible to answer - will it cost a little or a lot to track down the fault ?

ICBM

Could be a cracked solder joint on a filter cap, or a failed one. If so it's not a big job. I've seen this more than once on Eden amps (and many others).

Don't count on it though.

Paul_C

so might it be worth me re-flowing all the solder joints (I'm assuming I can at least manage that without causing any damage/ injuring myself   ) ?

jpfamps

ICBM said:

Could be a cracked solder joint on a filter cap, or a failed one. If so it's not a big job. I've seen this more than once on Eden amps (and many others).

Don't count on it though.

I concur.

That's the most likely fault, in which case it shouldn't cost too much.

ICBM

Paul_C said:

so might it be worth me re-flowing all the solder joints (I'm assuming I can at least manage that without causing any damage/ injuring myself   ) ?

Not really, just look for the cracked one. Re-flowing everything can do more harm than good if your soldering isn't top-notch - no offence intended!

It's also worth seeing why it came loose, if this is the cause - the usual reason is not being fitted tightly enough to the board so it can wobble around - in which case it *is* worth redoing the other joint first so you can get it tight down - and/or not enough hot glue to hold them firmly in place.

jpfamps

ICBM said:

Paul_C said:

so might it be worth me re-flowing all the solder joints (I'm assuming I can at least manage that without causing any damage/ injuring myself   ) ?

Not really, just look for the cracked one. Re-flowing everything can do more harm than good if your soldering isn't top-notch - no offence intended!

It's also worth seeing why it came loose, if this is the cause - the usual reason is not being fitted tightly enough to the board so it can wobble around - in which case it *is* worth redoing the other joint first so you can get it tight down - and/or not enough hot glue to hold them firmly in place.

I prefer to use silicone to secure caps rather than hot melt (or even low melt) glue as it doesn't go brittle, and is more adehesive whilst still being removable.

I hope this doesn't start a WD40 vs switch cleaner argument......

Often you'll notice one of the filter caps will be loose and wobble.

It's not a difficult repair once you've removed the PCB.

I've had a few customers bridle at the cost of "just resoldering a cap", not realizing the time spent doing this is mainly taking the amp apart and reassembling it.

Paul_C

so it's likely to be one or other, or both of those two blue beasties ?

jpfamps

Could be either.

Should be fairly easy to look at the other side of the PCB for broken solder joints, just 4 screws.

Do the caps wobble when pushed?

Note there's no glue on them to holder them in place.

ICBM

Interestingly that looks a lot better designed than the posher Edens I've worked on! Which probably explains why I haven't seen a broken one…

I agree with jpfamps that the caps should be glued to the board though - unfortunately they're arranged with the pins aligned horizontally in a line, so gluing the two of them together at the top won't create a rigid 'square' that can't wobble. I'd probably be inclined to really go over the top - desolder all four pins, and rivet them down onto the PCB before resoldering and gluing.

Paul_C

Neither of those two big capacitors appeared to move when wobbled, so I first re-flowed and then removed the solder and re-soldered (just in case ) but no joy, so I'll have to get a couple of replacements.

I need 8200µF / 80V and the ones in there (Su 'scon) don't appear to be too popular, any recommended brands/suppliers ?

ICBM

Paul_C said:

Neither of those two big capacitors appeared to move when wobbled, so I first re-flowed and then removed the solder and re-soldered (just in case ) but no joy, so I'll have to get a couple of replacements.

I need 8200µF / 80V and the ones in there (Su 'scon) don't appear to be too popular, any recommended brands/suppliers ?

Before you do, get a temporary replacement of roughly the right value (and at least the 80V rating!) and try connecting it across each one in turn - if neither is faulty, there's no point in changing them. Although hum is often a filter cap problem, there's no guarantee it is.

Be careful when doing this, they can hold a lot of charge and cause big sparks! A 1Kohm 5W resistor (or thereabouts) will come in handy...

Paul_C

the only way I'll get a replacement, temporary or otherwise, is to buy one (I can't think of another way) but I guess I could just buy the one and test with that first.

jpfamps

Paul_C said:

the only way I'll get a replacement, temporary or otherwise, is to buy one (I can't think of another way) but I guess I could just buy the one and test with that first.

If you can't find 8200uF / 80V caps, then 100V would be fine as would 10,000 uF.

Try to get 105degC rated caps as these will last longer.

Just make sure that the new caps aren't too big to fit.

Paul_C

ok, thanks

I discovered the other day that the caps have been glued (or at least, that's what it looks like) to the board, but I'll update as and when I get them replaced.

jpfamps

It's not a double-sided board so removal shouldn't be too much of a problem.

I'd apply some leaded solder to the joints and then remove the old solder with desoldering braid.

Paul_C

Old capacitors replaced with a couple of 10,000uF/100v, no difference

Paul_C

so, before I take it to my local repair guy, is there anything else that's more likely than not the cause ?

or would replacing every capacitor be worth a try ? or re-flowing all the solder joints?

I'd rather save money and fix it myself but if I'd be better off handing it over and maybe paying £50 then I could live with that, my only fear is that making it noise-free will cost me as much as it's worth (or worse).

Paul_C

Finally took it to a repair man and it turns out the noise is mechanical (the transformer windings are vibrating) which is why it didn't sound any different when turned up.

He's going to sit it on a pad in the hope of reducing some of the noise (No new transformers to be had according to Marshall, who are the distributors) and I'll see what the result is before deciding what to do next.

It does mean that it could be gigged without a problem, but it may be useless for home use.

jpfamps

Paul_C said:

Finally took it to a repair man and it turns out the noise is mechanical (the transformer windings are vibrating) which is why it didn't sound any different when turned up.

He's going to sit it on a pad in the hope of reducing some of the noise (No new transformers to be had according to Marshall, who are the distributors) and I'll see what the result is before deciding what to do next.

It does mean that it could be gigged without a problem, but it may be useless for home use.

Usually a buzz EI transformer is caused by loose laminations.

Tightening the bolts through the lams may cure the problem.

Sassafras

I know nothing about electronics but can transformers not be wax potted like pickups?

jpfamps

Sassafras said:

I know nothing about electronics but can transformers not be wax potted like pickups?

Your not allowed to use wax now, but old transformers such as those in the AC30 were.

Transformers are now usually varnish impregnated. This is more to ensure good electrical insulation.