Debugging a background noise issue with a Mk2B

The Boogie Board

Help Support The Boogie Board:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Joined
Jul 18, 2013
Messages
5
Reaction score
0
Hi,

I've got an '81 mk2B, there's an intermittent static background noise sounds like a bad connection or a bad line on a telephone. The noise is irregular crackling, static noise that and comes and goes but is nearly always audible. The noise can be made louder by cranking up the presence which typically I don't have set very high. When the presence is turned down the noise is harder to hear but it's definitely there.

I don't think it's the tubes the preamp and the power-amp tubes have been replaced fairly recently and the noise has persisted despite this.

Fwiw all the PSU electrolytics and the one on the preamp board have been recently replaced for preventative maintenance since the amp is 35years old and all but one of the caps was original.

The noise is there with nothing connected, even when all volume controls are at zero. I have a basic scope setup with a dummy load and I can see the noise there as well as hear it through the speaker.

I've cleaned (with deoxit) and re-tensioned the tube sockets but that doesn't appear to have helped. I've tried to find a loose connection using a plastic tool, but this hasn't yielded anything obvious as yet.

Does anyone have any advice as to what to do next to isolate, and narrow down the source of the noise - or has anyone experienced a similar problem and solved it?

Regards,

Stuart.
 
It could be some sort of grounding issue.Does your model have reverb?
 
Sometimes even new tubes can be problematic.Do you have any spares that you could try in each position to see if it eliminates the noise?
You could also try running a patch cable in the effects loop.Sometimes the jacks can become dirty and cause some problems.
 
Since you mention that the noise is there with all volumes at zero (including all masters?)...then your noise is born in the power/output section somewhere. If it is an intermittent noise like a crackling/static sound on startup and then goes away once the amp is warmed up, then your focus might be on a power dropping resistor. On a IIB these are the big 1k ohm 2w carbon composition resistors tied to the preamp electrolytics. I mention these because you said you recently had a cap job. Sometimes if there was excessive heat from a soldering iron removing the caps these resistors can develop thermal noise.

I have also had more than one standby switch go bad one early Mark series amps with an early sign being a static like noise as the switch contacts become intermittent.

Additionally, check all pots and Jacks to make sure they are snug on the control panels. As well as the ground lug on the transformer bolt.

There is a method to determining a source of noise that is fairly systematic. Isolating the power section from the preamp is helpful.
 
Since you mention that the noise is there with all volumes at zero (including all masters?)

Yep all front-panel controls on zero.

If it is an intermittent noise like a crackling/static sound on startup and then goes away once the amp is warmed up, then your focus might be on a power dropping resistor.

The noise starts when warmed up rather than it be there when cold and die away as it gets up to temperature. The noise issue was already present before the recapping too - but I suppose these could still be worth a look.

Thanks for all the tips, I'll see if I can narrow it down based on your pointers.
 
FWIW, an unusual one I had in a IIB was crap BETWEEN tracks and giving millivolts of DC into signal path.
In a Kitty Hawk M1 I had the same problem which turned out to be moisture on the PCB.
An old Fender trick is look at the PCB in total darkness - occasionally can see micro-sparks between tracks. We're talking TINY, but visible.
Simple clean with methylated spirits via cotton bud sorted it.
Heating with hair dryer can fix moisture - it fixed the Kitty. Also helps fibreboard Fender crackles if U suspect moisture.

That yours seems to disappear with heat makes me think cold joints, crap between tracks that's getting burned off, craps on sockets.. and similar stuff..

As lovetoboggie mentioned, confirm power or preamp, but before this I'd check puwer supply for ripple ( HV AND bias), and I guess a dodgy switch giving internal arcs will be easily found - wiggle it..! LOL

Backtrack from 6L6 inputs, through the PI and further towards input..
Or pull PI - does crackle go away ? If yes,CRO PI input, without tube - I'm betting noise is visible.

Again, as LTB mentioned, systematic is the key..

Let us know if pre or post PI and we can throw more ideas..]
cheers, Dave
 
Pulling the PI the noise continues.

Thanks for the pointers. I'll check out the power supply for ripple and look out for tiny sparks in darkness and report back.
 
U saved yrself a mountain of poss suspects by pulling the PI - nice !

From here I'd :
check ripple on HV, heaters AND bias.
scope the noise and see wot it looks like. In doing so you'll locate where it first appears.
Check for coupling caps leaking DC (which is usually more hum than static),
Check for noisy resistors - I've replaced many carbon comps in Fenders for this reason.

I wouldn't clean,adjust or replace anything until I found the source, proved it and resolved it.
If the source is indeed crap and gunk then so be it..

There's a few ideas, but as always, only ideas as we don't have the chassis in front of us..

Good luck.. I suspect you'll sort it easily..!! Dave
 
Turns out the culprit was a power tube after all. Despite it being rather new. Annoyingly my old set of power tubes had noise issues too so I was a little too quick to rule that out based on a similar noise being present with a new set. With step by step tube removal I found the noise moved with the tube and that helped pin it down.

Apologies to all who suggested that up front - I consider that a lesson learned to take more time to rule out the tubes in greater detail next time.
 

Latest posts

Back
Top