Vox AC30HWH noisy top boost section

[defcon.klaxon]

Hi guys,

I have a Vox AC30HWH (Vietnam built) on my bench, and I'm trying to track down the cause of a very noisy Top Boost section.  The Normal channel is actually very quiet, even when the volume is turned up 100%.  A little background hiss, but pretty much what I'd expect.  The Top Boost section comparatively is VERY noisy, there's a very loud pervasive hum that steadily increases as the volume is turned up.  I did some chopstick poking around and found something really interesting.  None of the components on the turret board seemed to be unduly noisy, but if I rap on the turret board itself, every once in a while the noise will go away entirely, then it comes back.  I also found that as I chopstick poke the leads from the Top Boost pots, sometimes the noise fades away.  But of course I don't remember which wire did it, and now I can't get it to do it again.  Also, strangely, when I went to rotate the chassis to take a look at the bottom of the tube sockets, the noise went away when the chassis started flexing.

To me, this sounds like it could be an intermittent grounding issue.  I did inspect and re-seat the signal star-ground near the speaker output jacks, but that didn't seem to have any affect.  In fact, after I re-secured those I can't get the noise to go away with the chopstick tapping like I used to. 

I'm not super familiar with AC30 circuitry, plus it's been difficult to find the exact schematic that this amp seems to have; many come close in one aspect or another but the only HWH schematic that I've found is back from 2002 or so, obviously a different amp since the components don't match.  I've asked Korg for a copy, but I haven't heard back yet.

Wondering if anyone else has had issues similar to this, and/or what you'd suggest checking for while troubleshooting?  The first place I can find the noise (via oscilloscope) is on the plates on the phase inverter, if that helps.

Thanks for any help!

[st]

If it's in one channel only, and consistently, it's not the phase inverter. Have you considered the possibility that it's a tube (socket) or switching jack? Just to rule out the simple things first, clean the preamp tube sockets and temporarily replace/research the tubes. Any changes? And does this noise happen with an instrument plugged in? If not, clean the switches on the input jacks and make sure they have good contact. Press on them with the amp on (chopstick), any change?

[defcon.klaxon]

Hi st, thanks for the reply.

I worked on the amp last night and have the following to add:

1.  I found a cold solder joint in the Top Boost input jack area; when nothing was plugged into either jack, resistance to ground was 1M.  I fixed this, and this changed the sound of the noise but did not fix it.
2.  I have moved tubes around, haven't noticed any change.  I've put one of my gold standards, an old Telefunken ECC83 in various locations and the noise is still there and quite loud.  I haven't tried cleaning the sockets yet, I do have some Deoxit which I can try.
3.  Just for grins, I replaced the plate resistors (which were carbon comp) with new carbon film on the phase inverter (which did nothing, which isn't surprising) as well as the first tube sections for each preamp section.  Again, no notable change.
4.  I ran new shielded cable from the volume pot of the Top Boost channel to the section triode section.  Strangely, as this wire moves around the level of noise DOES change.
5.  I did note that as I turn the treble up in the Top Boost section, the noise decreases notably; not to the point of acceptable, but several dB down.  Bass adjustment has zero effect.  Noise is much louder in "Hot" mode, which according to the Vox literature bypasses the tone stack (but not entirely); in "Cool" mode, which is stock Top Boost tone stack, the noise is reduced several dB but I wouldn't say to acceptable levels.
6.  I'll mess around with an instrument plugged in today and report back what I find.  Will also short an input jack and see what happens (I have a shorted jack *somewhere* but couldn't find it last night).

[pompeiisneaks]

The fact that flexing the board changes sound, and finding the solder connection problem, seems to imply to me that it's a solder connection somewhere that's not making great connectivity.  I'd try tapping on the wires coming to/from etc.  Do you have a scope to trace where the noise is arriving?  If not try pulling one tube at a time in the flow to see which is the one that is causing the noise.  From this schematic (if it's the right one) https://el34world.com/charts/Schematics/files/Vox/Manual_vox_ac30h2_ac30h2l.pdf it seems to show V2 and V3 are the tubes in the chain, try V2 first, if that doesn't change the tone replace and put it back and pull V3.  If that does it, you know which tube components are related to the problem, and narrows down what you'll need to try and fix.

~Phil

[defcon.klaxon]

Hi Phil,

Thanks for the suggestions.  The tapping and noise elimination has been very sporadic, and not easily repeatable.  Pulling tubes does help a bit, it's definitely V1 or V2 but Top boost is using the second triode of V1 and both of V2, so it still leaves a fair bit of circuit to go through (Normal channel uses first triode of V1 and then goes directly to phase inverter).

Korg won't release the schematic to me since I'm not an authorized dealer, and the schematic you linked to is similar but not quite the same (the amp on my bench isn't the EF86 version, and the tone stack doesn't match...the tone stack seems closer to this schematichttp://www.mediafire.com/?147chgaw792n6gl, which is the old version of the AC30HW from 2001 (though the one on my bench is from 2008 or so, doesn't have reverb/tremolo, and many other circuit components don't match and some topology doesn't match either; seems like this 2008 HW version is a weird hodge podge of various previous circuits).

I do indeed have a scope, and so far the first place I can find the noise is on the plates of the phase inverter, but I don't think that's where the noise is originating.  What I'm going to have to do is bite the bullet, and draw out the schematic manually and then I think I'll have better luck searching for the noise source.  I was hoping I could find it but it's just not showing up, so I'll go through the motion and maybe it'll help someone else out in the future.

Thanks for the help, definitely sounds like a bad solder joint/bad ground connection, somewhere, that's intermittent and not easily isolated.  I have a feeling that when I finally do find it, it's going to be a major victory.

[pompeiisneaks]

If the noise appears on the plates of the PI, and isn't on the output of the previous stage, that means it has to somehow arrive from the PI, it could be 'sourced' in the other areas though.  It has to appear somewhere before the PI otherwise or your issue is with the PI.

~Phil

[Ritchie200]

Did you happen to clean/re-tension the grounding contacts on the top boost input jacks?


Jim

[pdf64]

That the noise calms down when the treble control as turned down may be significant (does the 'tone cut' have the same effect?).
These things have a lot of total system gain available, with little to no stability mitigation; regarding the noise first seeming to appear at the LTP phase splitter, bear in mind that as there's no NFB loop, there's consequently a lot more gain than normal there. So anything untoward that you can notice at the LTP plates will be at least ~30 times smaller earlier in the circuit. Hence it might be difficult to zoom in on.

On the same theme, maybe the noises are artifacts of some sort of oscillation; bear in mind that an easily identifiable, free running howl/squeal is just a subset of many possible ways that might manifest.
So I suggest to try rolling off the preamp HF response a bit and review, eg tack ~220pF caps across R5 and R15.

[st]

That the noise calms down when the treble control as turned down may be significant (does the 'tone cut' have the same effect?).

Noise goes down with treble UP = grounding issue.