Another Mesa (DC-2) problem

[goldenGeek]

I recently got this Mesa DC-2 (studio caliber) in for a repair. I've been through a lot of Mesas the last few years and I really love them when they work, but they are usually a real bi*$& to work at. So - I thought this one only had bad tubes, it had a lot of white noise and crackling and popping going on. So I ordered new tubes and installed. The crackling and popping went away but there is still way too much hiss going on. There is actually more hiss in the clean channel than in the lead channel. I have rolled tubes and put in tubes that I know work well in other amps, but no luck, of course there is a tiny difference between different tubes but only slight, as you'd expect. I have also tried to reflow the tube socket pads.

I have linked to a DropBox-folder with a sound clip and the schematic (the DC-3 has the same pre-amp) In the clip I first turn the master up, then the channel master and finally the gain and then fiddling a bit between about 7-10 which cause a bit of scratching. Reverb is off, the other controls is at 5. Then I switch to the lead channel and repeat. The scratching is not as present in this channel I think, and hiss is also more "normal" here, though I think it is a bit too much. Finally I switch back to the clean channel... the hiss is louder.

I used to own a DC-3 some years back and that one did not have this excessive hiss. I think the only difference between DC-2 and 3 is that the DC3 has the graphic EQ and it sports 4xEL84. The DC2 has 2xEL84 and has a fixed set of inductors to "simulate" the V-shape on the parametric EQ (called Contour).

Is there anyone who got a clue where to start? Chopsticking make the usual high pf ceramic capacitors pop, but that is fairly normal, right? Other than that there is nothing sticking out. The 1meg resistor right before V1A pop also, but I've tried to solder that one out without any change. The owner have modified the circuit to have a serial FX-loop instead of a parallel, I haven't closely inspected that part but it seems okay at a glance, but coud there be something there?


https://www.dropbox.com/sh/6sjkgbihlsbq ... r0Jna?dl=0

[Phil_S]

I'd pull all the preamp tubes and see if the hiss goes away. Yes? Then the hiss comes from the preamp. Install tubes one at a time starting at the one closest to the PI, then power and listen for the hiss. When you hear the hiss, then you know which section (it's in front of the tube) is producing the hiss. I'd look for a bad plate load resistor.

[goldenGeek]

I'd pull all the preamp tubes and see if the hiss goes away. Yes? Then the hiss comes from the preamp. Install tubes one at a time starting at the one closest to the PI, then power and listen for the hiss. When you hear the hiss, then you know which section (it's in front of the tube) is producing the hiss. I'd look for a bad plate load resistor.

Sorry, I forgot to mention that I tried that. If we look at the signal chain for the clean channel it uses V1a before gain, then V4b and V4a before channel master, then into V3b and finally into V6a/b for PI. Pulling V6 remove all hiss. Pulling V3 remove all hiss. Pulling V4 remove almost all hiss, and pulling V1 remove a lot of hiss, so it seems like might be building up along the path. Pulling tubes has about the same effect as turning the gain, channel master and main master in the sound clip.

[Phil_S]

If pulling V1 removes a lot of hiss, I suggest the other stages are amplifying what is produced by V1. I'd look for the problem between the input jack and V1. I'd start by changing the plate load resistor. Use a 1W resistor if it will fit.

[goldenGeek]

If pulling V1 removes a lot of hiss, I suggest the other stages are amplifying what is produced by V1. I'd look for the problem between the input jack and V1. I'd start by changing the plate load resistor. Use a 1W resistor if it will fit.

I replaced both plate load resistors on V1 (100k and 62k), but that did nothing unfortunately. For some reason I think theres more 50/100hz hum now..

[Phil_S]

I think I hear 100Hz. Bad filter caps?

[Phil_S]

I thought you said you're in Canada? That would be 60/120Hz, wouldn't it? I played 120Hz along with your clip. I vote for 120Hz.

[goldenGeek]

I thought you said you're in Canada? That would be 60/120Hz, wouldn't it? I played 120Hz along with your clip. I vote for 120Hz.

Im in norway, the power supply here is 230V/50hz.. I’m off for work a few days, I’ll dig deeper on Friday I measured the two 30uf filter caps (in circuit) and the readings were fairly normal I think. I know the hum can be caused by the filter caps, but is it likely the hiss is related..?

[goldenGeek]

So, as the my last post stated, we have 230V/50 hz so 50/100hz hum is likely. However, I have studied the schematic a bit more and I have found that V1 has a DC-supply for the heaters, and the same supply is dirctly connected to a relay - could there be something here causing excessive hiss..? I feel pretty stupid to not have seen this before, but it is kind of hidden away at the switching-matrix page in the schematics (there is also some missing diodes in the DC2 schematic, but they are pretty obvious and also visible in the DC3 schematic).

[goldenGeek]

When I was about to rip everything out of the chassis today for more troubleshooting I noticed that the potentiometers was really greasy... do I have the soution there? It seems like someone have sprayed the pots with some kind of lubricant... my hands got really sticky and it was hard to wash off. Or is this some kind og pot-lubricant thats just fine?

Edit: also there seems to be one 6800uf cap missing from the DC-supply, but installing the missing one did very little.

[goldenGeek]

Update- I got the board out, the missing 6800uf was on the bottom side But I desoldered both 6800uf and they are way out of spec, measuring about half of that and the measurement was jumping also. I had new caps around that measures within spec so I’ll install those. What concerns me more is the look at the bottom side of the pcb, there’s lots of ugly stuff on the soldering of the tube sockets See attached pic.

[sluckey]

What concerns me more is the look at the bottom side of the pcb, there’s lots of ugly stuff on the soldering of the tube sockets.

That's just solder flux. That's just flux. Not harmful. Clean up with mineral spirits and acid brush. The majority of that board was soldered using an automated system called wave solder. But the sockets were soldered by hand.

[John_G]

Hi,
Two things, with the amp chassis exposed i.e out on the bench, just your body and other devices in close proximity can induce hum.
Re noise I find most times in MB amps that after all the tubes swap outs and anode resistors changed, there is often still varying amounts of residual hiss, and usually on the clean ch. You would normally think that because on its lower gain it would be quieter.
Not the case though ! I don't remember specificly working on the DC series amps but I find when Boogie uses HUGE series grid resistors they are are usually the source of noise and sometimes the source of hum.
Try bridging the input of the clean ch series grid resistor with a 10K that is if the hiss is pre volume control.
John

[goldenGeek]

So i meased around some more today. Here’s something interesting - it was working perfectly for a little while. I have tried to replace the input 1.5M/180p paralell with a 33k, nothing. I have tried to replace the plate resistor on V1, still hissing like a swimming pool with snakes. I have tried to replace the potentiometers, no change. Then I replaced V1 for like the 100th time. Everything is fine, very little to no hiss. So I played it for some minutes and turned it off and thought “funny it did not work the last time I tried”. So I put some strips on the wiring that I had cut loose and decided to give it one more go befor putting it back in the combo. Hsssccccchhhh, like never before What the f? I then tried to wiggle and move wires, cut strips open again. No go. Does this make any sense at all? Can it be some bad transistor/relay/opto that decided to function for a while..? I have absolutely no clue. After this I once more tested to pull V1, and now There is next to no change in hiss after pulling the tube. Since most of the hiss goes away when turning down the gain, I would think it is in the tone stack... what do you recon? The gain pot is a bit scratchy, and another effect is that when turning bass/middle/treble all the way down and gain all the way up, it is pretty silent. My mind is about to implode I think, but I’m at the point that I will not leave this amp until it is fixed

Edit: there seems to be a very big voltage difference on V1 (pin 1 measured) with and without a tube installed. The schematic calls for 140v if I recall, it is about 144v with a tube in place. Without it was above 260v... I have never noticed/seen this before. Can this be a clue? I am sorry fir all this back and forth, I guess i should take three steps back and try to document and measure everything systematically...

[John_G]

Hi,
Have you swapped out the the tone stack 250pF treble cap ?
I have found that if it is a brownish colored Silver Mica cap, then they can become leaky.....I change these mica"s out on spec now days especially if it is used in a high voltage situation.(plate connection)
My further .02 cents
John