Troubleshooting Noise from Crate Solid State Amp (Power amp section?)

[WSKCONDOR]

(location is Southern California, normal US line voltage)
I have been searching various sites looking for troubleshooting expertise on noise my (relatively new) Crate CA6110D (Gunnison) Amp is producing. I did find several informative posts, and so I am providing where I have gotten in troubleshooting as well as a sound clip of this noise. I know that it is not "normal" as it did not make this noise when new, and it is only about 3 years old. First, What I believe I have narrowed it to / information on the situation:

-noise seems related to main power supply (i.e. 60Hz or 120 Hz -listen to sound file attached. I turn off the power mid sound bite so you can hear the power up and noise when I turn it back on)

-Amplifier is "working normally" in all functions outside of this hum. i.e. the inputs work with mic or guitar, and if you are relatively loud, you can't hear the hum because it does not increase in volume with the rest of the program. It is at low levels or between playing/singing that the hum is noticeable.

-I do not believe it is in the preamp section(s), due to the following troubleshooting: 1) noise is present and same intensity regardless of whether anything is plugged in to any input or not. 2) noise is present and same intensity regardless of gain levels of any channels or main volume setting. 3) with a guitar plugged in, I took effects send output into a separate amp and the hum is not present (just normal guitar noise). I also did the same with the overall line output from the back, and again, noise hiss levels are normal, but this oscillating hum not present - this seems to point to the Power amp section, since a clean signal can be achieved past the preamp and all the internal effects sections.

-Finally, and this is a weird one, I "think" I made the noise go away briefly. I opened the case and just pushed on/reseated connectors as a first troubleshooting effort since I have done that with a different SS amp and it actually was the issue. Then when I turned the power on, the hum was gone but then I bumped the amp, pulling the plug out-when I plugged it back in and turned it on, the hum was present again and continues to be present. This made me think that perhaps one component was simply "loose". I disassembled the amp to the point of taking the power amp out completely and examined both sides of the board-but I see no indications of any loose or broken solders or traces and no inactions of anything burned or otherwise damaged.

QUESTIONS:

1) Any guess as to the cause of the noise?
2) If it is a diagnosis of a filter capacitor, what capacitor(s) are likely the culprit(s)? (schematic attached).
3) If it is a filter capacitor(s) is there any way to test in place, or do I just need to remove and replace with new ones and then re-test? (all caps are pretty well glued around with silicone of some kind, but I can cut that away if needed)
4) If it is the power transformer, wouldn't that noise be present in the pre-amp output or line output as well as the main speaker output?

Any help or expertise is welcome. No need for cautions about voltages, please, I understand that part of it and won't electrocute myself. I do not have extensive equipment, just a pretty good multimeter, soldering capability, and other basic tools-but no oscilloscope.

V/R,
Bill

[nickb]

This is definitely the kind of thing that could be caused by a bad ground connection somewhere, that doesn't mean that it is in this case. Unplug J19 and see what happens to the noise.

[52 Bill]

Welcome to the place.

If the noise seems to be mechanically started and stopped, the loose part or bad solder joint is a good place to start. Not all bad solder joints are visible to the naked eye.

My first thought would be the two large filter caps C6 and C7 on the schematic. If you push on them, can you start and stop the hum?

[WSKCONDOR]

This is definitely the kind of thing that could be caused by a bad ground connection somewhere, that doesn't mean that it is in this case. Unplug J19 and see what happens to the noise.

I did try unplugging J19. Problem is, there is a power cutoff of some kind in that connector (or power is supplied to the power amp) and when/if you unplug J-19 (which looked like it was just signals going from pre-to power amp, but really it also has some voltage)-the power amp turns off completely. I would have to sort and cut out just signal inputs within the J-19 connector to test this further, and I wanted to avoid that.

Seems like if it was a ground problem...it would be some conductor broken somewhere wouldn't it?, and I can't see any evidence of that. This is a pretty new amp and didn't get tossed around much. And it was working fine originally..

[WSKCONDOR]

I did push on those two large caps (C6 and C7). I guess because....they were the largest ones on the board. But there was no change in the hum. I tried "wiggling" them as well as other caps and did not change the hum.

Seems like what I am hearing is that there is no good way to test capacitors while they are soldered into a board? Is that a fair statement?

[52 Bill]

I did push on those two large caps (C6 and C7). I guess because....they were the largest ones on the board. But there was no change in the hum. I tried "wiggling" them as well as other caps and did not change the hum.

Seems like what I am hearing is that there is no good way to test capacitors while they are soldered into a board? Is that a fair statement?

Have you tried tapping or pushing on different parts on the board? Often plug terminals break loose from the board. And check for loose connectors all through the amp.

Usually, cap meters will work with caps in circuit.

[nickb]

I did try unplugging J19. Problem is, there is a power cutoff of some kind in that connector (or power is supplied to the power amp) and when/if you unplug J-19 (which looked like it was just signals going from pre-to power amp, but really it also has some voltage)-the power amp turns off completely. I would have to sort and cut out just signal inputs within the J-19 connector to test this further, and I wanted to avoid that.

Seems like if it was a ground problem...it would be some conductor broken somewhere wouldn't it?, and I can't see any evidence of that. This is a pretty new amp and didn't get tossed around much. And it was working fine originally..

You will need to short the MUTE pin to -13V on J19 to allow the power amp to work.

Even unwanted tiny resistances in ground paths can lead to noise, for example a connector pin in the right spot that is a little bit dirty will do it. To me it sounds like a mixture of hum and something digital and this model does have a DSP. I would clean J19 - both ends if appropriate. Is the DSP on a sub board ? Check/clean it's connections. After that I'd start looking at the filter caps on he preamp.

Can you post the entire schematic please?

[WSKCONDOR]

I kind of fiddled with the whole board as well as the other sub-boards. Tapping and generally seeing if anything was loose-and could not get a change.
I think next I need to test the caps.

[WSKCONDOR]

You will need to short the MUTE pin to -13V on J19 to allow the power amp to work.

Even unwanted tiny resistances in ground paths can lead to noise, for example a connector pin in the right spot that is a little bit dirty will do it. To me it sounds like a mixture of hum and something digital and this model does have a DSP. I would clean J19 - both ends if appropriate. Is the DSP on a sub board ? Check/clean it's connections. After that I'd start looking at the filter caps on he preamp.

Can you post the entire schematic please?

I will have to try shorting tag mute pin.

One thing on the rest of the theory-the DSP works prior to the line out. Not the effects send, but the line out. I tried both effects send and line out into a separate amplifier and this noise is not present from either of them. i.e. with DSP in the loop, there is no noise output, which makes me think the issue is not the DSP. Also, same goes for the caps on the preamp. If they were the culprit, wouldn't I hear the noise in the effects send or at least in the combined line out? There is no noise in either signal.

Standby for the rest of the schematics....

[WSKCONDOR]

The Preamp schematics are attached. This and the previously posted power amp schematics are all I have for this amp.

[WSKCONDOR]

Update:
So I ordered a capacitance meter, and started checking the power amp section (I had removed it from the amp).
It appears that all of the caps I have tested so far look ok (they are reading the capacitance value printed on the side of the cap). So I think I need to move off that board perhaps. I still am at a loss as to why I can get a "clean" signal from the line out point and from the effects send point. Could the transformer (main line transformer) be giving the power amp board a dirty signal? Are all the filtering caps on the board or is something "semi clean" expected out of the main transformer (i.e.is there something I could check on the main transformer with it plugged into the wall?)
Do you think I'd need to buy a whole oscilloscope?
really I am doing this out of curiosity now. SLM won't sell me a new power amp board, and really I can just buy the exact same amp (but cosmetically beat up) for less than 200 bucks and trade the good guts for the bad guts and have a clean pretty amp again. But this is somewhat interesting to try to find the culprit.

Any more ideas?

[52 Bill]

SLM won't sell me a new power amp board, and really I can just buy the exact same amp (but cosmetically beat up) for less than 200 bucks and trade the good guts for the bad guts and have a clean pretty amp again.

There really isn't much to the power amp board other than the power amp chip and the few components that support it. Rather than replace the board just change the chip and see if that cures your problem.

[Enzo]

Transformer is fine, move on from that. Cap meters are useful, but the cap value is not going to cause a problem.

I test caps in circuit by their function. For example, if the cap is a filter cap, its job is to smooth the DC voltage, so I scope the DC supply to see if there is ripple or if it is smooth.

You get a clean signal from the preamp out because the preamp circuits are clean. Nothing more cosmic than that. Plug a signal into the FX return, if the power amp then sounds bad, you have indeed isolated the problem to the power amp.

You have the power amp out? What numbers are on the board. I don't see a power amp in my files either. Don't know if my Loud Tech password still works.

The power amp has main power supplies, which you can check for ripple, but they often have low voltage supplies, typically 15v, which can also be rippled.

[WSKCONDOR]

OK. thanks a million and some great suggestions. Nothing is hitting a eureka bell yet, but I think I might try the "replace the power amp chip." idea. Largely because I seem to be able to find it online and removing and replacing that is something I can do myself and feel pretty confident about. Not sure what weird project I will use the old chip for if the new chip sounds the same, but at least it is isolating one more item.

[nickb]

I don't see where the problem was isolated to the power amp. Last step in this process was post 9 Did you ever do the unmute and test just the power amp? What was the result?