Crackling Static on volume control

[spacebiscuit]

I hope my amp qualifies as "vintage" and doesn't break any rules!

I have a power amp from the late 90's (for PA / music hire use) a real heavy indutrial lump. One of the channel crackles until the volume is turned up to about 1/3. I thought the issue was the potentiometer which I replaced but the new one didn't improve matters. I wasn't sure if my solder work was the best so I tried another potentiometer, but in doing so I think I may have damaged a capacitor with my iron, but in doing so I may have inadvertently found the real cause.

Each channel has it's own circuit board - here is one of the channels and shows the back side of the pot which I changed:


https://www.vintage-radio.net/forum/...1&d=1643009121

This is the damaged capacitor(?):


https://www.vintage-radio.net/forum/...1&d=1643009121

When I took a closer look I noticed on the board written "vol" and "mod" - this is the good channel:


https://www.vintage-radio.net/forum/...1&d=1643009121

Being a newbie this might sound like a silly question, but the blue capacitor I am guessing is key to how the volume is controlled? Is this a possible cause of the original crackling/static sound?

Since I damaged it I can barely hear any audio at all, I have to turn the source (a mixer) up really high and even then I can still only barely hear anything. So I think I need to replace this capacitor:


https://www.vintage-radio.net/forum/...1&d=1643009121

Is anyone able to help identify the component here, I see 1k but what is the "63". Again apologies if this is an obvious newbie question.

Thank you in advance.

[superjust]

Looks like a standard 63V, 100 nF capacitor to me.

[Richardgr]

If it was marked 2u2, that is 2.2uF, so u1 is 0.1uF, or 100nF.
K is the construction, MKT, or plastic film.
63 is the voltage rating.

[Silicon]

Potentiometers used as volume controls can be noisy if there is any DC voltage across the carbon track.

A healthy film capacitor is usually fitted to the 'signal in' terminal of the potentiometer to block the DC and prevent the crackling noise.

[knobtwiddler]

Silicon is right, and DC across the track was my first thought. However, that appears to be a film cap, and it's rare to see film caps start to pass DC. It'll be easy to replace and find out, though. As others have suggested, it's probably a polyester film cap. Polyester or polypropylene ought to work.

Do you know the maker / model no. of amp? If so, we can have a look for a schematic.

Once you've changed the cap, if the problem's still there, I'd check for oscillation. Some stages can oscillate when fed with a higher source resistance, i.e. when pot is at high cutoff* Try putting a 10K square wave to the input and looking at the output with a 'scope. Do you see a nice square on both channels?

Another possibility is that the blue cap is a red herring, and another cap (an electroytic?) that's up or downstream of the pot is leaking DC.

* and a volume control can sometimes be used in a feedback loop of a stage, not just as a passive level control. If this is the case, it could oscillate at lower gains if the stability margin isn't great.

[spacebiscuit]

Many thanks - I have ordered a replacement. I'll post the results of the repair once down!

[spacebiscuit]

Seems the amp is a Mosfet T400

I have put the new resistor in situ (small grey bottom right of the pot) but my soldering iron element has gone so I was unable to solder and test until tomorrow.

Does the photo of pcb help to id any other potential causes of the crackling volume control (if the resistor swap doesn't work)?

Thanks,