Troubleshooting occasional popping from solid state amp?

What's weird is that other amps didn't react to the fridge or lights, could be a design flaw?

It may not be grounded or not have a class x suppressor cap across the line. Fitting one may help.

thanks for the suggestions, I noticed now also when switching light on and off it causes popping. It seems the amp has some kind of grounding or shielding issues since I didn't have this problem with other amps? I've read others having popping due to refrigerator. Any idea why the amp could be so sensitive to other electronics?

Op
I would spray liberally with a good electrical contact cleaner, all the pots the amp has, volume, balance, treble bass etc, whatever as dirt and dust gets built up behind these very easily.
As far as popping when the fridge turns on and off that is interference over the mains from the fridge compressor kicking on and off.
Can you place the amp on a totally different circuit to the fridge?
If not you would need to get some type of power conditioner/isolator for your audio circuit to prevent the pickup from the mains noise.
Good luck

Identify which channel is involved. If it is both you have a power supply problem. I doubt a cold solder joint as that would have caused trouble a long time ago. I'd be more inclined to look at capacitors. You can replace them yourself by getting the value and rating then you can order the part from Digikey. Do not just heat up the joints and try to pull the cap out. Use a solder wick or sucker and remove all the solder then straighten the wire and gently remove the cap. You do not want to break the copper foil. 

Good luck,
Mike.

I noticed also the amp pops when the fridge turns on and off, any solution to this?

Yes. Could be a bad connection on the pot. Replace the pot - or, try contact cleaner spray if you're happy it's not a bad joint.

What I noticed is that the volume pot makes a pop when it's touched, could this mean the pot is the problem? Could be a grounding problem of the pot? Any advice on how to check it?

Open up amp on bench. Connect to small test speaker. Power up. Use insulator like thin dry wooden rod to push on board and connections until faulty joint is found. Use freezer spray too. Look for leaky electrolytic capacitors. Consider replacing them.

Hi,
 First disconnect everything and try to identify if this popping is coming only from the amp. Cold joints are not always easy to trace and you have to dismantle the unit to check thoroughly but normally they keep the unit working or not. 
A 20 year old product needs some attention if you want to keep it.
Pops like that are sudden short bursts in the circuit. Better checked.

I picked up a ‘90’s era old HARMON KARDON integrated amp and tuner locally on the Uber-cheap Craigslist- type ad with a view to refresh them and pass on to a senior citizen transitioning into a retirement home.

initially they were working fine for me for a couple of weeks, but then the amp suddenly developed the intermittent occasional popping and subtle cracklings noise. I popped the cover , but other than the usual dust from an old unit , I could not see any obvious issue to the untrained eye.

i ignored them initially while I assessed next steps forward, until there was suddenly a louder sequence. Now my antenna was raised to the point that I took it in to a local professional tech for a necessary look-over.

Good thing I did to avoid a potential FUHBAR in the making.
The tech popped open the box and identified a power supply failing section that was causing an intermittent electrical arcing ...ergo, a hazardous fire hazard in the making. I didn’t see it when I first popped open the cover before taking it in . He said it was an age-related failure typical of old units.
A $150 comparatively cheap cost-of-insurance fix IMO. Be careful out there.

Trouble, shooting and occasional popping are becoming an all too frequency occurrence in today’s society.