Old Fisher amp crackling in left channel

[carcinogen]

I'd like to save this unit as it has fairly good sound quality and I'm flat broke. I use it to amp a phono and computer input out to a pair of 150w speakers. It's a Fisher CA-854.

A couple of months ago, I had this crackling/distortion in the right output channel. Sounds like a blown speaker. It is independent of the volume setting and seemed to be proportional to mid range sounds. It was an intermittent problem that would go away for about 5/10 mins if I turned the amp volume all the way up and played some music for a second. Then I grounded the unit and it went away completely, until now.
Now, there's the same crackling/distortion in the left output channel! It's independent of the speakers (as was before - switched the wires) and has the same "temporary" fix (a good bass beat at full volume will make it go away temporarily).
I'm thinking some kind of corroded connection, but I've poked around inside and I haven't seen anything out of the ordinary, besides some corrosion on the output fuses. Didn't fix it.

Does anyone have any ideas? The problem occurs regardless of inputs (aux or phono produces same results) or A vs B channel outputs.

TIA

[HayZee518]

the way a speaker is constructed, you have the frame which supports everything. on the back is a permanent magnet of varying gauss. on the frame is a magnet pole piece which is a solid rod. glued to the surface is a mesh spider which centers the voice coil around this rod magnet. the voice coil consists of a fine wire wound around it. it is terminated in a braided wire jumper which goes to the terminals. a dust cover is glued to the baffle front which keeps stuff out of the air gap of the magnet - voice coil. the baffle is a composite paper which forms the cone of the speaker. this is glued to the out side of the frame. when a varying frequency voltage is applied the coil is energized and moves in or out around the voice coil. this movement in and out is what produces sound. if the voice coil is rubbing ANYWHERE on the magnet it will cause distortion. varying temperatures and humidity will expand the voice coil form and cause rubbing. mishandling might disrupt the spider and mis align the voice coil. when a speaker "blows" the voice coil burns out - it opens. some windings might weld themselves together changing the impedance of the speaker and cause rubbing on the magnet piece. turning up the bass temporarily fixed it by moving the voice coil over a large range of frequency but will return to "normal." I don't think your amp is at fault I'd look to the speakers.

[carcinogen]

I took your advice and fetched another pair of (75w, good condition) speakers from the basement. I even ran some new wire. Same problem! Just the left channel!
What the heck is going on?

[carcinogen]

So, I just got a nice static electricity shock from the receiver (yes, it's still grounded, even!!) and the crackling went away, again for only a couple of minutes. Does this have something to do with a poorly grounded transformer or something like that, perhaps? Since I don't know my way around the inside of an amp, can someone point me to any possible culprits?

[HayZee518]

I'm not familiar with your amplifier in particular however if it uses tubes, then what you may be feeling [as far as shocks] go is the B+ voltage on the plates of the tubes. This voltage can be as high as 180 volts DC or higher!. The tube filaments operate at around 6.3 volts. AC is fed from the line into a filament and B+ transformer where it is stepped up and down for its use within the system. It is then fed to a bridge rectifier which changes the AC to DC. A dual diode tube is what changes the AC to DC. Everything in the circuit except for the power supply capacitors should be running "above" ground. The negative of the filtering capacitors are grounded to the case. Essentially everything metal is grounded to the case. If the transformer shorts out or part of its winding shorts to ground it will charge the case to line voltage [ which is dangerous] if you are grounded. there are other components of tube systems which can contribute to your problem. the control grid has a DC voltage on it to vary the amount of electrons that are given off by the filament from reaching the plate which is how a tube can amplify. there's other parts too - a suppression grid etc within the tube.
A leaky filter capacitor can contribute to extraneous sound or crackle.