The Ghost of FM said:
I use products from Caig Laboratories for cleaning pots and switches.
Crackling pots can also mean some DC bias voltage on them.
I'm not sure of the circuit of this deck, but as its an older one it may well use a single rail with the audio signal lifted with a DC offset voltage. Newer equipment runs on a split rail, so that the AC audio waveform swings around zero volts between (say) +15 and -15 volt rails. On older gear (like my 70's Akai) this waveform swings around, say, 6 or 12 volts on a rail upwards of 20 volts.
These voltages are blocked from the input and output using caps, and you'd put caps between stages and before and after pots. Seeing you've had other problems with caps, and noise which could relate to caps, its a good place to start. Use a multimeter to check for DC voltage on the pots - if its there, that's you're problem.
Another possibility with the hiss problems could just be an old input stage. It depends on how bad the hiss is. I have used some very noisy home brew gear from the 70's (i.e. stuff dad built then!) based on circuits that were supposed to be 'low noise' at the time - an example being a balanced mic preamp using two BC109's followed by an LM301. Very nice sound, but noisy as anything.
If you find voltage, replace the caps (I'm guessing on gear of this vintage it shouldn't be too difficult a job, just make sure you get the values right and have a schematic and parts list nearby - and be prepared to have to realign all of the levels, bias levels etc) and see if this hiss disappears.
Hope this helps!
Cya
Andrew
EDIT: Just re-read the post detailing the problem with noise. If it doesn't change with volume knobs, the noise doesn't relate to old design in the preamp, so some of the above doesn't apply. Feed a signal in and see how the noise level compares to the signal. If you blow holes in your eardrums you just had the level up too loud, otherwise we go back to caps etc. after those level controls; try changing the blocking caps directly after them first.