Audio crackling - please help!

[builtclever]

Are you asking me? Because if you read my post clearly, I distinctly said I (don't) think that a bad cap will cause crackling. Also, I asked you to report on whether the crackling is present on 1 or 2 or all 4 of the drivers. You said you'll report back with findings but never followed up.

I can now confirm that the crackle is evident in all speakers, which I tested separately.

 

[XLVII]

Thanks for the vid. The crackling is totally different than what I expected to hear. Have you used your multimeater to check your power supply/transformer?

 

[Superduper]

This boombox has 4 separate amps. 2 for each of the left and right woofers, and 2 for the 2 super woofers. That's why I asked if the crackling is coming from every speaker, the purpose is to try and isolate the circuit that is giving you issues. Unless the other speakers are silenced, it's often difficult or impossible to know if the crackling is coming from one or all speakers. Prior to the output circuits, the preamps also has dual parallel circuitry. That means the left and right signal paths are separate from each other. A cap or resistor in one signal path (i.e. R)is not going to affect the sister channel (L). So if you are correct in that every speaker is having this issue, then the problem is global as opposed to local, meaning whatever is the issue must be common to everything, and the only parts of the boombox that is common to everything is pretty much the power supply, and maybe the main filter capacitors for the amplifiers. Focusing on a single cap or resistor that only affects a local channel (L or R) or speaker, or amp would be the wrong path since it's highly unlikely the exact same problem affects the other channels in the same way at the exact same time. Also, I'm not familiar enough with the 767 but does it also have line-output jacks? Because if it does and the crackling exists there too, then we can surmise that the issue also is in the preamp circuitry too. The headphone/internal speaker thing doesn't matter because both are powered by the output amplifier chips, so the only thing that could tell us is whether the issue is poor connections at the headphone jack. Without having direct access to your boombox to look, or confirm or test anything, it's hard to give a real informed suggestion since you obviously don't have access to advanced testing tools and equipment. But you might want to try and replace the stiffening caps that beefs up the supply rail to your amplifier chips. Specifically C814 and C843 which are large 3300uf capacitor and 47uf capacitors typically near the 2 amplifier chips. Also there is the possibility that the glue that holds the capacitors down might have gone conductive. The glue that Sansui used on some of their receivers were notorious for this, and gives technicians fits. That you "cleaned" the boards and condition improved suggest this may be a possibility. The resolution would be to clean off all of the residue and the caps that have this glue would have to be removed first to access the underside where the glue resides. Obviously on equipment this old, once the caps are off, you should replace them with new caps since stuffing 45 year old caps back in after they've been removed is ill advised. This glue is usually only on the larger caps.