Rockford p8004 lost channel sound, one by one

[Dack Wallace]

Hi everyone. I'm a long time fan and reader, first time poster. I have a peculiar problem with my old 4 channel rockford punch. Around 6 months ago I was getting noise mostly on the two rear channels for quite a while. Then one day both rear channels kicked out completely. I could swap RCAs and still no sound in rear. Recently, the front right channel had stopped working, then the front left two days later.
I verified all wiring and I tested the amp with a different head unit to no avail. I cracked open the case and a few of the larger 4700 microfarad 50 watt caps seem to have a rather bloated top (filter caps (?), not the smaller ones nearer to the power supply). I desoldered one and discharged, then tested for a short and it was fine, but I couldn't get a good reading of capacitance with my multimeter.
There are no leaking caps and no other visible problem besides a transistor with a loosely soldered first leg, which I resoldered (although I still have no sound).
Is there any chance replacing these caps might restore sound to my amp? I still get power light but no sound at all. I can include pictures of the internals if needed. I have a 300 watt rockford from the same time and series that is still working fine, although sometimes it kicks out, and increasing volume on the head unit kicks it back on.

I've been stressing this and any help is greatly appreciated. Thank you!

[kaboom]

Re: Rockford p8004 lost channel sound, one by one

Where is this amp mounted? If in the trunk, any subs nearby to rattle it?

Check it for bad/cracked solder joints, especially the opamps in the early stages. Those big caps are for the positive and negative supplies for the amplifier proper.

Like any conventional amp, not so-called class D, which are really PWM switching amplifiers, you have maybe a +/-45V supply to the power amp itself. Then, either extra windings on the toroidal transformer or resistors/zeners from the amp's supply, power all the opamps.

You'll have to replace the caps, but you can determine if you need anything else for it by 'hot testing' it.

Here's what I'd do if you can run this from a bench supply and want to play around. Hook any old speakers up to it and use anything as a source. Even a portable CD player or one of those "I-things." You don't need full input voltage for this. With it hooked up to the bench supply and bottom cover removed, get a wooden dowel and gently push on the board and opamps in the input section. Do the same around the power amp, but make sure you're using speakers you don't care about. Poking a bad joint in the differential amp can really make a 'pop.' I think resoldering the whole input section should get you someplace. Otherwise, go after the other joints around that one transistor lead you resoldered.

Does the amp seem to get hotter now at zero signal than it did years ago?


Does the other amp, when it cuts out, go out with a 'pop'? Or just no sound suddenly? When you turn up the input level, again, does it come back with a pop, or start playing again at once? Again, I think 'bad connection' somewhere. Once in a while, that symptom ends up being a bad transistor, but that was more common on older equipment. Just solder it up if you want a go at it.

And if you can, post pix so that members here can look things over. There may even be a trouble spot that screams out that one of us can show you.


Good luck,
-Paul

[Dack Wallace]

Re: Rockford p8004 lost channel sound, one by one

Thank you so much for the great reply Kaboom. As soon as I get out of work, I will take photos and answer your questions in detail. Thanks again! =)