On/Off Speaker POP......but only on one channel

[King Forrest]

1996 Ford Taurus/JBL Rear Control Unit F6DF-18C851-EF:
4ch out amp with discrete ST 70023AB amp ICs (F5VF-14A608-JD/PWB4082)
Logic board w/ Integrated Control Panel/cassette/CD inputs, output to powered sub
Digital AM/FM tuner

Background: Ever since I bought it (on eBay), the right-rear speaker would "POP" loudly when I turn it on, it would then immediately produce sound before the other speakers, but would sound perfectly groovy afterwards, untill I turned it off and heard "POP" again...Turned out (so I thought) to be a bad solder joint on pin 11 of the IC, used to control the on/off mute.

Where now: After that fix, I'm getting a much quieter pop, and it seems delayed. But the actual sound starts when its supposed to.

Now, I have a basic understanding of circuits and components, but I can't make sense of it all. The power and control circuitry seems to be shared between all the ICs, and Id hate to take off every singe cap just to test it. I did swap two ICs to see if that was the problem...it wasn't. I hastily checked all the resisters and diodes and they seem fine, I also swapped logic boards in this and my backup amp (80w MACH), it seems to be the JBL amp.

This model is pretty rare, I've spent 3 years getting the whole '96 JBL set together, so Junkyarding might not yield me much...

 

[Bluejets]

Some of these units if not part of the original gear, apparently use differing turn on voltages to the head unit which can cause your symptoms all-be-it in both at once.
Could be a line to follow.

 

[King Forrest]

I doubt it, Ford basically combined the head unit and amp in one box, and the dashboard unit is basically a big remote control. But I did try and swap the logic boards between this and my spare amp, which was made for my car, and I've pretty well narrowed it down to the amp board.

 

[Bluejets]

 

[PETERDECO]

Check if there is DC voltage on the speaker terminals. You might need an amp.

 

[King Forrest]

None of this is relevant. It's a factory amp and is already using the same turn-on voltage as the head.

 

[Bluejets]

Well, you know it all then.
Good luck.

 

[King Forrest]

Well I still don't quite get how the amp circuit works and how id go about diagnosing it.

 

[King Forrest]

Update: I pulled some large caps, I believe used for power delivery, rated at 2200μf @ 16v, but my capacitance meter was all over the place on all of them, nothing close to a stable reading. Not sure if its something i'm doing wrong or if they really are completely bad.

 

[shrtrnd]

Most of the time I encounter this issue, it's the physical contact connection between the amp and the speaker.
Just something to look at if you haven't already. A loose connection, crimp, solder joint, something like that.
May not be your problem, but the first thing I'd check before troubleshooting the innards.
Good luck finding a quick fix.

 

[King Forrest]

I'll check, my backup amp works fine so the only spot it could be is the connection on the amp iself

In the meantime, there are a couple of 220pf ceramic discs I broke trying to test (woops). Figured I'd upgrade the caps to something more audiophile grade, but I don't know all the specks...they just say "220" with a small dot underneath. These seem to be mostly coupling caps.

 

[King Forrest]

So, i'm really at a loss on how this works. The + and - signals seem to be bridged by the caps and resistors. wtheck?

 

[shrtrnd]

Can't help feeling like I play like a broken record. Are you possibly not seeing the forest because of the trees?
Check the speaker itself if the connections to it are ok.
Maybe the previous owner liked his music loud.
Is the speaker cone ok, or torn, or holed, or worn, allowing the voice coil to bounce to places it shouldn't?
Reverse the speaker wires and see if the other speaker 'pops'.
I may be wrong, but I've troubleshot too many intricate circuits, only to find-out I should have checked the most
obvious things first.
I'm still thinking your speaker is the problem, and not your amp.
Kick me if I'm wrong.

 

[King Forrest]

I've checked that already. They work perfectly fine with my backups amps and even with this one the music is fine, it only pops when I turn it on or off.

 

[BobK]

A capacitor marked 220 is 22 pF, not 220.

Bob

 

[King Forrest]

Okay, I just found something weird. 22pf = .022nf, right? Well, my meter tested them at .22nf, same as my spare "221" caps. Wth?

 

[King Forrest]

Umm.....I did a dumb-dumb. I didn't take photos, and my parts tray got messed up...
There's no shcematics for this.
How screwed am I?

Is there a way to just kinda figure it out?

 

[shrtrnd]

Sounds like desperate times to me. You have basic troubleshooting skills, apparently.
You are convinced the connections to the speakers are ok: actual connections, wiring harness itself, speakers themselves.
Only thing I can think of is a capacitor issue. It may not help, but I'd take the plastic (non-conductive) end of a screwdriver or something similar, and start tapping the components on the board while the unit is working, to see if you can find a fault.
I would SUSPECT something like a bad actual capacitor lead between the capacitor and the board. You can't see those when
the cap is mounted on the board. Sometime during handling, maybe a cap took a physical shock that loosed the bond between
the cap body and the lead coming out of it. I'd be real surprised if you have an actual semiconductor problem (aside from an
actual solder-joint problem you may not have noticed).
Sounds like you really want this fixed, that's what I'd recommend trying next.