Logitech Z3 plate amplifier repair

[Dr_Krieger]

Last week my logitech Z3 speaker system decided to magically *cough* short a wire somewhere in the lines from the plate amp to a volume control thing, and since then I have been investigating the plate amp and volume control.
Getting good access to the amp was a pain in the ass, they sure do like their glue. :/
The heatsink covers all of the board except one capacitor and screws into the board at the 4 corners and also to the plate in 4 places, unfortunately the 4 screws holding the heatsink to the plate will not budge and so the board stays as is until I can cut them or something. All the screws have nuts and locking washer things on them.
Suggestions on removing them anybody?

Hasn't stopped me from finding out a bit about it though, seems pretty straight forward.
Input comes in and goes through a TL074CN op-amp as a preamp and I guess through band pass filters and then off to a TDA7377 connected in a stero bridge configuration, as shown in it's datasheet.
Based on the datasheet, I'm assuming the TDA7377 is OK, and something else, either on the plate amp board or volume control circuit (in a volume control pod thing, plugs in with a POS 9pin DIN on the back of the plate amp)

Tuesday:
I plugged in the amp and started probing voltages, transformer is stepping down to 13V AC and being fed into 4 diodes for rectification and cap for filtering.
I probe the diodes that I think should have DC voltage on them (has the filter cap connected to it and goes to Vcc pins on TDA7377) and my DMM shows 0.3-0.4v between ground and Vcc
Obviously not right and I'm not entirely sure what to do now. :/

Also, I don't know for certain what connection was shorted but V+ to ground short seems most probable, though I never actually saw any kind of short circuit.
So I guess it's possible for a diode or two to be damaged? Or would the TDA3733 go into stand-by and bring the voltage shown on the diode down?

Thursday:
 I opened the volume control and could smell that almost burnt plastic smell, but no obvious signs of magic smoke release.
Found an NJM4560 dual op-amp and voltage regulator on the bottom and a few caps and resistors, pot, blue led and power button, on the top.

Sunday:
Hooked up my bench power supply to the wires I found to be ground, V+ and V- and ground, and played music through the pcb and into a set of headphones.
Sound came through clear but kind of in and out and scratchy every now and then.
(most likely due to the fact my bench supply doesn't have a proper split rail setup, I just ran 14v through a voltage divider circuit I threw on a stripboard pcb)
I no longer think the volume control circuit is suspect in some way.

All I have for test tool(s) is my cen-tech DMM and bench power supply. Oh, and my brain...sometimes.

Pictures https://www.dropbox.com/sc/z5jow6uxh8abgm3/6j_60ILI_x

Anybody got ideas on what might be wrong or other things to check? This may be my repair project, but I'm more than happy to take suggestions.
What with the situation with the heatsink, desoldering parts for testing isn't an option. (except for a couple parts maybe)

[SeanB]

Use a good screwdriver and try tightening them, or apply a lot of heat to them from a soldering iron to cook the locking compound to the breaking temperature. Otherwise just brute force and shear the heads off.

[Dr_Krieger]

lol, after fighting so much with one of the screws so much last week, even applying heat with my soldering iron, and getting no where;  doing it today, it just unscrewed like there was nothing there.
The other 3 screws aren't coming out as easy though. I'm gonna see if I can find a small wrench I can fit under the heatsink and to the nuts and see what happens

[SeanB]

If they are that obstinate I generally drill the heads off and go from there on to remove the rest by some method or the other based either on drilling it out and retapping or using an extractor.

[Dr_Krieger]

After heating the nut and twisting the screw a bunch, going CW,CCW, with the screw driver eventually broke it free.
Unfortunately about 15-20 minutes after getting all the screws out and heatsink off the PCB, I got called away to put fires out in my field. (they have been put out of course)

I added a few pictures of the top of the PCB to the dropbox photo album, and here's a link to just the three of them. https://www.dropbox.com/sc/aeq3vo8i0dcgkzp/yBIbwtWoUg
They sure do like their hot glue...
Probably won't do anything else with it tonight, maybe trace some of the op-amp connections and try to find where the inputs are coming from and what have you.

[Dr_Krieger]

Since I had the board free, I hooked it up to 12v from my bench power supply and audio plays out perfectly fine.
I thought maybe it was the transformer, to check I hooked up the transformer to a rectifier diode IC I had sitting around and it put out the proper voltage.

Could it really be the rectifier diodes?
I checked each diode and they seem OK, diode check on my meter was showing .5 for each one.
The diodes on the amp board are IN5401's, although they appear good, I have 4 IN5408's sitting around somewhere that could replace them.
I also have a rectifier IC rated for up to 4A, I could wire it in their place. Would I want to replace the 4 diodes with an IC though? (I guess I could, it's my project, only thing stopping me is me really xD)

I'm gonna wait and maybe get some feedback from the community here,  (please? I love you long time...) before proceeding with replacing 4 diodes with anything.

[Dr_Krieger]

Soooo...
Of course I didn't start a new threat on the forum to ask a question, I just ended up wiring in that 4A rectifier IC last night around midnight. xD
And of course after putting it all back together, falling asleep, I wake up this morning to remember I forgot about the fuse that's soldered to the board. :/
Guess I'll pull it apart again later and replace the 4 diodes and have it back the way it was I guess...I dunno, might just take the fuse and wire it up off the PCB.

[Dr_Krieger]

Well this is interesting, I decided to go ahead and just replace the diodes, hook up AC to test and it's doing the same thing as before, only showing .14v
Checking AC voltage across the fuse shows 13v AC. :/

And now for something really weird. I hooked up the IC again with the fuse connected in line with one of the AC lines, DC output on the rectifier is 1v, remove fuse and it's 18v now. (drops to ~17v when load is applied)
WTF?

EDIT//
While the fuse looked OK (I guess, never seen a fuse exactly like it before), but when checked with ohm meter it was showing off the scale measurement, set to 2000k ohm.
Replaced with a 5A250v fuse from a power supply and it's happy. I figure there was enough current draw for long enough that the fuse sort of went out.
Reminder to self and all, don't just visually inspect fuses. :/