Electronics > Repair

Logitech Z3 plate amplifier repair

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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. :D
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.

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