Rokit 5 Powered Speakers - Blowing Fuses

[ocf]

Hello Guys and Gals,

The powered speaker blows fuse as soon as its turned on, it makes a loud low frequency noise and then blows. Inside of speaker smells distinctly smoky.

Looking inside the TDA2052 on the HF side looks like its had a lot of heat, thermal compound between IC and the heatsink has dried up.

Anyone have experience with this particular model, any schematics around?

There is quite a lot of black insulation compound used throughout the Mainboard and power PCB's, this looks like a black version of the regular white silicon stuff you get on PCB's to prevent arcing and movement of components.

All the black gunk appears wet, small beads have formed all over it and when wiped they come off on a cotton bud and stain it brown.

Doesn't smell good either, I read about a type of rubber that breaks down after time and produces butyric acid, butyric acid is supposed to smell like vomit and this stuff definitely has the same aroma.

Anyone have experience with this stuff looking/going bad, I cant see any signs of water damage in the speaker and the droplets are on the black gunk only.

thanks in advance

OCF

[CapLeaker]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Post some high resolution, straight shot pictures. First off you'd have to remove all that wet black gunk, whatever that is.

[Andrew F. Ali]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Output ICs shorted. The 'black' stuff could be from a burst e-cap. Rectifier cct probably affected also.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

the black stuff was definitely there from time of assembly and not from a leaked component. Its looks as if the solidified silicon sealant has sweated the liquid, it does not run and just sits on the black silicon in small beads untill wiped

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Images as requested, let me know if you need any more

Red lead on right side of board goes to the woofer and left side to the tweeter, the IC on the right which I assume to be dealing with HF because of its proximity to the HF feed is the one that looked burned.

Some of the leads of the HF and LF IC's looked dry so I retouched them and reapplied thermal paste between IC's and heatsink. Tested with a new fuse and it still blows immediately.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Output ICs shorted. The 'black' stuff could be from a burst e-cap. Rectifier cct probably affected also.

Could you elaborate, when you say output IC does this refer to the 7 leg IC's that control the HF and LF in the amp, the ones I pictured. The part number I'm seeing on them is TDA2052.

What is the rectifier CCT? I don't see any recognizable bridge rectifiers.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

confirmed, black goop used on pcb turns conductive after a while -

http://forum.cakewalk.com/Shade-Tree...-m3117829.aspx

[Andrew F. Ali]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Yes it is likely one of the TDA2052 might be shorted out. Possible too for the Pwr supply cct to have a short. Could we see pics of the Pwr supply?

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Dont see any other boards that look like they provide power, nonetheless pics below of remaining parts of amp -









The only other power supply parts I can see are the transformer and the filter caps, I don't see any other power IC's anywhere that could be shorted.

What does CCT stand for?

[Andrew F. Ali]

Re: Rokit 5 Powered Speakers - Blowing Fuses

CCT means circuit.....Check the rectifier connected to the transformer secondary.....If that is OK......then one of the output ICs is bad....I'd just replace them both....if I didn't know how to test them....most likely though the LF side may be the culprit.....see if there is a direct short across pin2 and pin 4

[budm]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Check the resistance between pin 1 and pin 2, pin 1 and pin 4.
BTW, your pictures are way to low of the resolution to see anything clearly when enlarged.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

High res images - http://postimg.org/gallery/1x8utslru/

LF IC shows almost no resistance between Pin 1-2 and pin 1-4

HF IC shows about 700ohm resistance 1-2 and open on pin 1-4

I am looking for the rectifier, will this be a couple of diodes back to back? cant spot anything that looks like a dedicated half/full rectifier.

[vinceroger69]

Re: Rokit 5 Powered Speakers - Blowing Fuses

Do the four diodes by the large capacitor with the yellow top on test ok?

[budm]

Re: Rokit 5 Powered Speakers - Blowing Fuses

You have two sets of bridge rectifiers setup.
BTW, does the fuse still blow if the secondary side of the transformer (RED/YELLOW/BLACK wires) not connected to the board?
The TDA with 700 Ohms reading is not right, remove it and test it off the board.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

High Frequency TDA -

Pin 1-2 - 660 OHM
Pin 1-4 588 OHM

Low frequency TDA -

Pin 1-2 - 21 OHM
Pin 1-4 - 3 OHM

I will remove from board and retest.

What should I be seeing between the pins if the IC is good?

[budm]

Re: Rokit 5 Powered Speakers - Blowing Fuses

It should show high resistance:
This one is bad for sure.
Low frequency TDA -

Pin 1-2 - 21 OHM
Pin 1-4 - 3 OHM
The measurement are for the Upper and Lower power transistors' Emitter-Collector junction inside the IC.

BTW, it was confusing when you have indicated "LF IC shows almost no resistance between Pin 1-2 and pin 1-4".

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

thanks for reply bud. I will be clearer in future, surprisingly difficult to troubleshoot and transport facts/figures concisely for others to view.

Appreciate all the help! I will test the High frequency TDA out of circuit and order a replacement for the low frequency.

I will also test the diodes that make up the dual rectifiers and report back test results. Is there an easier way to test the diodes that make up the rectifier than removing them from board?

[budm]

Re: Rokit 5 Powered Speakers - Blowing Fuses

You can test them first without removing them, if any of them does not read right then remove and retest it off the board.
I would test the HF one off the board or might as well replace it too.

[Th3_uN1Qu3]

Re: Rokit 5 Powered Speakers - Blowing Fuses

They're cheap, i would replace both. Worst case, if the HF amplifier IC is also bad, it may blow the tweeter also, something you definitely do not want.

[ocf]

Re: Rokit 5 Powered Speakers - Blowing Fuses

I tested all the diodes, the smaller ones on left all checked out good in circuit. Larger ones on right were showing a voltage drop both ways so I pulled them out of circuit and they seemed to test ok.

Two of the larger ones, the ones located closer to the Amp IC seemed to act a bit different than the other two. When I tested forward bias it showed up as 600mv then started to creep up a but, I probed for about 10 seconds during this time the reading went up to about 630mv.

Does this seem like normal diode operation?

I am going to order the TDA's soon