KRK Rokit 8 (G3?) -19v on the mute pin - Anyone have an older schematic?

[jondoe]

Hi All,

I have a Rokit 8 that was blowing it's main fuse, I found a short in the diode bridge for the tweeter side of the circuit, I changed all the diodes for new ones. This solved the problem fuse blowing, but on testing i found the TDA2052 was putting +3.4v on the Output pin. I've removed the TDA2052 and found the mute pin 3 is being supplied with - 19v, which can't be right. For good measure, I've removed loads of the black gunk, there is a mix of soft clear silicone and hard to remove rock hard back crap.

The correct schematic would be helpful, the only one I've found is for a newer and different board lay out. This board is dated 2013, Version A1.

Thanks!

[Jazz P Bass]

The 'Mute' circuit is the same as the main output mute.
To unmute the TDA2052, Pin #3 must be +2.7 V above -V rail for the IC.
That + voltage appears to come from an opamp before the header (preamp board?)
Rockit TDA2052 Circuit.pdf
RP8G2 AMP sch.pdf

[jondoe]

Thanks for those, I'd seen the first pdf, but not the second.

The board in this particular model is all in one, that is there is no separate preamp board. The only separate boards are for the DI/Balance and a 3 pin 5V standby circuit mounted behind the IEC mains input. There is no 9 pin header (CN4) anywhere. I'll check the op-amps and see if any have suspect readings.

[Jon Snell]

If you remove the black gunk that has gone electrically conductive, replace the corroded components, (in the worst cases) and it should function again.
They all suffer from this built in obsolescence.

[jondoe]

If you remove the black gunk that has gone electrically conductive, replace the corroded components, (in the worst cases) and it should function again.
They all suffer from this built in obsolescence.

Hi Jon, I've removed all but a small area of the black gunk, mainly because it's bonded to/around some of the interconnecting plug sockets, I don't want to break them if possible as I had a helluva job getting the plugs out in the first place due to the slathering of the black stuff in and around it. I only found one slightly corroded part under some of the black stuff, that was a 222 SMD resistor. I've cleaned it up, removed all gunk around it and checked it over, the resistor is fine and the circuit is connected to both sides, it's just a little discolored. The majority of this device has clear silicone holding the caps in place on the main board.



You can see the 222 center left. Removing the gunk took a little of the solder mask with it

I took a reading on the mute pin hole with the standby removed, it drops to -1.7v, reconnecting the standby see it's shoot back up to -19v. The actual separate standby module is putting out 5v correctly.

[jondoe]

I've done some further tinkering today, Jazz P Bass mentioned an opamp used in the mute circuit, so I looked at those for anything odd. One of the TL074C's is putting out -10v on outputs 3 and 4, it's near the mute circuit, although might not be part of it. I've also noticed there is a unmarked SMD 8 pin IC in the area of the mute/standby circuit, when probed it will change the voltage found at the TDA2052 mute pin, from -19v to -13v, this coincides with a nearby Zener diode that feeds part of the mute pin circuit (as far as i can tell looking at the traces) switching from a -v output to a +6v output.

[jondoe]

Just to update this thread and close it off, I ended up buying a complete spare part for the unit, which unfortunately included everything apart from the driver and cabinet , but that still worked out cheaper than a complete new unit, so it could be worse. The new spare part has a different design to the original and no longer has the TDA2052, the board is labeled as a Version 2.

[Mars Amp Repair]

If you remove the black gunk that has gone electrically conductive, replace the corroded components, (in the worst cases) and it should function again.
They all suffer from this built in obsolescence.

I agree with this post. I also have run into this newer black gunk version of the older tan colored glue that becomes conductive as well as corrosive as evidenced by the rusty look on the component leads after you remove this crap. Where the glue was a bit thicker I was actually able to read resistance between two points in the glue blob.
Not only must you chip this crap off but in some cases this crap is still a bit damp. You should use a stiff brush with solvent alcohol AND a rag like Kim Wipes between the brush & the board so you aren't just smearing the crap around on the board.
On one of these mod RPG2 Rokit 5: After removing the black glue the amp was in a mute condition. Owing to the fact that there is no schematic for this model while trying to draw the part of the ckt I suspected was for muting, I accidentally discovered that wiping solvent alcohol on R7 near the one IC U1 unmuted the amp. As a result that voltage on R7 became lowered just a few 10ths of a volt to allow the muting circuity to unmute and the sound came back until the alcohol evaporated.
I removed the filter cap C108 and re-cleaned the board. Once I did that, the unit came out of mute & functioned fine.
BTW, I had replaced C108 & 109 as they were bulged. There wasn't any cap electrolyte on bottom of the cap but it was glued in with that awful black stuff.
This just goes to how even meg ohms of leakage thru this black gunk they use to keep parts from vibrating can cause major issues.