Electronics > Repair
KRK Rokit 5 RPG2 speakers (again)
waste:
The speakers have the famous black goo problem. I removed so this problem is solved but I still have a very loud pop noise when I turn the power on. That probably means that the Limiter system that turns off the amplifiers during the first few seconds is compromised somehow, but :)
This board is different than the other board I have. The 1.2 revision board has the limiting circuit with optocoupler and such, but in the 2.0 revision board those parts are not populated at all.
So my question is, which method do these speakers use in order to eliminate the pop sound of turning on? The mute signal on the amplifier seems to work ok applying a bit of negative voltage to mute the amp for a few seconds and the positive again, but it doesn't seem to mute the pop sound. I have attached the two schematics, the Rokit5 is really bad scan but the ciruit is very similar with the Rokit 6.
Is it possible that version 2.0 as a cost saving method has no pop protection circuit at all, or they use another method? Any ideas?
The first picture is the 1.2 revision with the populated board on the limiting ciruit (the circuit looks better in the rokit6 schematic and the second picture the 2.0 revision with the unpopulated one
cte:
This is the mute circuitry:
R18 charges C16 until Q1 is turned on. This turns on Q2 which charges C17 via R22. Once the voltage on C17 reaches the threshold set by the resistors on the inverting input of the opamp, the output goes high.
D5 discharges C17 when the power is turned off.
Mark1406:
The problem I had in one monitor was the black goop ate through a resistor and the mute signal was staying on. Once I replaced those components the monitor worked fine.
When I compared it to the other monitor, the other monitor stopped working, and the repaired monitor worked like a treat.
I have removed the goop on the second monitor. It’s not working, though it’s not blowing fuses. I will have to check the rail voltages and the mute line in this monitor.
Thanks for the explanation of mute circuit, that’s really helpful.
Regards
Mark
waste:
@CTE
I recognized the mute circuit and checked it and it seems to function correctly . The voltage turns positive 1.9V in respect to VS- in the beginning so the mute is on and then it turns positive to 6.5V , but the pop still persists as if it's not affected by the mute action. In other amplifiers I have repaired it's usually a Relay that eliminates the pop as it's also in Rokit6 with the optocoupler
Here the description from the datasheet:
MUTE/STAND-BY FUNCTION
The pin 3 (MUTE/STAND-BY) controls the amplifier status by three different thresholds, referred
to -VS. When its voltage is lower than the first threshold (1V, with a +70mV hysteresis), the amplifier is in
STAND-BY and all the final stage current generators are off. Only the input MUTE stage is on in
order to prevent pop-on problems. At Vpin3 =1.8V the final stage current generators
are switched on and the amplifier operates in MUTE. For Vpin3 =2.7V the amplifier is definitely on (PLAY condition)
@mark1406 The board is pretty clean and fortunately the parts not corroded so I assume the muting circuit works as advertised, but there was black goo on the pins of the amplifier chip. But I will check the other speaker to see if it has the same attitude :)
PS by the way I found the limiter circuit in a daughter-board under the volume input control board
cte:
Can you measure the MUTE signal on pin 3 of the amp IC with an oscilloscope at the moment of switching it on? (need to use single trigger mode for this)
I've simulated the circuit in spice. See screenshot below.
Green is the power rail, blue is voltage on the op amp output, red signal would be expected voltage at pin 3. This would be roughly one second of delay at turn-on.
Is the pop sound audible on both channels?
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