KRK Rokit 5 RPG2 speakers (again) - Page 1
Forum section: electronics / repair

author: waste
author_rank: Contributor
timestamp: December 08, 2024, 07:53:06 pm
content: 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, butThis 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

author: cte
author_rank: Regular Contributor
timestamp: December 08, 2024, 09:59:36 pm
content: 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.

author: Mark1406
author_rank: Newbie
timestamp: December 09, 2024, 02:20:17 am
content: 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.RegardsMark

author: waste
author_rank: Contributor
timestamp: December 09, 2024, 08:33:01 am
content: @CTEI 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 optocouplerHere the description from the datasheet:MUTE/STAND-BY FUNCTIONThe pin 3 (MUTE/STAND-BY) controls the amplifier status by three different thresholds, referredto -VS. When its voltage is lower than the first threshold (1V, with a +70mV hysteresis), the amplifier is inSTAND-BY and all the final stage current generators are off. Only the input MUTE stage is on inorder to prevent pop-on problems. At Vpin3 =1.8V the final stage current generatorsare 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 attitudePS by the way I found the limiter circuit in a daughter-board under the volume input control board

author: cte
author_rank: Regular Contributor
timestamp: December 09, 2024, 10:16:56 am
content: 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?

author: waste
author_rank: Contributor
timestamp: December 09, 2024, 02:49:22 pm
content: shiiiit, you put so much effort to simulate it , I will bring out the oscilloscope and scope the mute pin.Rokit's only have one channel but it has two amplifiers for twitter and woofer. They both get the same signal to mute but the twitter doesn't make a sound .I'm bringing out the scope now, and report back (from my multimeter definately the whole oscillation to stable 6V (relative to Vs-) didn't take 4 seconds.thanks again for the effort

author: waste
author_rank: Contributor
timestamp: December 09, 2024, 04:13:44 pm
content: I bear newsI scoped the Vs- to the mute pin in 2 different Rokit5 .The one has the optocoupler and the TL074 limiter on the amplifier board (that's my no pop rokit) , board revision 1.2And the other one is the one that Pops. It has the limiter with the TL074 on a daughterboard but no optocoupler (does it use the transistors? there is no schematic for this board). Board revision 2.0 , probably they wanted to save money on the optocoupler? even though a seperate daughterboard doesn't sound a cheaper alternative.Why am I talking about the limiter circuit? Because actually the mute pin behaves on both the same way and totally in spec according to the datasheet so probably the problem is not there. Check on the pictures. And yes your simulation was correct, it actually takes around 4 seconds to turn fully on (at 6V+ relative to Vs-)

author: waste
author_rank: Contributor
timestamp: December 10, 2024, 07:37:30 pm
content: As I have to confess I can´t follow how the limiter works, and have no schematics for rokit5 without optocoupler (only rokit 6 with optocoupler) I came across this articlehttps://windowpaneaudio.wordpress.com/2019/05/09/rokit-5-g3-limiter-and-stand-by-mute-removal/where it actually states that the limiter circuit works all the time with just a transistor (no optocoupler) so I guess they eliminated the optocoupler in v2.0 and they passed this design on Generation 3 too. So If I get correctly the whole salad of opamps, the limiter limits the LowPass output in ver2.0 using only a transistor, so in my pop rokit the limiter doesn't limit anymore and is always on (dead transistor?). Is this train of thought any good, or am I just fantasizing like an AI?

author: cte
author_rank: Regular Contributor
timestamp: December 10, 2024, 09:25:27 pm
content: Actually, I've never seen or repaired this device before, so without a schematic I will have to look into my crystal ball...There might be a component named LCR-0202 on the PCB.Someonesaid, this was the culprit in his Gen2 device. Maybe this works for you.Full datasheet:https://電子うさぎ.com/wp-content/uploads/2020/09/lcr0202.pdf

author: waste
author_rank: Contributor
timestamp: December 10, 2024, 11:32:34 pm
content: Thanks that's what the ver 1.2 board has with the optocoupler and the opamp on the board.The ver 2.0 board unfortunately doesn't feature an optocoupler anymore, so that's why I fantasized that the same job was assigned to a lowly transistor as the guy in the article suggests about the rokit5 generation3Anyhow I will put the thing to rest for now and attack the issue some other future time.