HELP PLEASE- ROKIT 8 POWERED SPEAKERS

Discussion in 'Technical Repair' started by dougalere, Aug 19, 2016.

  1. dougalere

    Thread Starter Member

    Mar 4, 2015
    128
    5
    Hi

    I have been asked by a friend to repair a pair of ROKIT 8 powered speakers,

    He paid hundreds of pounds for these,and left them in the loft for afew months ,but we found that they had been ruined by damp on the boards,

    I am slowly bringing them back to life, replacing damaged and corroded components,

    And of course a good drying out. And are back working.

    I should add that as a hobbyist I’m a little out of my depth.

    I am now struggling with a loudish pop which comes from both lf and hf speakers on switch on, this is the same on both cabs.

    Each cab comprises two boards-a preamp board (which looks undamaged) and the main output amp board /rectifier etc

    Little info available, but A very similar schematic is here-(at bottom).

    http://music-electronics-forum.com/attachments/33751d1430277257-svparts_rp6g2scha1.pdf

    The output amp is two separate amp circuits- TDA2052 for the HF and aTDA7294 for the LF. the amplifer using the internal mute circuits for start up mute,

    And fed from what I assume is the delay circuit on the (undamaged?) preamp board- UP3B to pin 6 on the multiblock,which then splits to the amp chips, i have tried interchanging boards- no difference.
    the pop is on both LF and HF cones.

    also after being powered off for a few days, the speakers take some minutes before audio is heard, i dont know if this is related.
    if I cut the mute wire- no start up pop.

    Logic would suggest (an unlikely?). fault on the UP3B delay circuit, but am I barking up the wrong tree?,
    THANK YOU
    DOUG
     
    Last edited: Aug 19, 2016
  2. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    I agree. The start-up delay seems to be the bad guy.
    UPB3 (at the bottom) is the delay circuit...about 1 second.
    I would watch the voltage at the output of UBP3 as the amp starts up.
    It moves from page 1 to page 3 on pin 6 of the connector.
    It goes positive to about 9 volts after a time delay.
     
    Last edited: Aug 19, 2016
  3. dougalere

    Thread Starter Member

    Mar 4, 2015
    128
    5
    hi thanks for for your prompt reply,
    as you suspected the voltage on the mute wire is almost instantanious at 10 volts ,relative to power ground,
    with the mute wire cut ,
    but strangly the voltage on the mute wire from the power amp is over 20 volts dropping to10 when reconnected.
    i can soon knock up a delay circuit probably quicker than trouble shooting as i only have a multimeter
    but it seems strange that both preamp boards are messing up..
    thinking about it, the speakers were powered up while still wet, so i suppose that could have damaged the delay circuits
    tregards
    doug
     
  4. dougalere

    Thread Starter Member

    Mar 4, 2015
    128
    5
    ps be interesting to run the delay circuit on a simulator to see what the delay would or should be.
    doug
     
  5. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    No, that's not what I expected. R22 and C17 are supposed to delay the positive voltage by about a second. Check the input to UBP3 and see if the delay is happening on C17. If delay yes, then UBP3 is stuck high. If no delay, C17 is probably open. If both=false, I don't have enough measurements yet.
     
  6. dougalere

    Thread Starter Member

    Mar 4, 2015
    128
    5
    THANKS #12
    i.m putting togrther a new delay circuit, i need to keep my hand in.. i'll stick with 2SAs,
    althoug they are a bit pricey.
    a bit worrying, i noticed this morning when i rejoined the mute wire with power on, i got the speaker pops,
    but i'll see what happens with the new delay ckt.

    in the meantime, could i push my luck on the audio output issue, after being powered off for a couple of days
    the HF starts immediately on power on, but LF takes many minutes +5, to come on,
    but if i switch off during this period, i get a moment of full audio.
    after this period, audio comes on immediately when powered up,

    i suppose this could be the LF mute circuit on the output board, as this area did suffer the most water damage.

    regards
    doug
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    If the mute voltage is 10 V, after one second, I would suspect the 5.6V Zener as being shorted. Do you have an oscilloscope?

    On the HP Mute, don't forget the circuit is not connected to power ground but to -VDD of -19.5V.
     
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