BeoLab 8000: Power problem analysis, help needed

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  • #4560
    KolfMAKER
    BRONZE Member
    • The Netherlands
    • Topics Started 9
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    I am working on a power problem with a BeoLab 8000. I tried looking for information on BeoWorld and found this: https://archivedforum.beoworld.org/forums/p/10657/79327.aspx#79327 But I am not sure what to do.

    Problem

    After having functioned for a while pretty well, this speaker now remains dead. When powered the normal way, via C7 socket in the foot:

    • No Red or Green LED
    • No clicking of the relay
    • No sound form the speakers

    Checks

    This unit formerly had the old foam, but has been thoroughly cleaned: PCB, components, cables, connectors, sockets.
    After that I have checked the PCB’s for damaged traces, especially the ones that often fail after foam rot. But I have continuity on all that are checked. See ‘Tests’ for more about traces on the PSU PCB.
    Is is obvious that this unit has had work on before. One example is the manual repair of a trace (see blue box on 2nd picture).
    This unit has worked perfectly after the foam cleaning, for about a month.

    Tests

    • When power is directly connected to the pins of P4 (230v input on PCB1)  > the relay does click.
    • Same as a), but now also the two JST connectors (3 wire and 4 wire) are connected to the connection sockets on PCB 6 in the bottom of the aluminum pipe > the relay does not click, the LED does not turn Red or Green, but stays off.
    • Continuity test on the 230v wires from the C7 socket on the outside, to the connector on the inside that plugs into the PSU PCB > this shows positive continuity.
    • Continuity test on the relay > I found two pins on the relay that had positive continuity.
      The two fuses on PCB1/the transformer side, F1 and F2 > both show positive continuity.
    • I also measured all traces that have big white dot on them (there are 5 such dots, see 2nd picture). Two of those cover traces without continuity (red star). And three of them have continuity (green star).
    • The two fuses on PCB1/the transformer side, F1 and F2 > both show positive continuity.
    • LED test. I removed the PCB with the power LED from the bottom of the speaker and tested if the LED still works. Both Red and Green are fine.

    Considerations

    Since the relay clicks when 230v power goes directly to the PCB (instead of via the C7 in the foot), I wonder if there might be anything wrong on the PCB to which the JST connectors and the power LED are connected. I did a visual check if there might be shorts in the JST connectors, caused by remaining foam rot particles. But couldn’t see such.

    Questions

    1. I think I should measure if the PSU is generating all needed DC voltages. In the scheme I found: point a1 and a2 for + and – 50v, a7 for 20v, a3 and a4 for + and – 15v (see diagram B picture). Though, I have difficulty finding these points on the physical PCB. Where can I find them?
    2. If I make the measurements, should the JST connectors coming from the foot be connected to PCB02? Or can I do without these connections, and put the whole PSU/AMP block on my desk without these connections?
    3. Is this the right approach? Or should I focus on different things? Or does someone already know what is wrong? Please mention?

    All help is welcome!

     

    Schermafbeelding 2022-04-29 om 09.57.41

     

    PSU PCB - Traces & No Continuity measured b

    • This topic was modified 3 months, 1 week ago by KolfMAKER.
    • This topic was modified 3 months, 1 week ago by KolfMAKER.
    • This topic was modified 3 months, 1 week ago by KolfMAKER. Reason: Addition
    • This topic was modified 3 months, 1 week ago by KolfMAKER.
    • This topic was modified 3 months, 1 week ago by KolfMAKER. Reason: Addition
    • This topic was modified 3 months, 1 week ago by KolfMAKER. Reason: Addition
    #4601
    solderon29
    BRONZE Member
    • Topics Started 1
    • Total Posts 7

    It’s quite common for the standby supply transformer(T2)to fail,as it’s permanently connected to power.

    Check that you have the two standby supplies +15and -15 at the emitters of TR3 and Tr4.

    Check that there is about 23 volt dc at the relay coil.

    If none of the above are present,check for about 60 volt AC between P1-1 and P1-2 on pcb 01.

    If not present,then the transformer T2 has failed.This is a special part made “in house” by B&O,so you need to ensure that you order the correct one for your country.

    You can work on the amp/psu assembly by connecting ac power to P4 on pcb 01,but you should find the looms are just long enough to allow connection from the input sockets,if you prop something under the assembly when it’s out of the casing.

    It is possible for the relay RLI to stick open too.As you can see,only one set of contacts are employed.

    Nick

    • This reply was modified 3 months, 1 week ago by solderon29. Reason: typo
    • This reply was modified 3 months, 1 week ago by solderon29.
    #5250
    Beobuddy
    GOLD Member
    • Topics Started 0
    • Total Posts 12

    Looking at the picture, it is for sure that the T2/standby/small audio transformer has been replaced. On ebay they sell the wrong version. But I would start by measuring the voltages after T2 and it’s rectifier circuit.

     

    #5310
    KolfMAKER
    BRONZE Member
    • The Netherlands
    • Topics Started 9
    • Total Posts 51

    @Solderon29

    Thanks for your extensive response and guidance. This is very helpful!
    Some clarification questions from my side.

    1. For checking the 23v DC at the relay coil … I believe it has 6 pins. Which ones should I use to measure on?
    2. You mentioned it possible for the Relay RL1 to stick open. What should I (not) measure when it is stick open? And at which pins should I do the measurement?

    For reference, I added a picture showing the bottom of the PCB and the relay pins with numbers.PSU PCB - Relay RL1 Pins

    • This reply was modified 2 months, 2 weeks ago by KolfMAKER.
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