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Dead ONIX OA21 Power on LED

Discussion in 'd.i.y.' started by BrothersInArms, Sep 7, 2009.

  1. BrothersInArms

    BrothersInArms Active Member

    Hi chaps,

    The power on LED of my friend's Onix OA21 has died. He tried to replace the LED with an off the shelf 2V LED to no avail as the LED flashed and burned out within a second after turning the unit on. For now I presume he's obeyed the LED's polarity.

    Quick measuring in my OA21S revealed that the voltage at the LED's connection is 33V - way beyond off the shelf LEDs limits. In its vicinity there's some bulky resistor thing going on - maybe a current limiting resistor... I didn't check for that.

    Does anybody know which spec LED would work in this situation? Thanks!
  2. midcm1

    midcm1 pfm Member

  3. martin clark

    martin clark pinko bodger

    Without an LED in-circuit, you'll be measuring the open-circuit voltage - so 33v is quite reasonable. That voltage ahould collapse when the LED is in place, due to the impedance of the limiting resistor.

    Inserting an LED backwards at this sort of voltage will result in its instant death (they really cannot tolerate reverse voltage like other diodes)

    You really need to work-out if there is anything wrong with the limiting resistor, it's probably in the range 5K6-22Kohms. If so, pop in another anywhere in this range and try again. The voltage across a red LED will be about 1.8-1.9v , so you need a resistor that will pass, say, 3mA with (33-1.9)volts across it - 10K looks good, but anything in the range indicated will work fine.
  4. James Evans

    James Evans Bedroom Bodger

    My claymore had a 10k resistor in series with the red LED. It runs on the same DC voltage as the OA21, so that would seem a good place to start as Martin suggested.
  5. BrothersInArms

    BrothersInArms Active Member

    Measuring the voltage on the LED's connection of the faulty OA21 unit revealed just a few millivolts on it. After that we tried to check the limiting resistor and it seemed to have a "value" that my multimeter couldn't measure: >2000kOhms. Looks like this thing is knackered.

    We plan to overhaul the amp in the future concerning some coupling cap upgrades and so on and we'll look into this matter by then. Stripping the whole unit to cure just one resistor is a bit too laborious.

    One thing though:

    My friend's OA21 has a pre-amp to power-amp connection at the back of the unit, connected by an internally wired DIN plug. My OA21S doesn't have this and has a passive pre. My friend also mentioned the existence of another version, the OA21E or B?, mainly differing in the pre-amp layout. Does somebody know what this is about? Could the OA21 possibly have an active pre?
  6. James Evans

    James Evans Bedroom Bodger

    Do you have any photos? Does it look like this inside?


    That was mine. Under the pcb was marked OA21E. The connector on the rear is for the external SOAP psu in this case. Be interested to see photos of your friend's if different.


    The LED resistor may be measuring an odd value if still in circuit - try desoldering one leg and lifting it, or take it off the board completely.
  7. BrothersInArms

    BrothersInArms Active Member

    Nope, it's a different unit. I have photos, but not the opportunity to upload them.

    The phono-stage board of my friend's OA21 is longer and reaches almost from back to front. This board is inhabited with more than just the phono-stage, as the volume pot is connected to this board, rather than to what I'd call the main PCB (with power stage). The volume pot is also a different deal - two bare pots connected to each other, rather than a blue ALPS box. Your unit deals with RCA connections, my friend's unit concerns two DIN inputs, one DIN connector connecting pre to power and two RCA's of the phono input. It has also a connector for a SOAP or connecting the Onix tuner - not sure about that. The channel selector switch is the same.

    The inside of an OA21S is a bit more compact, with an upgraded power amp channel shortening circuit (Onix got rid of these fuses) and the channel selection is made at the PCB, instead of flying wires to a selector at the front panel.

    I'll try to mail my photos to you so you can upload them if you please.
  8. BrothersInArms

    BrothersInArms Active Member

    I managed to upload a few images.


  9. James Evans

    James Evans Bedroom Bodger

    It looks like the input switching is being done by the switch at top, rather than relays. The long section of board may well be an active pre, or could still be the MM/MC section. The power amp looks pretty much the same. There does look to be what could be a pre out wire link to the rectangular connector, along with the usual SOAP psu wiring, or it could be wiring for a separate pre psu supply from the soap - one regulated the other not perhaps. The pre regulator is attached to the pre board rather than under it.

    I'd email the photos to Tony Brady (the designer) at exposure for his input.


  10. BrothersInArms

    BrothersInArms Active Member

    At the back of the unit, of which I don't have a photo, there's a DIN connector concerning the connection between pre and power amp. This socket has to be plugged with an internally wired DIN connector. Obviously, if one removes this plug no sound is being produced. Maybe this thing is for connecting an external EQ of some sort, or connecting an external power amp.

    I remember Naim offered a mod for the Nait2 to convert it into a NAC72-ish pre amp, and they continue this feat to this day from Nait3 through the modern products.

    It has just three inputs: tuner, tape and disc. PLUS that pre-power "input".

    No need to look for a SOAP though, as we plan to remove the SOAP umbilical cord anyway IF my friend is planning to keep the unit. I removed it in my 21S to use the hole to fit a DIN input to match a Naim interlink.
  11. James Evans

    James Evans Bedroom Bodger

    ah, ok, makes sense re the pre/power din. Must have been to connect to an external power amp as you say. Never seen one of these old OA21s before.
  12. Andre

    Andre Member

    Can i ask do you guys get a thud on power up with the OA21? I've just changed all the relays but it still persists!!
  13. Elephantears

    Elephantears Trunkated Aesthete

    I've always had a thud on power up with my OA21s, and get the same with the 21 that I just found recently. I've read elsewhere that this is standard. I seem to think I also read that the early Naits made the same sound and some Exposure amps until recently.
  14. Andre

    Andre Member

    Ah that reassuring then, i'm just worried about the long term damage to the speakers, measuring the DC offset it's not that bad really but still worrying, i was wondering if the power switch was part of the issue as it's a bit cheap & nasty? My friends Rega turntable gives out a right old clunk when switched one with the decks nasty little switch.
  15. BrothersInArms

    BrothersInArms Active Member

    The manual learns that you should switch on all your sources before switching on the amp.;)

    My Onix is also not dead silent when switching on the turntable, especially when I dial into the turntable input first, before switching on the unit. No need to worry about that either. It's just a bit of electromechanical interference you're hearing.
  16. BrothersInArms

    BrothersInArms Active Member

    Yah, James.

    My friend now uses an external passive pre with that "power in" input. Plays beautifully!
  17. jason68

    jason68 New Member

    Slight hijacking of your discussion, but I'm trying to get into my OA20 (c1987 No.1248). Any chance you know which screws I need to undo? There are some stripped screws (it was repaired by Onix in 1992) but I'm hoping that it's not those..


  18. flatpopely

    flatpopely Prog Rock/Moderator

    IIRC its four hex bolts on the underside. The case then slides off.
  19. jason68

    jason68 New Member

    Great, thanks, just checked those and they look fine.
  20. jason68

    jason68 New Member

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