[Premium-Rx] Cubic R3030 Help

Mon Dec 1 08:33:35 EST 2008

I know what caused it to fail that dumm kopf Lee forgot to set the S1 switch  110V/220V to 220V BTW the device
is 275V and so is the other. It looks I'll have to repair the pcb the little swine would not come out.
  ----- Original Message ----- 
  From: GandalfG8 at aol.com 
  To: premium-rx at mailman.qth.net 
  Sent: Monday, December 01, 2008 11:11 PM
  Subject: Re: [Premium-Rx] Cubic R3030 Help

  In a message dated 01/12/2008 11:32:59 GMT Standard Time, ufp at optusnet.com.au writes:
    Well I managed to blow the side off a component in my power supply. I have have never used one these before.
    There are two of them, one across the ac input the  other across dc side of the bridge rectifer. This CR12
    right under switch S1 What is it? I need to replace it and I don't know what to order. Is this what they use in place of a fuse?
    Why didn't they have a fuse?
  ----------------------
  Hi Lee

  CR14, across the mains input, and CR12 across the bridge rectifier are metal oxide varistors.
  They are there to absorb transients introduced via the mains supply and do not take the place of fuses.

  An MOV has a non linear voltage current characteristic and functions by lowering it's resistance as the voltage increases above the design threshold for the device, thereby absorbing power from the transient.

  Whilst this is ok for short transients it does mean problems can arise when over voltage conditions are prolonged or transients are particularly severe, as the reduced resistance and increased voltage over a longer time means the device will dissipates significantly more power and can overheat to the point of catching fire or exploding if the energy surge is rapid.

  That's the theory, in practice it's not unknown for such devices to fail and self destruct under seemingly normal operating or switch on conditions.
  What you decribe happened to me on one of my 3030s very soon after purchase but I was lucky enough to get the PSU swapped out so didn't investigate further.
  I say "seemingly normal" but other fault conditions might have been present that I wasn't aware of.

  One thing you must be aware of though is that the Cubic PSU is a switch mode device with no mains input transformer. All this input area is live to the mains.

  If an MOV fails to the point of significant destruction it's likely to end up as an open circuit, but this does depend on how much has been blown away and the severity of the energy release and I suppose it's possible at least that the leads could fuse resulting in a permanent short.

  Anyway, if either MOV is removed from the circuit the PSU should function without it.
  I'm not recommending this long term but, because it does just normally sit across the supply doing not a lot, it's presence isn't critical to the operation of the PSU.
  If you're lucky, it might just have been a faulty MOV that's self destructed at "normal" voltage levels.
  However, if after removing the remains and checking for any track damage or left over deposits on the PCB, the unit does not function as normal, then you have other problems with the PSU.

  It should really be tested outside of the receiver rather than risk incorrect voltage being passed into the set but please DO be aware of that lack of transformer isolation, especially if you're in an area with 230 volt mains supplies !!!
  Ideally an isolating transformer should be used but, if you have any doubts at all about what you're doing, just get it checked by someone who knows.

  Death by electricity can be very uncomfortable and has often been known to prove fatal.

  Assuming you have been lucky, and all seems to be working OK without the MOV, then you only need to source a replacement.
  The manual shows these as 430 volt devices, which should be fine, but it doesn't indicate energy or current capacity.
  Because of the nature of the beast size is probably going to be a reasonable indication of the latter so just go for a 430 volt device of similar size.
  Using one smaller shouldn't matter too much as long as not excessively smaller, it probably won't have the same capacity to absorb energy but should still do the job.

  regards

  Nigel
  GM8PZR

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