[Premium-Rx] Cubic R3030 Help

[GandalfG8 at aol.com]

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  ma
ins.
 
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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