Metronic Dualis 441350 R108 value

[deltatango]

Trying to fix a Dualis box which has a burnt out R108 resistor on its power supply which feeds a zener diode.

Seems to be known about, have tried to email (France) Metronic but they remain silent on this issue, so can anyone help me please.

Have fixed the rest of the supply though, suprised to find 85deg caps used in a switch mode supply which explains why a couple had puffed up, so a lot a better now with 105deg types and low esr.

Many thanks from David

 

[deltatango]

Well for now I have settled on 470ohm as a suitable value (one day I will find out its true value).

This was chosen by using a variable resistor of under 1K and checking the current through the zener, up to 7mA caused its power supply no problem in holding the feed voltage to it.
While 13mA started to make it fall fast, given this zener is 1Watt rated I thought it must pass a reasonable current so in true technical style chose 10mA as a round figure which is also a good level which ensures it is at a stable voltage point of operating. Checking that its feed supply voltage had only dropped a few hundred millivolts at this point seemed reasonable (you don't get owt for nowt).

Checking the resistor value with a meter which came to near 470ohm @ 50mW, so I hope this helps anyone else fix their unit, please note this is not the official value so any repairs are at your own risk (Just covering the bases).

David

 

[PaulR]

Thanks for posting your results.

As you might have guessed no-one was able to help you but your post may be able to help someone else.

 

[deltatango]

Sadly Metronic are sticking to their guns and will not say what its value is, I will give one last try but I doubt they will answer.

That said I think my chosen value will not harm it in any way, manufacturers I have seen though can be more cruel to their products, leaving components to overheat with no heatsinks etc, (thats my gripe over) to save money.

I will continue to seek an answer (when its no longer needed anymore) to this question just because it irritates me not to know.

Best regards to all for now from David

 

[Glinfilm]

Well for now I have settled on 470ohm as a suitable value (one day I will find out its true value).

Dear David,
I'm French, and this is the 1st post of my life, excuse if protocols aren't all respected !
As Paul said and at last, your post may have been helpful to someone, though you were expecting the other way ! You seem to be my saver : my Dualis just burned off a few days ago, R108 grilled and unreadable and what I called condensator beside (EC100) exploded, paper everywhere.
I read on its remaining cap 47µF 50V 85°, could you confirm condensator or Zener diode ? Is it polarised ?
I hope your 470 ohms resistor fixed the problem. I'll wait for an answer for a while before trying.
You made exactly what I thought about though I haven't much knowledge in electronics and no equipment for... Your test was surely more professional and safe !

I of course tried to ask Metronic as well, but their interest is in making themselves the reparation, or selling another receiver... They won't give an help, much less to a Frenchman !
I already ordered a Terbox (TV addiction...;-) and we shall have to afford special receivers anyway in a few month here) but I'd like to fix it just because I don't like to dispose of things that can be repaired, especially when it's less than 3 years old !

Thanks again, hope you'll get this message. Don't know if posts are also going automatically on your mailbox

[fieldset=Admin]posting of email addresses is not allowed as per the rules[/fieldset]

Cheers,
Eric

 

[deltatango]

Ok in my case the zener diode had shorted out which then burned out R108.

The zener (glass bulb item near to R108) thats fitted has a 1Nxxxx type code and I think (memory fades) was a 30 or 33volt at 1.3W range, I mention this because that capacitor that popped must have got very hot.
This takes place if the voltage or ripple current through it becomes higher than it can tolerate so one thought is this zener diode may be open circuit (o/c) because it would consume some of this extra energy. If one end is unsoldered and an analogue meter (not digital) on high ohms scale you should only get a reading in one direction only, if you get none, or in both directions then it is faulty. (This is based on most meters not able to test with above 22.5V ( old AVO meters) on high ohms scale).

If you are still with me, then that capacitor is polarised and I tend to prefer fitting 105deg type capacitors instead of 85deg as they are more durable.

Now we get complicated.......

Many items we buy that work from mains used switch mode power supplies, fine no problem with that, but they used to be fitted with quality type capacitors that have "low ESR" (able to stand high action without getting hot inside). Now it seems to save money they fit them only in critical places and wonder why power supplies fail after a few years!
As these capacitors fail, they are unable to remove spikes and ripples which then inflict damage to other parts.

I would look at the other capacitors on that board and check for top swellings, they should be nice and flat, but bow upwards when failing, some also ooze their paste from below but this is harder to spot.

If you do have any that have swelled then change them for 105deg Low ESR types and also change the one (I think) at the transformer side or near the mains area which would be a 10,22,33 or 47uF mostly, as this controls critical settings on the power supply activity.

Its been a while to remember exactly what I did, if need be I will take the top off of mine to recall.

Please be careful with mains present, you can fire up the supply without it being connected to its main board and should get a 30+ volt a 12v and two or so at 5volt and maybe one at 5.6volt (I may be slightly in error as many other things have cropped up so things blur a bit over time)

Sing out if you need me again and please take care..

Regards from deltatango

 

[Glinfilm]

Hi Dave,

I keep in touch to let you know. Just replaced yesterday night the resistor as well as the capacitor exploded and 2 or 3 others that tops showed wills to blow.
Unfortunately the Dualis didn't start again.
I read your thead again and I think I will unsoldered the Zener you talk about to check it. Will have to wait longer as I must find the time to do it then (if it is the problem) to go afford a new one.
Cross the fingers ! Cheers, take care.

 

[deltatango]

With the exploded and problem capacitors I am concerned that either a short circuit may exist on the secondary side (low voltage side) or as may be possible a blown primary side (mains area) which tend to destroy several parts or some power supplies, but I have seen seem also go quietly (just the driver/output chip or mosfet) leaving fuse etc intact.

It is hard to explain in simple terms faults that are favourite with these power supply types but I will take a picture of my units supply and add in some tips to help later on.

 

[deltatango]

I have uploaded a picture of my power supply to help with identification, I still worry that your caps popped which is a sign of heat or distress, these supplies can generate spikes when their capacitors fail.
I would first check the 18volt zener with an ohm meter and see if it is short circuit, make sure mains power is removed for a few minutes first. If it is shorted then your IC has also blown, check also the resistor to its right (current Limiter sense).
Change the critical capacitors and the primary one these hold the key to its correct start functions.
Another favourite fault of these type of supplies is the large mains capacitor or the two resistors for open circuit, which will give no smooth DC voltage or no kick start.
If you have not changed the Primary capacitor 1000uf I would do so for a 105deg low ESR type this is important as it provides "feedback" to tell the chip data it needs to function in a stable manner via the smaller parts on the right side of its transformer.

I hope this helps regards from deltatango.