Diode replacement

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falen5's picture
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Joined: Jan 1 2017
Posts: 58

Hello again folks.

Recapping another II powersupply

Noticed that D12 looks burnt and the PCB is burnt under it as well - so i want to replace it

As far as I can tell the markings on the diode are RGP 15B

I dont under these markings - from what i gather the 15 means 15 volts. A diode is basically a 'one way valve' isnt it?

so will any 15 volt diode do as a replacement

if so do i just get any diode that says 15 or higher?

again I am probally completely wrong......But I have the good sense to check with you good people first

thanks people

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speedyG's picture
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Joined: Nov 16 2011
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Re: Diode replacement

the 15 is rather not related to 15 Volt but rather more to 15 Ampere current....

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falen5's picture
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Re: Diode replacement

Just as well i asked SpeedyG

I found an exact replacement on ebay - will have to wait again

I thought I might be able to use a diode from something else. Ah sure!! paitence is a virtue

why would a diode burst. The one i am replacing ( after removing it) has burst underneath . The .1uf foil cap had exploded as usual.
Im thinking there may be some other problem, as this is the first diode I have found in a II power supply to have failed ( i have not powered up this psu. i am just assuming both components have failed.....as they both have physically broken open!!)

all other components appear to be ok.

Its just the .1uf foil cap and D12 diode that have failed. Would their demise be related to each other?

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Joined: May 27 2013
Posts: 794
Re: Diode replacement

falen5 wrote:

why would a diode burst.

Probably the reverse breakdown voltage. This is the specification of a diode that determines the largest reverse voltage that can be put into a diode without causing an increase in the leakage current in the diode. As long as the current is low enough, exceeding this breakdown voltage of the diode doesn't blow it out.

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Joined: Dec 4 2015
Posts: 43
Re: Diode replacement

DigiKey has them but a waiting period. Specs are general purpose, 1.5A 100V 150ns recovery. I think 15 marking is 150ns switching speed.
I would check R31 22 ohm next to it since they are paralleled and share current load. Could have been running hot since bad (open) caps caused
the output transistor Q2 to generate more power to compensate. You could substitute a 600V or 1KV 1.5A diode as long as the recovery time is <= 150ns.
Voltage rating is peak inverse voltage (PIV) which is max reverse voltage before diode junction breaks down. 100V PIV is uncommon nowadays.

Larry G

speedyG's picture
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Posts: 2407
Re: Diode replacement

One of the most reasons for such damages are caused by so called "Spikes"
in old communities with old electric installations ( and there are plenty such
communities out there) the electric companies saved a lot money in their installations
and refused to add better filtering in the local community setup.....
( refering to comunities where the electricity is from 1950 or earlier... )
in such comunities for example lightning strikes are not filtered well and
then in areas around 1 to 3 kilometers are filtered that bad that so called
spikes happen in the local electricity facilities.... and reach from the wall outlet
the powersuplies behind the wall outlet....
If the suplies of electricity is that old you may more often recognize such damaghes...
In such case it's a good recomendation to use between the power outlet at the wall
and your old computer a setup with modern spikefiltering capabilities...
( often called "overvoltageprotection" with very fast protection diodes )integrated inside the box that offers addtional cablepelugins )
speedyG

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Joined: May 27 2013
Posts: 794
Re: Diode replacement

Speedy is right.

My computers used to get lots of problems with the power supplies getting themselves messed up.

So I started using surge protectors instead of power strips.

And I connected all of my computers to Uninterruptable Power Supplies with the correct computer program running in the background so that my system is switched to battery power when the supply voltage is too high or low - the computer program can do this but Windows can't (Windows native power management).

speedyG's picture
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Re: Diode replacement

The story behind the posting is quite amazing....

i moved several years ago fom Munich to what you may call a "pre urban community"
about 20 km upfront of Munich (population of the village 20.000 )...

after that move several evidences leaded me to some investigation:
In summertime i had a radioclock that often became "reset" to 00:00 clock when i turned back home.... few months later i added a RAID5 system to my workspcecomputer at home and discovered bunches of entries in the log-file indicating the computer to have performed synchronisation......

after i discovered that i got curious to find what's happening in that village....
several months later i recieved a chance to view the main powerfaacility of that village....

( That's the facility where the main powersource of the entire village is transformed from
15000Volt to 1500 Volt by large transformers... and we found in that facility a bill for spare
parts demanding payment in "Reichs Mark" a currency that indicated that the major part of powerwires in that village originate from 1924 to 1948 ..... Nowadays i can confirm that the major power
installation in that village originate from short after WW1 ( 2924 till Black Friday 1932 !!!

That was the very day where i decided to add up modern filtering and powersource management devices between my workspace and every poweroutlet of the wall from local powersupplies....
ancient computers are very gratefull for achieving such precautions.... LOL

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....