Re: [Amps] L4B HV diodes

[craxd <craxd1@ezwv.com>]

Ian is correct. You might get lucky replacing just the one but if its 
values is different than the others, it will most likely lead to 
premature failure. Anytime I have one rectifier in a HV string bad, I 
replace them all. Even though the others may check good, they could have 
been weakened by the stress placed on them by the one failing. Then, in 
a short time, you'd be replacing the other anyhow. It's best to just go 
ahead and upgrade the whole board to 1N5408's. You know they're new and 
matched if they're off the same spool or band. To get really technical, 
you could check each with a hi-pot but that may be a little overkill for 
your application. It will most likely be hard to find those older 
rectifiers anyhow.

Will Matney



Will M wrote:

>>Not necessarily, you may get by with replacing the one diode with the
>>1N5408. I think the 1N5406 is the 600 V, 3 amp. They should all have a
>>drop of about 0.7 volts a piece. I only found one data sheet for this
>>on the web and it was a pay site. It's a wonder they didn't all fail

>from the domino effect. But, if you want to replace them all, get some

>>good solder wick and go at it. To find an original M4G5, you'd probably
>>to search about all the surplus houses. A Google search didn't find any
>>for sale anywhere. I still don't think it would cause too much of a
>>problem just replacing that one, if it's the only bad one in the
>>string. About the only way to tell for sure if they're good or not is
>>run a hi-pot test for leakage on each.
>>

As Will implies, it's not good practice to replace part of a diode
string with one of a completely different kind. It is almost guaranteed
to cause unequal division of reverse voltages, and will be fighting
against any equalizing Rs and Cs that are provided on the board.

The hardest part of the whole job is removing the rectifier board so you
can work on it. You don't want to have to repeat that, so why not
replace all the rectifiers with 1N5408s (new, branded and supplied on a
bandolier). They are reliable, readily available and excellent value,
which makes them the industry standard for this type of application.


-- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) 
http://www.ifwtech.co.uk/g3sek
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