[Amps] SB 200 blown diodes in power supply

[Carl]

That is an easy assembly to fix yourself Kathy. However why did they blow? 
The usual cause is old original and leaking filter caps. Another possibility 
is a run away parasitic condition that exceeds the diode ratings before the 
internal breakers pop.

For repair you can use 1N4007's or go a step up to 1N5408's which will 
require the mounting holes to be drilled out a bit.

Those mounting style caps are no longer available except as high priced 
speciality items. The common Snap-In cap in 105*C rating can be adapted with 
a little work or if you are really creative you can restuff the old cans 
with them.

Also replace those heat generating equalizer resistors with 100K 3-5W metal 
oxide.

All done the PS will last another ~40 years and you can be proud of doing 
it.

Carl
KM1H


----- Original Message ----- 
From: "Kathy Bookmiller" <wb2aio at yahoo.com>
To: <amps at contesting.com>
Sent: Thursday, March 24, 2011 11:46 AM
Subject: [Amps] SB 200 blown diodes in power supply


>I know many of you probably consider a SB200 as a driver for your real 
>amps, but for us QRPers, it is as good as it gets:)
> I just blew a diode in HV chain. Now one friend I respect says I should 
> put in the Harbough board, soft start, etc. Trouble is, that's money I 
> don't wish to spend if I don't have to. Put in new board is easy to say 
> when you have money, but us poor underpaid nurses....
> Taking a look at mine, I noticed there are 3 diodes already replaced in 
> the two strings by placing them under neath or on the bottom of the board. 
> When I first looked at it, I thought they had been vaporized:), so this 
> isn't a new problem.
> I'm hoping to find something at Radio Shack I could use and just change 
> out the newly blown diode, maybe even using the trick the previous owner 
> employed of just putting in the new one underneath to save taking it apart 
> to get at the top of the board.
> So was wondering what you guys think or your experiences with the SB200. I 
> just acquired it last summer and otherwise has worked really well when 
> I've needed it.
> 73,
> Kathy, RN
>
>
>
>
>
>
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