Im not a fan of that HV board and recommend rebuilding the original with
1N5408's (drill out the holes) and adding the 1N5408 bias diodes on perf
board. The metering resistors can be 5M 7.5KV Ohmites. The caps can be CDE
330 uF 450V 381LX series.
Carl
KM1H
----- Original Message -----
From: "Bill VanAlstyne W5WVO" <w5wvo@cybermesa.net>
To: "[Amps]" <amps@contesting.com>
Sent: Tuesday, August 10, 2010 4:43 PM
Subject: [Amps] Replacing bias string diodes in Harbach rectifier board
>I had a bit of an accident with my converted 6m SB-220 (flashover somewhere
>while transmitting, not sure where yet) that resulted in all the rectifiers
>in the operating bias string (Harbach rectifier board) being blown short.
>The diodes that were in there are 251Ds. They are 2.5A 1KV diodes -- pretty
>decent. The original Harbach diodes were supposedly 1N4005s, which are not
>as beefy as the 251Ds.
>
> I don't have any rectifier diodes of this size in my parts bin at the
> moment. What I do have is quite a few new Fairchild 1N5408s. These diodes
> won't physically fit on the PCB (they are the same diode used in the HV
> string, but not in the bias string) but I can easily just wire 8 of them
> together in series and cobble it up ugly-wise. They are silicon diodes, so
> I would THINK the voltage drop across them would be the same as the
> 251D/1N4005. Please advise if using the 5408s in the bias supply is NOT a
> good idea for some reason.
>
> This is the second time the bias supply on the Harbach board has blown up
> when something else went wrong. Maybe the original Heath zener wasn't such
> a bad idea after all. With the Harbach diode string design, I'm thinking
> maybe I ought to put a fuse in there. I'm not sure what a safe fuse
> current rating would be -- something that would absorb any surge/peak
> current but would still protect the diodes in case of a short. Ideas?
>
> Bill W5WVO
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