Thats the first time I ever heard that a ~3KV dead short wouldnt blow
something in a SB-220. Do you have a reference to that?
First time I did that it took out the zener, several diodes, and the .82
Ohm resistor by the time a breaker popped. Luckily the Multimeter was
not in the grid position. Replaced parts and it ran fine for years as
the second radio contest amp.
The second time it blew almost every HV diode and again they were
replaced with higher rated ones, 1N5408's in this case. Finally, and not
trusting my gray matter at 3AM I just bent the bar down.
Never did have to replace a filter cap and that amp got beat on pretty
hard.
Also note that those original diodes are only rated at 600ma.
There is a difference in the failure depending if you are on 120 or
240V. On 240V the filament transformer was also at risk. Heath had a
Change Bulletin on that.
Carl
KM1H
----- Original Message -----
From: "Scott McGrath" <mcgrath@fas.harvard.edu>
To: <n1miw@cox.net>
Cc: <amps@contesting.com>
Sent: Thursday, February 12, 2009 1:45 PM
Subject: Re: [Amps] SB-220 troubles
> Bad caps - blown diode was your first clue HVPS is designed to take
> the
> interlock crowbar on the plate supply without failure. You could
> have
> a fresh internal short on the tubes as well. To test once you get
> new
> PS remove the tubes and bring up HV normally should be ok. Check
> the
> tubes with a ohmmeter to ensure you have no resistive paths anywhere
> but
> the filament if ok then reinstall in amp AFTER CHECKING HV WITH
> VOLTMETER TO MAKE SURE ITS REALLY NOT THERE!!! We don't need any
> accidental SK's time takes enough of us as it is. Then bring up HV
> but
> be ready to shut down QUICK
>
> Scott N1JIN
>
> n1miw@cox.net wrote:
>> I recently purchased an SB-220, and I went through it to see what
>> might have been changed/modified over the years before I powered it
>> up for the first time - in particular, the wiring change to the relay
>> to protect the filament transformer (which was done). When I felt
>> everything looked good enough for the first power-up, I plugged it in
>> and turned it on - without the perforated shield in place to open the
>> interlock. I ended up losing one of the diodes on the PS. It didn't
>> seem like too big a deal - I just replaced the diode.
>>
>> Upon firing it up again (interlock bypassed permanently now), I
>> had about 2600V in the tune position. I shut it off, put the covers
>> on, and connected it up to the rig to see what condition the tubes
>> were in. I was mostly concerned with 10 meters, but the output is
>> right around 1000 watts, so I believe the tubes are fine (weak tubes
>> have lower output on 10, right?). I shut the 220 off again to
>> reposition the amp better on the shelf, turned it on again, and heard
>> 3 loud snaps, and then 1 loud BANG. Little tendrils of smoke came out
>> of the case around the PS board area, so I took it apart to inspect
>> the damage.
>>
>> I saw the 3 resistors on the upper right side of the original
>> board were literally blown in half, chunks missing, etc. I now have a
>> new PS board on order.
>>
>> I'm curious why this happened AFTER I was able to see the amp
>> working, putting out strong RF on all bands? I want to believe that
>> when I first shorted out the PS via the interlock, I damaged that 1
>> diode, but weakened other components on the board. With that last
>> startup, the inrush was enough to blow the PS completely. Inrush
>> protection on order as well. Looking through the archives, I didn't
>> see anything relating to an interlock "mishap", so I'm hoping you
>> guys here can have an answer.
>>
>> This is what I get for being "slightly careless" with this amp,
>> and not thinking everything through before throwing the switch.
>> ...Larry N1MIW
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>
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