[Amps] SB-220 Troubleshooting Question

[Carl]

----- Original Message ----- 
From: "Mark Cherry" <macherry54 at gmail.com>
To: <Amps at contesting.com>
Sent: Sunday, February 05, 2012 10:17 AM
Subject: [Amps] SB-220 Troubleshooting Question


> Hi all.
>
>
>
> I have recently had my SB-220 end its useful life.  I keyed it for CW, it
> groaned, hummed and popped one of the two breakers!  During this event, it
> also took out one of the two power resistors of the Harbach Soft Start.



Thats fairly normal once a relay welds its contacts.



> I started 'shooting it in stages.  The filament xfmr and circuitry is
> functioning.  There are no shorts from the rectifier board through the 
> plate
> circuitry.  All diodes have been checked with an ohmmeter and look good, 
> as
> do the resistors on that same board.  The SCR for the bias voltage tests
> good as well with an ohmmeter.


Thats a zener diode


>
>
>
> The HV xfmr shows a secondary resistance of 56k ohms (fully disconnected)
> and from everything I have found, it should be about 12 ohms.


Cockpit error.


Without any
> load the HV xfmr gives about 850VAC running on 120V mains, for testing.


Normal


> When connected to the Cap bank (no load after that), it resumes to blowing
> the softstart power resistor.


Back to Step 1


>
>
>
> I have tested caps with ohmmeter only and know that loaded, they may still
> fail, but, my big concern right now is the 56k secondary resistance.


Thats impossible since you have HV.
A shorted cap will usually take out some diodes.


Disconnect and bypass the soft start. Remove the tubes. Wire the jumper 
board for 240 and plug into120. Place switch in CW.  Turn on and see what 
happens, hopefully you will have half voltage available and no smoke. A 
variac or a LV transformer would be better but if not just go with what you 
have and be prepared to quickly shut down. Sometimes another pair of hands 
ready to yank the plug is a help.

If the caps are original its time to replace them. I use CDE 381LX series 
105C from Mouser. They have 220 and 330uF 40mm diameter sizes that will fit 
the blocks perfectly. Use 100K 3W MOX for equalizers.

>
> I am looking for any hints or thoughts as to the direction I should take 
> or
> tests I could perform.

Just be careful.

Carl
KM1H

>
>
>
> Thanks in advance for any help and for your time.
>
>
>
>
>
> Mark
>
> WT6P - Kingman, AZ
>
> NA6E Memorial A.R.C.
>
> Arizona Outlaws Contest Club
>
>
>
> "Borrow money from a pessimist, they don't expect repayment"
>
>
>
>
>
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