[AMPS] SB220 Failure-More info

[Denis]

Dennis Ashworth wrote:
> 
> To answer a couple of questions about the posting below:
> 
> Yes, the step start does engage the HV circuit ... the HV transformer is
> current limited for a 3 seconds by a resistor in the primary. The failure
> occurred AFTER the resistor in the primary was bypassed by a time delay
> relay. Everything seemed normal up to that point.
> 
> RF drive was not applied. Plus, the amp has never shown an inclination to
> oscillate.
> 
> Upon closer inspection, R3 (looks like B- to ground) is burned open.
> Obviously too much current going somewhere. The capacitor bank appears
> okay visually, and with an ohmmeter, but this says nothing about what
> happens with working voltage applied.
> 
> Ideas? Other things to try before taking a wild guess and assuming it's
> the capacitor bank?
> 
> Dennis, K7FL
> 
> ==== begin original message ====
> 
> Hi Folks:
> 
> I finally got around to wiring the shack in our new home for 220VAC, and
> plugged in my vintage SB220 for 48 hours of CW contesting. After the
> step-start circuit disengaged, there was a load pop and flash from the
> high voltage section behind the meters. The amp had been off for six
> months. I checked the
> AC mains, and found 245V! The old QTH had 220-225V ... and the amp had
> run flawlessly at this voltage for 15 years. My theory (and I am asking
> your opinion) is that the old filter caps probably decided the 10%+ extra
> voltage was too much! BTW, the diode board was replaced last year with
> one of Harbach's.
> 
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Hi Dennis, this is how I would trouble shoot the PS.  I would disconnect
the capacitor from the diode bridge and turn the amp on and see if the
transformer and bridge are OK. If they are I would shut the amp. off,
discharge everything and connect the diode doubler to the capacitor with
two 1 amp fuses. I would  disconnect the load ( the lead going from the
+ side of the power supply to the amplifier.
Hands clear and turn it on again. Check to see if the fuses are blowing.
This will check out if the Power supply is good. 

If the fuse(s) are blowing the problem is in the capacitors. 

If everything is OK, connect the HV + to the load but take out both
3-500z. If everything is OK, insert one tube and test by loading the
amp. This way you can check each tube out. 

Look for simple things like heat on wire ends. Just move each diode lead
a bit and each capacitor lead a bit. Blow it out with compressed air if
you can.  It is usually the simple things.  Let me know how you make out

DEnis VE6AQ



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