[AMPS] SB220 Failure

[km1h@juno.com]

On Sat, 28 Nov 1998 01:55:04 -0800 Rich Measures <measures@vc.net>
writes:
>
>>
>>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.
>
>Why speculate?  In less than five minutes, you could be looking at the 
>
>electrolytics'  vent holes.  If you see white deposits, the caps are 
>history.  While you have it open, to reduce filter cap. temperature, 
>the 
>eight 30k 7w resistors should be beplaced with Matsushita 3w 100k ohm 
>mof 
>resistors, or something of equal quality.   

BAD advice IMO. The caps should be checked with the original resistors
first for voltage drop. A good string will show a 10-20V variation at the
most. Replacing resistors across well aged caps could be asking for a
wide voltage variation and more failures. 

73  Carl  KM1H



.  If the caps are ok, I 
>would check the 0.82 ohm grid current shunt R.  If the resistor is 
>damaged, either a tube is gassy or an intermittent vhf parasite took 
>place.  .  If the resistance of either one of the vhf supp. resistors 
>is 
>more than 50% too high, a parasitic is indicated.   
>>
>-  later, Dennis
>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  
>
>
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