[Amps] SB220 step start blows fuses

Jim Thomson jim.thom at telus.net
Thu Jul 6 16:42:56 EDT 2017

Date: Thu, 6 Jul 2017 00:07:50 -0600
From: Warren Volz <warren at warrenvolz.com>
To: amps <amps at contesting.com>
Subject: Re: [Amps] SB220 step start blows fuses

<Without tubes and with the HV transformer unplugged, the fan and meter lights work. In addition, the step-start fuses don?t blow!

<That leads me to suspect the tubes + HV supply. I checked the windings on the HV transformer to look for shorts:

BLK-YEL = .7 Ohm
BLKGRN - BLU = .8 Ohm
BLU - BLKRED = 0 Ohm

Do any of those look off?

Also, would these steps (or something similar) make sense?

1) Re-enable HV supply, power on, if fuses are ok power off and go to step 2
2) Once HV dissipates, add tube 1, if fuses are ok, power off and go to step 3
3) Assume tube 2 is shorted, else tube 1 is bad


###  With  NOTHING  connected to the SEC winding of the plate xfmr,  you should see  aprx
9-10 ohms of dc resistance.    You could also hook it all back up, then, with tubes REMOVED
from sockets...OR disconnect the B+ from the top of each tube..... try it again.    If normal B+  appears,
its one or both tubes.   You really need to isolate stuff.   IF  plate xfmr is good, the trbl could be in the 
FW Doubler  diodes.   IF diodes are ok,  it could be the 8 x filter caps, or eq resistors across each filter cap.
It could be a bad HV  interlock assy,  it could be the plate choke assy, it could be anything..including a 
internal shorted, or partially shorted  plate xfmr.   Or  any HV inter connecting wiring between  any of the above
components.   Or it could be your new  step start assy itself. 

Jim  VE7RF   

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