[Amps] SB220 step start blows fuses
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
BLK-BLKYEL = .6 Ohm
BLKYEL-YEL = 0 Ohm
BLKGRN - BLU = .8 Ohm
BLKGRN - BLKRED = .6 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.
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