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[Amps] SB-200 constantly keyed after flash

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Subject: [Amps] SB-200 constantly keyed after flash
From: "Larry" <>
Date: Sun, 16 Mar 2008 19:22:58 -0400
List-post: <">>
     OK, now I am starting to see why the seller got rid of this amp so
cheap. Anyways, I cannot seem to get the amp back to its idle state. I was
tuning up on 10 meters with a low power input (10 watts), everything seemed
fine, so I went to full power (100 watts), adjusted for the max output, then
BAM! 5 seconds later, I saw a flash from the top of the case, and the amp
will not switch back to receive. I did NOT try to xmit with it again. I do
not see any burn marks, no smoke, no little pieces anywhere.

     I have been troubleshooting all afternoon. I removed the softkey
module - no change. I checked all diodes and resistors by the antenna
relay - they appear fine. I checked the meter positions - in grid, the meter
moves BACKWARDS. Plate shows 70ma. Rel Pwr shows no deflection. HV is at
1900V (which is what it reads when keyed) - with the amp in idle before the
mishap, I would see 2250V. I also noticed one of the tubes has a blue color
on the top - almost looks like a haze. The other tube does not do this. I
even disconnected one of the coil wires to the relay so it wouldn't transfer
when I powered up, and I'm still showing the same readings as before. The
antenna is a G5RV, matched with a SA-2060 at 100 watts before turning on the
amplifier. Frequency was 29.500 FM from an FT-847. I also remember that
while tuning, the wattmeter stopped increasing on the 2060 at ~50 watts
input from the rig with ~250 watts output to a perfect match. The tubes
are/were 3 weeks old - matched pair. Not sure if this makes a difference as
to what went wrong. I was seeing ~600 watts on 80 meters earlier. Hoping
someone can help. 73's... Larry N1MIW/AG

PS - I haven't looked at the schematic yet, but why does the tube on the
left, when looking down at the top, only have 3 of the 4 pins connected? I
will be looking at the manual to see if this is correct. Even if it is, it
may still be good to know why it's like this.

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