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Re: [Amps] SB-200 constantly keyed after flash

To: <>, <>
Subject: Re: [Amps] SB-200 constantly keyed after flash
From: "jeremy-ca" <>
Date: Sun, 16 Mar 2008 19:46:35 -0400
List-post: <">>
Well, you have at least one blown tube, the flash being from inside it.

Remove the tube with the auroral glow and check things again.

But first with both tubes removed check that you have -125VDC at the tube 
grid pins in standby. Also be sure that both 33 Ohm resistors going to the 
grids are OK and that the 200pf caps going from each grid to ground are not 
blown apart.

You will also most likely have one or both parasitic suppressors either 
blown or the resistors have gone way up in value. Defective suppressors and 
excessive (long) 10M tuneup will blow things every time, especially with 
some Chinese tubes.

If you look at the schematic you will be able to see why that pin is empty. 
The 572B is a triode so you have a filament, grid and plate and there are 4 
pins plus a plate cap. Voila, that pin isnt used. On the other tube it is 
simply a convenient tie point for the input power lead from the switch.


----- Original Message ----- 
From: "Larry" <>
To: <>
Sent: Sunday, March 16, 2008 7:22 PM
Subject: [Amps] SB-200 constantly keyed after flash

>     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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