I'd add a comment to what Carl says: your tuneup procedure is
questionable. 10 watts isn't likely to give enough of an indication of
anything, and 100 watts is way to much to drive the SB200. If you really
need that much drive for a 500 watt output, then you need to check the
input circuits. Over the years, I've had to replace several of the coils
on several bands, not a difficult thing to do. Maybe Carl can explain why
the coils themselves go south; the only thing I can think of is that
somehow the powdered iron slugs get saturated. Every time I've diagnosed a
problem with the input circuit, and wound a new coil form, the problem was
solved. It's an easy matter to tune the coils for max. amp output for a
given input. This is something you should do with the existing coils
before putting in any new ones. That may solve your problem. My SB200
gives full output with about 50 watts of drive.
GL and 73,
> [Original Message]
> From: jeremy-ca <email@example.com>
> To: <firstname.lastname@example.org>; <email@example.com>
> Date: 3/16/2008 7:47:04 PM
> Subject: [!! SPAM] Re: [Amps] SB-200 constantly keyed after flash
> 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
> grids are OK and that the 200pf caps going from each grid to ground are
> 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
> 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
> The 572B is a triode so you have a filament, grid and plate and there are
> 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" <firstname.lastname@example.org>
> To: <email@example.com>
> 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
> > 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
> > will not switch back to receive. I did NOT try to xmit with it again. I
> > 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.
> > 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.
> > antenna is a G5RV, matched with a SA-2060 at 100 watts before turning
> > 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
> > 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
> > will be looking at the manual to see if this is correct. Even if it is,
> > may still be good to know why it's like this.
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