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Author Topic: SB-221 No power out on 80 meter. Where do I start?  (Read 365 times)
KB0PJR
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« on: November 07, 2008, 07:36:59 PM »

80 meter band moving tonight.. Turned on SB-221 amp as I have in past. Keyed up into B&W 1500 watt dummy load with Icom 756 ProIII set at 3.875- zero power out. Plate amp meter excessively high, .8,.9 1.0 dial RF power back to .3 adjust load/tune/load tune, plate voltage .12kv, grid current 120mA, load/tune/load tune. Powermaster 3kw meter reads zero power out. So I check 40m and get 1100 watt out, 20m gives 1035 watt out, 10m 800 watt out to dummy load..... Where do I start looking for problems and what am I looking for? Thanks.....Larry Andrew W0LEA
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K6SI
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« Reply #1 on: November 07, 2008, 07:44:50 PM »

Appears you are not in resonance at 80 meter, why high plate current, no output.

Could be bad section contact on 80m band switch. Open part of 80 tank coil.

You need to be sure you are getting 80 m drive into the cathodes of the amp tube... not 80 meter drive, no signal for output to tune on.

Do you get response on grid current when you drive, not-drive, from the Icom..... maybe no drive into amp.

Don't run it very long on non resonance output, or you will cook the tubes.
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KB0PJR
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« Reply #2 on: November 07, 2008, 08:18:29 PM »

Do you get response on grid current when you drive, not-drive, from the Icom..... maybe no drive into amp.

Yes.
No drive via key radio plate amp zero/grid current zero.
When I drive via key radio plate amp .3, grid current 1100 kv. Meters will rise & lower as I turn up/down RF power button on radio.
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KB0PJR
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« Reply #3 on: November 07, 2008, 11:15:09 PM »

I find the band switch is toasted. Now to find a new one!
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W8JI
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« Reply #4 on: November 08, 2008, 03:48:11 AM »

I think I still have some wafers around here that will work if you get in a bind finding one.
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KB9CRY
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« Reply #5 on: November 08, 2008, 07:50:53 AM »

Harbach
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KB0PJR
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« Reply #6 on: November 08, 2008, 09:42:46 AM »

Thanks for the offer!  I see that Jeff at Harbach will be handling replacements shortly so I
will source there first. Thanks again.

73
Larry Andrew W0LEA
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WB2WIK
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« Reply #7 on: November 09, 2008, 11:07:03 AM »

>RE: SB-221 No power out on 80 meter. Where do I st       Reply
by KB0PJR on November 7, 2008    Mail this to a friend!

No drive via key radio plate amp zero/grid current zero.
When I drive via key radio plate amp .3, grid current 1100 kv.<

::1100 kv is surely a lot of grid current!

;-)

WB2WIK/6
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KB0PJR
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« Reply #8 on: November 11, 2008, 07:29:55 AM »

Update!
Dis-assembled coils and bandswitch assy. Switch contacts not arced. They were dark/tarnished and looked burnt but were not burnt. Cleaned and adjusted for better contacts. As I re-assambled I noticed wire lead at lowest solder joint on vertical coil was "floating" within contact area with no solder attached. Ahaaaa! There was the arcing issue. Must have been poor solder job to start with. Resoldered and reassembled. Tested. No power out on any band.............Damn.........I had power out on 40m,20m,15m and 10m prior to tearing down. Back to the manual....trouble shooting. No power out....check bandsitch reversed. But I did not unsolder switch. How can that be? After inspection... I had re assembled the switch "out of time" or clocked 180 degrees on the shaft.Soooo pulled all apart again and reset switch to proper position. Finished all above last night at 1:00 AM. We will smoke test tonight!!!!!!!!
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