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Author Topic: SB-221 burnt choke, updates?  (Read 1287 times)
AJ4CU
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Posts: 75




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« on: June 15, 2008, 05:20:18 AM »

Hi All!

Repairing an SB-221 due to the rf choke on the grid having burned up, this is what I have done so far.

Replaced the capacitor bank with a harbach upgrade.
Replaced the .82 resistor on the meter board.
I am looking for the large stud mount zener diode heath part # 56-82 crosses to 1N3996A 5.1v 10w I am having little luck finding this...

Anyway I hear rumors about directly grounding the grids leaving out the chokes and caps, I have done a search and have found nothing related, is there anyone familiar with this mod?

The Tubes show no shorts so I am wondering what may have caused this?

Anything else I should be aware of?

Thanks for any and all help.

73  ES  CUL  DE  AJ4CU

Hari
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KB1LKR
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Posts: 1899




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« Reply #1 on: June 15, 2008, 03:25:45 PM »

W8JI has some material on his web site on 3-500Z grid grounding: http://www.w8ji.com/grid_resonance.htm Perhaps he'll pipe in here too.
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K0BG
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Posts: 9867


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« Reply #2 on: June 16, 2008, 05:51:32 AM »

First answer, Mouser has the zener. Second answer, hard ground them is the preferred method. The reference to www.w8ji.com is a good one.

Alan, KØBG
www.k0bg.com
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AI4NS
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« Reply #3 on: June 16, 2008, 03:24:04 PM »

If you get the RM-220 Replacement Rectifier/Metering Board from Harbach, it will have everything you need to fix the bias, losing the requirement for the zener. It also replaces the metering resistors with more 1% resistors to limit the voltage drop across each one. Better regulation, better metering, etc.

Mike
AI4NS
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W1QJ
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Posts: 1444




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« Reply #4 on: June 17, 2008, 07:43:10 AM »

Any number of things could cause the grid chokes to burn open.  Usually its a due to high grid current draw and the rf choke acts like a fuse.  This could be casued by a gassy tube burbing for a split second, so fast you don't notice it.  There could be other casuses, but basically boils down to the same problem high grid current draw.  It usually happens so fast.  Sometimes the .82 ohm grid resistor will open instead, it depends on if the problem was with the tube or something else.  Sometimes the choke and the .82 ohm resistor opens, sometimes one or the  other.  You can directly ground the grids and the amp will work just as well, but under certain fault  conditions you won't have the choke as a fuse, so hopefully the .82 resistors opens.  If the amp does not have any stability problems grounding the grids I don't think would help.  Sometimes it does help if you have stability problems, but I would assume mostly on 10 meters.  People who don't operate 10 meters with Sb-220's usually don't have many problems with them.  10 meters is rough on the Sb-220 amps along with many other simnilar type amps.  Most "cheaper" amps have a very high Q on 10 meters and the circualting currents are very high, mistune the amp on 10 and bad things happen qucikly.  I have probably autopsied more Sb-220's than most anyone.  The numbers are in the hundreds!  Statistically about 7 out of 10 Sb-220's autposied has at least one burnt bandwitch contact.  Usually the 10 meter one.  Since the switch has reduntant contacts the amp still works, but comes harder and harder to load up without problems.  If you blow the zener diode I would switch over to using 7 1N5408 1000piv 3A diodes in series in reverse bias in place of the zener.  It works fine and is a pretty cheap fix.  I do it all the time when needed.  The Harbach replacement rectifier board automatically does this for you if you buy one of them.  Lou
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