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Author Topic: Kenwood TL-922A Filament Transformer or “dead” unit?  (Read 2458 times)
G3RZP
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Posts: 8123




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« Reply #15 on: October 25, 2017, 02:29:20 AM »

One problem with a small SMPS is that it has to start when loaded with the cold resistance of the filaments, which with a pair of parallel 3-500Z, is very low. You would probably need at least a 60 Amp supply, if not bigger.
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W1QJ
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« Reply #16 on: October 25, 2017, 03:14:14 AM »

Mark, the OP never stated why the filament transformer needs to be replaced.  We haven’t heard that part of the story.
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N1UK
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Posts: 2078




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« Reply #17 on: October 25, 2017, 06:22:57 AM »

Very true but it cant do any harm to add a fuse for protection in the future.  A nuisance trip is certainly a lot cheaper than replacing the transformer.


73 Mark N1UK
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K1ZJH
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« Reply #18 on: October 25, 2017, 07:11:15 AM »

Mark, the OP never stated why the filament transformer needs to be replaced.  We haven’t heard that part of the story.

Does it have the same problem as the SB-220 with a bias winding on the filament transformer?

Pete
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N1UK
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Posts: 2078




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« Reply #19 on: October 25, 2017, 09:34:55 AM »

On the 922 a grid to cathode tube short will short out the 80v filament transformer winding and after a while the transformer will overheat and be damaged.  I smelled mine and was able to turn off the power before any permanent damage occurred.   I think the circuit on the SB220 is similar.


Mark N1UK
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KM1H
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« Reply #20 on: October 25, 2017, 09:53:32 AM »

The early SB-220 had that problem only on 240V but a simple Service Bulletin corrected it.

Carl
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KK4BFN
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Posts: 13




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« Reply #21 on: October 28, 2017, 08:45:53 AM »

When you have the amp fixed you might want to consider adding a slow blow 1 amp fuse to the primary of the filament transformer. I also added the same fuse to the secondary 80V winding. This will protect the transformer from damage when you have a bad tube with a shorted grid to cathode.


Mark N1UK

Do you have any further info on this? I have a 922A that I just picked up recently, and would hate to lose it due to a bad tube taking out the xfmr.
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Kevin - KK4BFN

"No Matter Where You Go, There You Are..."
N1UK
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Posts: 2078




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« Reply #22 on: October 31, 2017, 04:01:57 PM »

I used a leaded axial 1 amp slow blow 250V fuse like this

https://www.ebay.com/itm/50pcs-1A-250V-Slow-Blow-Glass-Axial-Leaded-Fuse-3-6x10mm-1AMP/252257141495?hash=item3abbb282f7:g:dnEAAOSwI-BWQEu1


and mounted it as shown in the picture

https://photos.shutterfly.com/album/10179575982/full/13441374590

Remove the red wire from the transformer terminal and solder it onto the adjacent unused tag strip. Connect the fuse between the tag strip terminal and the now unused transformer terminal .   

73 Mark N1UK
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K6AER
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Posts: 4665




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« Reply #23 on: October 31, 2017, 05:01:53 PM »

One problem with a small SMPS is that it has to start when loaded with the cold resistance of the filaments, which with a pair of parallel 3-500Z, is very low. You would probably need at least a 60 Amp supply, if not bigger.

I just wired a 15 amp, 10 volt supply to a pair of 3-500Z's in series and they lit just fine.  The supply is 3 inches by 4 inches by 2 inches in size.
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W1QJ
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« Reply #24 on: October 31, 2017, 07:38:23 PM »

Why not just switch the amplifier over to “ self cut off bias” and be done with it?  Fixed the problem of burning up the 120vdc supply winding and is just a matter of switching some wiring on the bias relay.  Takes just minutes to do with no added parts.  The simple mod is explained and showed with a photo at W8JI.com. 
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G3RZP
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Posts: 8123




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« Reply #25 on: November 01, 2017, 05:23:42 AM »

I got a small 5 volt 15 amp supply: it wouldn't start on a 4-250A filament - which is the same as 3-500Z.
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N1UK
Member

Posts: 2078




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« Reply #26 on: November 01, 2017, 07:08:42 AM »

Quote
Why not just switch the amplifier over to “ self cut off bias” and be done with it?  Fixed the problem of burning up the 120vdc supply winding and is just a matter of switching some wiring on the bias relay.  Takes just minutes to do with no added parts.  The simple mod is explained and showed with a photo at W8JI.com. 

Yes you could do that as well.  I chose to add a fuse.  Either method will work.



Mark N1UK
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