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SB220 series 3-500 grid RF Chokes Sourcing




 
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Author Topic: SB220 series 3-500 grid RF Chokes Sourcing  (Read 462 times)
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ns7h
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« on: May 11, 2019, 10:15:55 PM »

Hello - I have had a failure in the Heathkit HL2200 with all the Harbach mods - with at least one release of smoke.  I am researching the source of the failure but do have as evidence a burned grid RFC, 1 mh, 4 stacked series coils (I think it is similar to a Hammond Engineering 1534A which is not made anymore.  I was not aware how rare these are since all the parts houses I use have this item as obsolete. With the large population of these and similar amplifiers and also burned out RFC's I would think someone has an alternate or has a reliable source.   I am monitoring ebay now for such a NOS unit and have the word out to local hams/hamfests.

Thanks for any help in pointing me towards one (or two).

Bob - NS7H

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KD6VXI
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« Reply #1 on: May 12, 2019, 10:28:15 AM »

Those typically act as fuses when you have a grid to filament short.  The 500Z does that, usually under operating temp.

Most people ditch those chokes because they are no bueno.  Makes bias yoyo up and down as well as the amp harder to drive.

Also, change the cut off method to resistor cut off.  Will protect the transformer in case of future shorts.

Ground the grids directly and drive level decreases and imd drops a few pct.  Even W8JI (supposedly behind the design according to some) advocates ditching the resistors in the grid circuit.

If you want mo protection, install 1 ohm 1 Watt fuse resistors.

--Shane
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DMOD
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« Reply #2 on: May 12, 2019, 02:14:58 PM »

Hello - I have had a failure in the Heathkit HL2200 with all the Harbach mods - with at least one release of smoke.  I am researching the source of the failure but do have as evidence a burned grid RFC, 1 mh, 4 stacked series coils (I think it is similar to a Hammond Engineering 1534A which is not made anymore.  I was not aware how rare these are since all the parts houses I use have this item as obsolete. With the large population of these and similar amplifiers and also burned out RFC's I would think someone has an alternate or has a reliable source.   I am monitoring ebay now for such a NOS unit and have the word out to local hams/hamfests.

Thanks for any help in pointing me towards one (or two).

Bob - NS7H


Hi Bob,

The first thing I would do is to get rid of all the Harbach junk, because you don't know if it was installed properly.

Secondly, remove all of the grid components and replace them with 1/4" copper strips directly to the closest ground point.

The third thing to do is to replace ZD1 with a 12V 50W unit, such as the NTE5254A.


Phil - AC0OB

 
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« Reply #3 on: May 13, 2019, 09:08:38 AM »

please add fil xfmr separate fuse .... I see a lot of burned up ones
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Beefus

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« Reply #4 on: May 13, 2019, 10:20:58 AM »

Beefus,

The fils xformer burning up is usually caused by grid to fil shorts.

Heath used a 110 volt winding on the fils xformer to bias the tubes into cutoff.

While I agree a fuse should be added, the cut off method should also be changed to using a 10k or other high value resistor in series with the center tap of the filament transformer.  Then use the PTT relay to short said resistor out of the circuit on key down.

Safe and sane.

--Shane
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