duh, why don't it work -- another cadaver on the slab

[Mike O'Dell]

D'OH!

I get it now. It's a full-wave rectifier without a center-tapped transformer
and only a half-bridge of diodes. The "center tap" is exactly the common
between the two stacked filters and the two legs of the half-bridge put
+400 on the top cap and -400 on the bottom cap through the ground
connection between the bottom of the bottom cap and the "vertical"
leg of the half-bridge.  Add the voltage and presto! 800VDC.

the fact the transformer center-tap can be interchanged with a
capacitor center-tap is not a duality which had occurred to me.
i don't normally think of capacitors having a "center tap", but
the same duality happens in oscillators, too: Colpitts vs Hartley.

     -mo


On 3/16/13 3:55 PM, Mike O'Dell wrote:
> My brother just picked up a Kenwood TS-820S for beer money
> but it was missing the three tubes (12BY7 driver and 2 6146 finals).
> He gently tried firing up the power supply while waiting for the
> mail-order tubes and was greeted by smoke from the power transformer.
> he pulled the transformer loose from everything else, powered it back up
> and was rewarded by no smoke and good voltages on all the windings.
> (He scored a replacement at a reasonable price anyway.)
>
> moving on with diagnosis, it turns out someone seems to have tried
> replacing the Plate HV electrolytics before but managed to wire them
> in some manner other than shown on the schematic. he's still checking
> caps and diodes and such now.
>
> I admit that the Plate HV supply is somewhat puzzling. you can see
> the schematic at <http://www.k4eaa.com/820_main.pdf>. It looks
> like a half-bridge fed by the 800V winding top end, but then the
> filter caps are stacked (with equalizers) in series with the "center-tap"
> between the two caps returning to the bottom end of the 800V winding.
>
> I understand stacking the 100mfd 500V caps to get 50mfd at 1000V
> to withstand the 800VDC output, but i do *not* understand the
> bottom end of the winding going to the center-tap of the capacitors
> instead of just going to ground. When I first looked at it, I thought
> it was a voltage-doubler, but the finals want 800VDC on the
> plates and the transformer has an 800V winding.
>
> can someone please explain what i'm obviously missing in this picture?
>
>     -mo
>
>
>
>
> On 3/16/13 2:54 PM, Richard O'Neill wrote:
>> On 3/16/2013 1:48 PM, Alex Fraser wrote:
>>> When you buy stuff used (and cheap) you assume the worse I guess.
>>
>>  Yep, like many ham fest guarantees, "Well, it worked the last time I 
>> plugged it in." ... "of course that was thirty years ago." .... he 
>> thinks to himself.
>>
>>
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