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

[Mike O'Dell]

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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