[Johnson] Johnson Adventurer question

[David C. Hallam]

I don't know the specs on your transformer, but if it was a choke input as
Johnson used on most of their power supplies you need about 1500 VAC CT to
get 650 VDC in a full wave center tap configuration.  650 volts seems to be
pushing a 5U4 rather hard.  Not having a schematic I can't say but are you
sure the tube is not supposed to be a 5R4?

I don't think you will find a 700V electrolytic.  You will have to series a
couple of 350 or 450V capacitors to get 700V.

David
KC2JD/4

> -----Original Message-----
> From: johnson-bounces at mailman.qth.net
> [mailto:johnson-bounces at mailman.qth.net]On Behalf Of Weiss
> Sent: Tuesday, July 17, 2007 4:14 PM
> To: johnson at mailman.qth.net
> Subject: [Johnson] Johnson Adventurer question
>
>
>      A few nights ago one of the 8 mfd capacitors in my Johnson
> Adventurer developed a short. In so doing, it burned the power
> transformer to a crisp before I could yank the power cord out. In
> a macabre humor sort of way, just as I grabbed hold of the line
> cord amidst the thick plume of smoke, the fuse opened (It would
> have been nice if the fuse had opened a bit earlier to protect
> the secondary of the transformer but such things don't seem to
> work that way although you can bet I am adding a fuse to the
> secondary of the high voltage winding.
>
>      Anyway, does anyone know what the original Adventurer power
> transformer voltages are? Oddly my Adventurer manual is lacking
> in any voltage information except for the 650 DC it says belongs
> on the 807 plate. Obviously I realize there is a 5 volt rectifier
> filament winding and a 6.3 volt filament winding. The RCA Tube
> Manual states the plates of the 5U4 run "between 350 and 500
> volts." I jury-rigged an old TV transformer that creates only 600
> volts across its secondary for a 5U4 plate voltage of around 300
> volts for each of the rectifier's plates. It is not enough
> horsepower to pump the Adventurer output up to its previous 28
> watts RF output with 110 ma on its 807's plate. I am thinking the
> original transformer must have been a 800 or possibly 1,000 volt
> secondary or so. The original transformer is stamped "P2758" but
> I have been unable to locate anything in any of the online
> Thordarson or Stancor lookups. It might not even be a Thord or
> Stancor transformer.
>
>      Also, does anyone know where you can find "700 volt"
> electrolytics these days? 8 mfd at 450 WVDC and even 500 WVDC are
> easy to find but the 700 volt caps are a challenge.
>
>      Thanks for any help.
>
>
>
>
>
>
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