[R-390] The Penultimate R-390* Ballast Replacement

[Bob Camp]

Hi

The tube swap process works fine as long as all of your radios have the 
same mod in them. The problem comes when you blindly swap modules 
between a ballast tube radio and a 12 volt filament radio.

	Take Care

		Bob Camp
		KB8TQ


On Feb 13, 2005, at 4:28 PM, Bruce Hagen wrote:

> I find myself somewhat mystified with all of the ballast tube 
> solutions.
> Most very clever and well thought out but were am I missing it? It 
> seems to
> me that an inch or so of wire bent into a "U" shape for the ballast 
> and then
> pulling two 6.3 volt tubes and replacing them with two readily 
> available and
> cheap 12.6 volt tubes is the logical and easiest solution if you do 
> not want
> to spend a few dollars and buy a 3TF7. Bruce
>
>
> ----- Original Message -----
> From: <ToddRoberts2001 at aol.com>
> To: <r-390 at mailman.qth.net>
> Sent: Sunday, February 13, 2005 9:54 AM
> Subject: Re: [R-390] The Penultimate R-390* Ballast Replacement
>
>
>> Most of the 3TF7 substitutes I have read about involve buildng 
>> something
>> small enough to plug directly into the 3TF7 socket or on a nearby 
>> bracket.
>> This
>> comes with problems of heat dissipation or else installing unsightly
>> brackets
>> near the I.F. subchassis. Some of the recent ideas got me thinking - 
>> Why
>> not
>> build a small separate power supply module with a
>> well-regulated/filtered/bypassed 12.6 VDC output that could be placed 
>> next
>> to the receiver and the only
>> connection would be a small umbilical cable with a 9-pin plug that 
>> plugs
>> directly
>> into the 3TF7 socket? You could run the umbilical through the side of 
>> the
>> R390A
>> chassis thru one of the large holes and tuck the power supply and 
>> cord out
>> of
>> the way next to the receiver. This layout is similar to the way some
>> audiophile preamps use a separate power-supply module with an 
>> umbilical.
>> You could
>> modify one or two pins of the 3TF7 socket with a jumper to ground to
>> provide a
>> ground return for the 12.6VDC so you wouldn't have to tie down a 
>> separate
>> ground
>> lead with a terminal and screw somewhere else on the chassis. When you
>> plug
>> in the umbilical it breaks the 25.2 VAC circuit and connects the 
>> 12.6VDC
>> circuit and ground. The ground pins would have no effect on the 
>> original
>> 3TF7 if you
>> wanted to plug one back in.  This way you could build a nice little 
>> husky
>> separate  regulated/filtered/bypassed 12.6 VDC power supply and not 
>> have
>> to
>> miniaturize it or compromize the performance. If you want to go back 
>> to a
>> 3TF7 just
>> unplug the DC supply 9 pin plug and put the 3TF7 back in. No unsightly
>> permanent wires or brackets hanging off the I.F. subchassis.  The
>> regulated DC
>> supply should give the ultimate in stability and pure DC on the 
>> filaments
>> of the
>> BFO/PTO tubes. 73 Todd WD4NGG
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