[R-390] R-390A ballast replacement

[Barry]

Sounds like a good reason to add four more 26Z5W's  :~P

Barry(III) - N4BUQ

----- Original Message -----
From: "Bob Camp" <ham at cq.nu>
To: "charles bolland" <ka4prf at peoplepc.com>; "R-390 HF Receiver List"
<r-390 at mailman.qth.net>
Sent: Saturday, February 12, 2005 10:19 AM
Subject: Re: [R-390] R-390A ballast replacement


> Hi
>
> The only issue with the solid state ballast tube replacements is that
> the ones that are easy to build all rectify the filament voltage. With
> modern diodes this generates RFI on the filament circuit.
>
> Depending on how your particular radio is wired and bypassed this may
> be more or less of a problem to you. There are several postings in the
> archives about hum modulation on CW signals that tracked back to
> various mods that rectify the filament voltage. Simply put you are
> doing something that the original designers of the radio did not
> expect. Since they did not expect it the bypassing was not set up
> specifically to handle it.
>
> If you want to get into the technical details here's more or less what
> is going on:
>
> If you put in a full wave rectifier bridge ( 4 diodes) and then attach
> a resistor to the output of the bridge current will flow as long as the
> diodes in the bridge are forward biased. With normal diodes this
> happens somewhere in the  1 to 1.5 volt range. When you are below the
> turn on voltage no current is flowing. Turning the current on and off,
> even at a 1 volt level generates noise.
>
> If you put a capacitor across the resistor then current only flows when
> the AC voltage is greater than the DC voltage on the capacitor plus the
> turn on voltage of the diodes. If the capacitor is charged to say 70%
> or the peak AC voltage then the current is flowing less than half the
> time. This generates even more switching noise since the current it
> turning on and off at a higher voltage.
>
> Now if you put a solid state gizmo on the capacitor you *may* even
> increase the turn on voltage a bit more. More is not a good thing in
> this case.
>
> Bypassing and grounding and filtering is a possibility. Since the
> bypassing has to go to the ballast tube socket you will only be able to
> do just so well.
>
> The question is weather it's all worth it. A fixed resistor soldered to
> a tube base works pretty darn well with normal line voltage variations.
> They also are very reliable. I have never heard of a wire wound
> resistor melting and taking out the wiring harness of an R-390. Of
> course I have not heard of any of the solid state mods doing that
> either ....
>
>     Take Care!
>
>         Bob Camp
>         KB8TQ
>
>
>
>
>
> On Feb 12, 2005, at 9:32 AM, charles bolland wrote:
>
> > Hi all,
> >
> > Yesterday I went to the archieves and found a circuit diagram for a
> > ballast
> > replacement by K0CQ.
> > It has as it's main componenet the  LM317T.  I was wondering if anyone
> > build
> > this circuit and if so, are they still using it in place of a ballast
> > tube?
> > Is it an improvement as stated in the building instructions?   Other
> > comments?
> >
> > Thanks
> >
> > Chuck
> > KA4PRF
> > ka4prfnospam at peoplepc.com
> >
> >
> >
> >
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