[R-390] R-390A ballast replacement

[Bob Camp]

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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