[R-390] R-390A ballast replacement

[Bob Camp]

Hi

Actually using 26Z5's would be the perfect solution to the noise aspect 
of the problem. They are soft turn on devices and only cut in up in the 
20+ volt range. They are high resistance at that point so the current 
would be quite low. With no current to the ballast tube filament string 
you have no noise out of the radio. With the oscillators running at DC 
(no output) there is no drift what so ever.

Obviously this is the ultimate ballast tube replacement scheme!

	Enjoy!

		Bob Camp
		KB8TQ


On Feb 13, 2005, at 2:32 PM, Barry wrote:

> 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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>>
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