[R-390] R-390A ballast replacement

[Bob Camp]

Hi

I'll bet you still have some electrolytic capacitors with black goo 
inside them.

I still claim *that* is the winner in the "deadest horse that has ever 
been beaten" category.

	Enjoy!

		Bob Camp
		KB8TQ


On Feb 13, 2005, at 2:34 PM, Llgpt at aol.com wrote:

>
> This has to be the "deadest horse that has ever been beaten,"
>
> Just put a resistor in there and be done with a useless tube.
>
> Les Locklear
> Monitoring from the Gulf of  Mexico
> Ten Tec  RX-340
> Ten Tec RX-350D
> Alpha Delta Sloper
> Quantum QX Loop
> Various  Longwires
> CU-2279/BRC  Multicoupler
> http://www.hammarlund.info/homepage.html
>
> PS: Yep, no more  R-390xx's, so my 2 cents won't count anymore. No 
> tubes
> either..........
>
>
>
> In a message dated 2/13/2005 1:30:23 PM Central Standard Time,  
> N4BUQ at aol.com
> writes:
>
> 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
>>
>
>
>
>
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