[ARC5] Receiver current drain

[J. Forster]

A Zener diode string to limit the HV until the filaments warm up?

-John

============


> On 5 Jan 2011 at 19:56, Mike Hanz wrote:
>
>> Like everything in life, "it all depends", Ken.  A clue is provided in
>> the 15mA cushion for external devices.  If the B+ is 240v, you are
>> pushing the audio output of the receiver to a full 400mW audio output
>> shown in the bench test procedures, and the BFO is on, the total
>> current drain can approach 45mA.  I seem to recall 35-40mA in normal
>> operation, but it all depends on the conditions.  I'm curious as to
>> why you might need it so precisely?
>
> Well, I have a number of power supplies which were built and sold by Fair
> Radio Sales back in the 1970's time frame. They built several versions of
> these, around some power transformers which they had had especially built
> for them.
>
> One of the end-bells has FAIR 818 stamped in it, in fact.
>
> Their earlier versions were built using 5Y3 rectifier tubes, but their
> later
> versions used a pair of diodes in place of the 5Y3.
>
> These were built on stripped dynamotor bottom plates, so all you had to do
> was to snap them into the dyno mounts and turn them on.
>
> Output was some +HV and 24 -28 VAC for filaments.
>
> The power supplies are properly designed....except for one thing: the B+
> is
> around 350 VDC when unloaded, despite their having included a bleeder
> resistor across the output, and a resistor in series with the diodes.
>
> The filter cap is a dual-30 MFD at 450 VDC can-type with a 2.5K resistor
> between the two sections.
>
> What I wanted to do was to load the output with a power resistor of the
> equivalent resistance to mimic the receiver load in order to discover how
> much of a voltage drop might occur.
>
> I used a 4167 ohm power resistor (250 VDC at 60 mA), and the voltage
> drops to 165 VDC...which is a pretty big drop!
>
> The transformers are fairly big, and don't get terribly hot at that load
> (about
> 40 mA), but I am concerned about the damage that could occur to the old
> can-caps in the ARC-5 receiver caused by the too-high voltage when first
> turned on.
>
> The older 5Y3 version probably wouldn't have had much of a problem that
> way since its filament has to warm up too, but the diodes DO cause a
> problem.
>
> I think what I am going to have to do is to add a switch to turn on the B+
> after
> the filaments have warmed up.
>
> I dunno....
>
> If I use your figure for current drain of about 40 mA, then I need to
> adjust my
> resistor to 6250 ohms.
>
> But what am I going to do about the too-high voltage at turn-on?
>
> Ken W7EKB
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>