Re: {Collins} Need RCA SK3089 diode data (312B-4 mod)



I fully agree!

I was just saying that those tantalum capacitors are a HUGE improvement
over the stock 312B-4. (At least that's true with the microphone I use,
which doesn't have the LF response --and greater meter swing-- that some
other mics have.)

I wish I could find where I got the ideas for those capacitor and diode
modifications. It was before the Internet.

I will certainly check into that board. Thanks.

73, Mike
www.w0btu.com


On Fri, May 1, 2015 at 2:54 PM, Colin Lamb <k7fm@xxxxxxxxxxxx> wrote:

> I agree with Bill about the PEP wattmeter board.  I have added similar
> boards to a number of wattmeters.  Simply adding an electrolytic capacitor
> to the circuit will not work.  Analog meters have a problem with
> instantaneous signals because of the mass of the meter movement.  So, when
> you have a voice peak, it will not fully indicate what the true reading
> is.  Each meter and voice varies.  And, if you attempt to turn the gain up
> high enough so that the meter peak will read the same as it did in cw/tune
> mode, you will overdrive the amplifier and often splatter.
>
> Some manuals specify that a normal voice may indicate 50% or some other
> value of full scale during normal voice operation.  But, that is just a
> wild guess.  Many years ago I put a scope on the output and soon noted that
> I reached full output when the meter read about 25%.  So, the gain was
> turned down.
>
> Adding a capacitor to the meter does not solve the problem.  It will store
> the charge, but it will not charge instanteously, so you are still likely
> to turn the gain up too high.
>
> A peak reading wattmeter circuit is normally a high impedance amplifier
> that does not load the input, but then is able to hold the peak value.  It
> works like an s-meter in a good avc system.
>
> Using an analog output meter without a true peak reading circuit must be
> interpreted by the operator who understands what information the meter is
> giving.
>
> 73,  Colin  K7FM
>



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