Recently, Somebody Somewhere wrote these words
>
> Wasn't it Winston that said we are two countries separated by a
> common language? hFE is NOT = to hfe. Gain bandwidth product (Ft)
> (specified in MHz) is only mildly related to hFE (specified as a
> ratio). By JEDEC definition upper case notation is used for DC
> parameters and lower case are ac parameters. In short that is not
> the end of the horse that you use to check his teeth. I have never
> heard of a RFI problem that was related to DC parameters. I am
> talking about ac parameters which are largely unspecified for this
> type device. This is where you get into problems when substituting
> different types or even the new for the old same type. Newer
> devices almost always have a higher Ft because the manufacturer has
> upgraded their processes. The manufacturers can upgrade their
> product without changing the specification because the maximums
> were never specified in the first place. Declan Moriarty wrote:
>
> Recently, Somebody Somewhere wrote these words
>
>
> I'm having a lot trouble understanding what you are saying. Let me
> state my belief. The aluminum case TO3s that you replaced the
> existing transistors with probably have a gain bandwidth product
> (Ft) 10 to 1000 times greater than the old transistors. This means
> that instead of driving the transducer with a sinusoid waveform it
> is now a nice crisp square wave. This could cause ringing and RFI.
> Furthermore, the hfe is now greater while the parasitic capacitance
> is now reduced by a factor of up to 100. This now permits parasitic
> oscillations that are heard as RFI. Poor grounding, inductive
> spikes, and poor earthing are the red herrings. Don't get me wrong.
> These are issues to be dealt with but not the source of RFI which I
> see as the subject of this posting. Fred Townsend
>
>
>
>
> I checked the gains. The Manufacturer's transistors died (my fault),
> and I now have the replacements in place.
>
>
> No you have the substitutions in place.
>
> BUX 80 (The manufacturer's job) has a minimum hFE of 3 BU225s (The
> Replacements) have a minimum gain of 10.
>
>
> You are up side down. It's not the minimums but the maximums one
> worries about for problems relating to RFI. Yes one worries about
> minimums to make the basic circuit work but RFI, in this type
> circuit, is almost always caused by too much, ac, gain.
Ah! That feels more like it. Somebody disagreeing with me. That's when
learning starts. No manufacturer specifies the maximum gain. You are
perfectly right, the response at the frequency is the issue.
>
> One can affect the transmission path by attacking the antenna but
> one normally kills the transmitter to suppress the transmission.
>
Thank you for pointing that out :-/.
> Ever use a cell phone inside a car? The car is a steel case.
Yes, a car is a steel case with at least six windows, and the
frequencies are much higher.
>
>
> Suspect the diodes as being the source of the problem or merely in
> the transmission path? Fred Townsend
I see the source of the problem as the exication of two big coils under
the base of this thing, following the rough equation
Power consumed in a noisy fashion ---> noise made
I also see the diodes as the transmission path. I'll have to look them
up. There is a complete lack of a mains filter, and what I really can't
see is any sensible way of killing quite so much noise.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
---------------------------------------------------------------------
To REMOVE yourself from this mailing list, send an E-Mail message
to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in
the message BODY, include a line containing: UNSUB CHIPDIR-L
(or the name of mailing list you want to be removed from). You may
also send the HELP command for other information (like subscribing).
