[ARC5] The value of MO and PA tank components.

[Leslie Smith]

  Hello Neil,
  As you know, we Aussie guys are at war with you New Zealand blokes -
  and especially near the Eden oval in Auckland during the Bledesloe Cup
  rugby match!  I'd like to thank you (the terrible beatings the "all
  blacks" give us not-with-standing) for such precise and practical
  advice.

You wrote:
> The BC-457 (4.0 - 5.3 Mc/s) can be brought down to cover
> the 80m band very simply by resetting the existing padding
> capacitors that are already in the set. I know it's possible
> because I did that with my very first set (BC-457) in 1967.

In particular, when someone tells me,  "I did that ..."  I sit up and
take notice.
Nothing - nothing - beats practical experience.

You make this observation:
> With the 1625 plate & screen supply disconnected, I set the
> dial to 4.0 Mc/s (PA and VFO caps fully meshed), then
> unlocked the padding cap on the VFO and meshed it more
> fully until the VFO signal came down to 3500 KHz.

I have a question.  Why did you adjust the tuning CAPACITOR to bring the
frequency down?
I imagined (I already thought about how I would do this) I would adjust
the slug in the coil (I assume there is one) to bring the low end of the
band down, and then increase the padder to bring the "top" frequency
down.   Why did you do it this way?  That's my question.

> Then I switched off, unlocked the PA tank padding capacitor,
> reconnected the PA plate & screen supply, switched on again
> and with the transmitter operating into an antenna, meshed
> the padding cap more fully until the plate current dipped (still
> at the same 4.0 dial setting). Small adjustments were needed
> to the capacitor's setting later until I was satisfied that it was
> correct at various tuneup conditions.

Yes, I understand the need to "tweak" the settings several times to get
them right.
When I have followed Mr. Stinson's procedure with the HV caps (and so
on) and my '457 is putting out enough power to light a 25 watt globe,
I'll bring the frequency down to 80m.

> I forget how good the tracking was across the ZL band,
> 3500-3900 KHz, but I was able to use that transmitter up
> and down the band to make contacts with no problems.

Here is my idea on tracking.  First, I have a lot of confidence in the
maths that relate frequency to the tank coil and cap values.  Next,
without doubt, the capacitors in the set will 'track' well (I mean the
capacity (pF) will be the same at the same angular rotation.)   Knowing
the tank coil inductance the 'tracking' should follow the calculated
figure well enough, and if that isn't so - there must be a reason.  

> 
> At no stage did I adjust the slugs in the PA and VFO coils.
> If you use the method I have just described, you could
> adjust the slug in the PA coil to make the PA track the
> VFO. Follow the instructions in the manual for that.

Now this is an interesting point!  I never imagined I would find enough
'spare' capacity to do this in the way you described.  This shows the
advantage to be got from experience!

> 
> By all means get your figures and do the calcs, but sooner
> or later you'll have to use the empirical method. Hope this
> helped.

I remember reading (somewhere) that the Greek mathematicians (or maybe
it was only Archimedes) used "theory" to advance their "practice" and
vice versa.  As for your observations - yes, I found them very helpful.

   73 to you (in ZL - the land of the dreaded "all blacks")  de Les
   Smith
   vk2bcu at operamail.com

PS.  One day we'll do you fellows "in" for the Bledesloe Cup and we'll
shout so loud (in Sydney) you'll hear us in Auckland.
I read that the longest you've held the cup (without defeat) is 28
years.  1951 to 1978!
Out longest is 5 years!  Ouch!   -  Les


On Thu, Mar 24, 2016, at 12:51, AKLDGUY . wrote:
> The BC-457 (4.0 - 5.3 Mc/s) can be brought down to cover
> the 80m band very simply by resetting the existing padding
> capacitors that are already in the set. I know it's possible
> because I did that with my very first set (BC-457) in 1967.
> 
> With the 1625 plate & screen supply disconnected, I set the
> dial to 4.0 Mc/s (PA and VFO caps fully meshed), then
> unlocked the padding cap on the VFO and meshed it more
> fully until the VFO signal came down to 3500 KHz.
> 
> Then I switched off, unlocked the PA tank padding capacitor,
> reconnected the PA plate & screen supply, switched on again
> and with the transmitter operating into an antenna, meshed
> the padding cap more fully until the plate current dipped (still
> at the same 4.0 dial setting). Small adjustments were needed
> to the capacitor's setting later until I was satisfied that it was
> correct at various tuneup conditions.
> 
> I forget how good the tracking was across the ZL band,
> 3500-3900 KHz, but I was able to use that transmitter up
> and down the band to make contacts with no problems.
> 
> At no stage did I adjust the slugs in the PA and VFO coils.
> If you use the method I have just described, you could
> adjust the slug in the PA coil to make the PA track the
> VFO. Follow the instructions in the manual for that.

> 
> 73 de Neil ZL1ANM

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