[Milsurplus] BC-9 Loop Test: Scratching My Head

Sun Feb 12 00:41:10 EST 2017

I said nothing about the plate current meter resistance. What I did say,
if you had bothered to read slowly, was that the original use of a
moving iron meter may have placed a certain impedance in the plate
circuit, which has been ruined by the addition of a moving coil meter.
Nothing to do with the meter's resistance.

The meter is in circuit on receive only, and could be expected to
lower the receive frequency. With the meter changed, the receiver
could be running at a higher frequency, which is exactly the problem
that we're asked to solve.

Ergo, the problem may not be the transmitter running low, it may be
the receiver running high.

Why do I have to repeat myself to fools who simply will not read?

73 de Neil ZL1ANM

On Sun, Feb 12, 2017 at 5:47 PM, Hubert Miller <Kargo_cult at msn.com> wrote:

> It doesn’t matter if you call the offset an offset of the receive
> frequency or an offset of the transmit frequency; this doesn’t alter the
> understanding.
> The fact is, there is an additional 100k resistor in series with the RF
> plate under key-up, receive. The plate current meter resistance is
> immaterial compared
>
> to this. With the plate voltage ratio 2:1 between receive and transmit,
> there is no way to bring the two frequencies together. There is no point to
> looking to
>
> possible other mysterious influences and such is a futile effort. This is
> my contention and i believe i will be proven right.
>
>
>
> Stabilizing keyed VFOs was a popular subject of tube era ham magazine
> articles. This is a ‘special case’ and a tough nut to crack. Like a VFO
> with an extremely
>
> poorly regulated B+.  High-C tuned circuits counteract this effect to some
> extent but here you already have around  500 pF resonating the loop, so
> there’s not
>
> much leeway that direction.
>
> -Hue
>
>
>
>
>
>
>
>
>
> Everyone's thinking is along the line that the unit is faulty on transmit,
>
> because the frequency shifts when the set is keyed. As I pointed out
>
> earlier, the frequency offset may actually be apparent on receive, and
>
> the frequency shifts to the correct one when keyed.
>
>
>
> I am interested in meter M. I presume it was a moving * iron* type, since
>
> that was the historical type. If a moving *coil* type has been fitted, the
>
> impedance in the plate circuit of the detector is changed, perhaps
>
> causing a frequency offset on receive.
>
>
>
> Redrawn circuit:
>
> https://photos.google.com/share/AF1QipPaz-hyQC9eOuPrWPnKAwcHXcUz7x6k2J-
> 5N1Krbn2wR0C1zD7Anf_6C0nGE-mfRg?key=QWNoTTNGV2lwOW9uTGVoc3RjcVhBbF
> liVmFBN1RR
>
>
>
> 73 de Neil ZL1ANM
>
>
>
>
>
> On Sun, Feb 12, 2017 at 2:20 PM, Hubert Miller <Kargo_cult at msn.com> wrote:
>
> If you look at the partial manual that was posted, mentioned previously,
> what you are describing is the SCR-77-A, the early version. Except that
> they did run it at 120 volts, not 60.
> Altho no operating instructions are there for the SCR-77-A,  it's apparent
> the oscillator is continuously running, under receive, while for transmit,
> it's keyed.  The same power level
> for receive and transmit, no receiver dropping  resistor. You can imagine
> this
> might make it difficult when more than two stations were operating and one
> was quite a bit weaker. This set did NOT have the frequency offset Dave's
> improved version exhibits.
> The dropping resistor in Dave's later BC-9 was an improvement, so-called,
> to decrease the radiating receiver level. That brought other problems,
> which maybe even are more
> difficult to overcome. But i think they "could" communicate, altho the
> stations would be chasing each other across the dial.
> -Hue
>
>
> -----Original Message-----
> From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
> Kenneth G. Gordon
> Sent: Saturday, February 11, 2017 4:58 PM
> To: Richard <brunneraa1p at comcast.net>
> Cc: Milsurplus at mailman.qth.net
> Subject: Re: [Milsurplus] BC-9 Loop Test: Scratching My Head.
>
> On 11 Feb 2017 at 19:44, Richard wrote:
>
> >
> > I think someone said the last change was higher voltage on transmit.
> > Some dim bulb who knew a thing or two, but not enough, probably
> > proposed it for more output, and it was accepted by a supervisor who
> > knew less.  I've seen this happen on big electrical equipment, with
> disastrous results.
> > Richard, AA1P
>
> Good thinking.
>
> So, if David dropped the 120 V feed to 60 volts (which is the maximum
> rated plate voltage for a VT-1) then removed the capacitor and 100 K
> resistor, replacing them with a piece of wire, the system may work
> correctly?
>
> Hmmm.....well, I don't see it. The "oscillating detector" will be
> oscillating at the same level of output in either case, it seems to me.
>
> THe only thing the key would be doing is bypassing the meter and the audio
> stages by a bit.
>
> David?
>
> Ken W7EKB
>
> ---
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