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

Sun Feb 12 02:13:02 EST 2017

In the redrawn circuit,

https://photos.google.com/share/AF1QipPaz-hyQC9eOuPrWPnKAwcHXcUz7x6k2J-5N1Krbn2wR0C1zD7Anf_6C0nGE-mfRg?key=QWNoTTNGV2lwOW9uTGVoc3RjcVhBbFliVmFBN1RR

the meter forms part of the load on the detector plate, in receive,
together with the transformer.

The meter therefore affects the load on the detector tube. If the
meter has been changed in the intervening 96 years (worst case,
a change from moving iron type to moving coil), the receiver may
be running off-frequency.

Since the meter and the resistor/capacitor are shunted out during
key down, any change of meter has no effect on frequency with
key down.

That is why I'm suggesting that a wrong type of meter may be
causing the receiver frequency to run high, while with key down
the transmit frequency is actually correct.

I have no idea why we are debating this when David could settle
it once and for all by telling us whether the meter is original. It
seems nobody is taking me seriously and he therefore can't be
bothered telling us.

73 de Neil ZL1ANM

On Sun, Feb 12, 2017 at 7:28 PM, Hubert Miller <Kargo_cult at msn.com> wrote:

> Are you postulating the meter inductance or some dynamic impedance of the
> meter movement?
> You do see that the meter is isolated from the RF components and the tube
> plate, yes?
>
> I want you to explain how the meter accounts for a 10-12 kHz shift of
> frequency between two steady states.
>
> I can read and re-read your post and this point is still not explained
> except as “somehow!”
>
> Everything to the right of the RF tube is RF-isolated and is lumped
> together as one resistance in series with the B+ supply.
>
> Address the issue of the 2:1 voltage ratio and consequent greater current
> ratio between the two steady states and THEN
>
> address the inconsequential mysteriosa such as the current meter or audio
> stage oscillation. What i have addressed is
>
> BASIC and no amount of beating around other unrelated circuit components
> is going to address that.
>
> What i wrote was exactly true: the earlier model, with no dropping
> resistor, transmitted and received on the SAME
>
> frequency. It had to! The oscillator parameters were the same.
>
> -Hue
>
>
>
> *From:* Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] *On Behalf
> Of *AKLDGUY .
> *Sent:* Saturday, February 11, 2017 9:41 PM
> *To:* milsurplus <milsurplus at mailman.qth.net>
>
> *Subject:* Re: [Milsurplus] BC-9 Loop Test: Scratching My Head
>
>
>
> 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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