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

Sun Feb 12 01:28:23 EST 2017

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<mailto: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=QWNoTTNGV2lwOW9uTGVoc3RjcVhBbFliVmFBN1RR

73 de Neil ZL1ANM

On Sun, Feb 12, 2017 at 2:20 PM, Hubert Miller <Kargo_cult at msn.com<mailto: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<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<mailto:brunneraa1p at comcast.net>>
Cc: Milsurplus at mailman.qth.net<mailto: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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