[Milsurplus] BC-9: A Big Road Block

[AKLDGUY .]

Yes, I understand that the meter would get slammed. Or would it?

There was some doubt about the circuit diagram, with the part of it
in the vicinity of the key being missing and retrieved from somewhere
else.

I was thinking that there might be a mistake and the key should not
be connected as shown to the 120V + wire that runs to meter M but
instead should connect only to C6.

C6 would then decouple C5 / R5 when the key is pressed, stopping
regeneration. The maximum plate current that could then flow through
R5 would be 120 / 100K, or about 1.2 mA. The DC input to the VT1
would then be 120 x 1.2 mA, or about 144 mW.

The meter needle would then indicate but not bend, power output
might be about 100 mW, and the VT1 would be operating at about
the same DC conditions as it does in receive.

Try connecting the key straight to C6. There might not be meant to
be a connection to the 120V at that point. I think no harm can come
by trying it.

73 de Neil ZL1ANM


On Sat, Jan 21, 2017 at 10:35 AM, David Stinson <arc5 at ix.netcom.com> wrote:

>
> ----- Original Message ----- From: "AKLDGUY ." <neilb0627 at gmail.com>
> Subject: Re: [Milsurplus] BC-9: A Big Road Block
>
>
> The key is the switch symbol above and to the left of meter M,
>> isn't it?
>>
> Yes it is.
>
> How come it shorts meter M when pressed?
>> Is there
>> no indication of plate current when sending?
>>
> The meter is there to monitor and test for receiving
> oscillation and is marked in tenths of milliamps.
> On transmit, this meter would be slammed.
> I suppose they didn't want to pay for a second meter
> or make the case larger.
>
>
>> 73 de Neil ZL1ANM
>>
>> On Sat, Jan 21, 2017 at 4:57 AM, David Stinson <arc5 at ix.netcom.com>
>> wrote:
>>
>> By using an external tank coil in place of the loop and link-coupling, I'm
>>> getting good, sensitive receive and
>>> am able to get a good Watt of power
>>> out on 80 Meter CW.  https://goo.gl/photos/SthdKR1rmwxXkZ6z6
>>>
>>> I had this dream of making a few contacts with it during Novice Rig
>>> Roundup.  But now there's a big problem I should have seen coming.  Take
>>> a
>>> look again at the diagram:
>>> https://goo.gl/photos/THcWRvYScQfnp5gg9
>>>
>>> The RF stage is regenerative through the plate
>>> resistor, R5.  This reduces the plate voltage on
>>> the RF tube during receive from 120 VDC to
>>> 70 VDC.   To transmit, the plate resistor is shorted,
>>> the full 120 VDC is applied resulting in a strong
>>> oscillation for transmitting.  And here's the rub:  What happens with a
>>> non-
>>> crystal tube oscillator when one raises the B+
>>> from 70V to 120V?  Right- the transmit freq
>>> jumps higher by several KC.
>>> Some movement is to be expected, but I suspect
>>> that the original VT-1 didn't move as much as
>>> the substitute 6AQ5; otherwise the rig wouldn't
>>> have stayed in service for a decade.
>>>
>>> Irreversable changes are absolutely forbidden.
>>> Not sure how I'm going to address this.  Always
>>> feel frustrated if I don't get at least one QSO out
>>> of a rig.   Perhaps something can be done in
>>> the substitute tube mounting.  I'm open to suggestions?
>>>
>>> TNX OM ES 73 DE Dave AB5S
>>>
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>>
>>
>> -----
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>
>
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