[Milsurplus] TCS Receiver: "More than one way to skin..."

[AKLDGUY .]

I see two problems with the 0.047 capacitor across the zener.
The first is that at the instant the B+ is turned on, V2 is subjected
to the full 220V B+,
since the capacitor is essentially a dead short until charging gets underway.
That high voltage may have a detrimental effect on V2's life.

The second is that a capacitor placed across a zener or voltage
regulator tube may
create a relaxation oscillator if the timing constants happen to favour it.
The mechanism is: the capacitor initially represents a dead short and the zener
doesn't trigger. The voltage across the capacitor rises as it charges
until at some
point it reaches the zener's trigger threshold voltage (always greater
than the zener's
marked operating voltage).
The zener then triggers, rapidly discharging the capacitor, at which
point the zener
drops out of triggered mode again. The capacitor voltage then rises again to the
point where the zener triggers again. This is a well-documented
relaxation oscillator
effect where the zener or voltage regulator tube oscillates between trigger and
marked voltage states.
If you've simply checked V2's plate voltage with an analog DC meter, I
suggest that
you verify with a 'scope that oscillation is not taking place. If you
haven't yet noticed
any ill effects, it's probably not oscillating, but better to be sure.

73 de Neil ZL1ANM


On 2/14/16, David Stinson <arc5 at ix.netcom.com> wrote:
> Tinkered around some more and found a way to "down-size" and
> simplify the Collins 353-series Mechanical Filter adaptor to fit
> inside the TCS receiver.  Yes; I could make some solid-state
> fancy thing.  But if I did that, I'd feel soiled.  No way.  So
> it's tubes or nothing.
>
> First- there's so much gain "head room" here that triodes do the
> job just fine.  I used two 6026 sub-mini triodes.  Why, when a
> single 12AU7 or similar would do the job?  Just because I enjoy
> tinkering with these tiny tubes, they don't need sockets and I
> have them laying around.  Nothing more complex than that.
> But then, a 12AU7 might not provide enough isolation between
> input and output.  I dunno; try it and see.
>
> There was a "hitch" (isn't there always): The TCS B+ is 220
> Volts, and sub-minis want max 150.  Boogering under the chassis
> was again forbidden; the adaptor must plug in place of the tube
> with no other changes.  A 1N5378, 100 Volt Zener with a .047
> across it to bypass it for RF connected as shown in the diagram
> fixed the issue.  220 Volts on one side, 115 on the other.  Mr.
> Mini Tube is happy.
>
> The input tube runs with very low plate voltage- about 19 Volts.
> This is because it is powered from the Screen buss and must do so
> to isolated it from the Plate buss, which powers the other tube.
> V1 draws 4 mils and the TCS screen dropping resistor is 47K, so
> that only leaves about 19 Volts.  Fortunately, the stage works
> fine at that level.  One could adjust the screen resistor, but I
> opted not to start diddling under the chassis when it clearly
> wasn't needed.  If you adapt this to other rigs, you many need to
> adjust screen resistor values at the tube socket.
>
> I'm going to build this up with two, switchable filters- one for
> .8 KC CW, one for 6 KC Phone.
> Note:  For this adaptor to work, one must align the radio
> properly.  The old "ham" trick of turning-on the BFO and
> diddling/twiddling everything to maximum noise will not work.
> The BFO needs to be 455.0 KC when the knob is pointing to "0" and
> the IFs need to be aligned to 455 KC per the manual.  If you go
> to the trouble to build this and do not have a properly-aligned
> set, you shall be unhappy.
>
> Here is the circuit.  Simple and it works.  Many improvements
> possible.
> http://home.netcom.com/~arc5/TCS/TCSFilterAdaptor1.jpg
>
> This, plus the "ear saver" clipper, makes the radio excellent on
> CW.
> Still tunes fast, but, as my Daddy would say: "Well for Heaven's
> sake, son! What do you want? Egg in your beer?" (I don't know
> why egg in a beer would be a good thing, but I loved my Daddy's
> smile when he said it).  So tune slow.  It ain't hard.
>
> Looking forward to adapting this for an ARC-5 receiver.
>
> 73 DE Dave AB5S
>
>
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