[MRCA] MAK Transmitter osc

[B. Smith]

Ray
Does this problem occur on other  channels?
Z

On 2/17/2020 1:24 PM, Ray Fantini wrote:
>
> Have been playing around with this oscillator circuit in my MAK 
> transceiver for a while now and have been having an issue or two. The 
> problem is that the MAK is intended for use in the field powered by a 
> car battery. I do not have the original power supply that provided 
> +175 to the receiver and the TX oscillator and built up a solid state 
> supply for both the B+ and the HV for the PA and modulator.
>
> I am running around 150 volts for the B+ and with 13.5 volts on the 
> input of the power supply the radio works without issue but below 11.5 
> or so the B+ drops down to 125 volts and the transmitter oscillator 
> will not start every time. At 10.5 the oscillator fails to start at all.
>
> This radio has a crystal in the receiver also and the receiver works 
> without issue down to 10 volts although it’s not as loud it still 
> works. The issue is with the transmitter.
>
> The question is am I dealing with a B+ issue or something else? At 
> first I thought it was B+ related but not one hundred percent of that. 
> The original transmitter oscillator used huge primitive crystals and 
> now I am using modern rocks and am wondering if this is not a crystal 
> capacitance issue that causing the oscillator not to start under less 
> than ideal conditions?
>
> I am attaching a copy of the schematic of the oscillator. It’s a 12A6 
> tube with a grounded cathode and a tuned resonance circuit in the 
> plate that directly feeds the PA tube grid. There is no directly 
> visible feedback path at least that I can see but I have seen this 
> same approach used before in prewar and ww2 design. Did designers back 
> then count on the added capacity of the huge primitive rocks and the 
> modern crystals just don’t provide that? Would a small amount of 
> capacity across the crystal help it start oscillation? Or is this just 
> an issue due to the reduced B+ voltage?
>
> Or maybe it’s something completely different like reduced filament 
> voltage cuts the gain of the tube below the point of self-exciting?
>
> An additional item of interest is that the PA plate tuning affects 
> oscillation. If I tune dead center on the plate tank dip the 
> oscillator will not fire up the next time I key the microphone but by 
> tuning a little on either side of the dip the oscillator works just 
> fine, except the plate voltage issue.
>
> Maybe when you consider the PA and its tank that is what is kicking 
> this thing into oscillation in kind of some weird form of Armstrong 
> oscillator? Although there is tons of shielding between the PA tank 
> and the oscillator tank.
>
> Ray F/KA3EKH
>
>
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