[MRCA] MAK Transmitter osc

[Ray Fantini]

Have not thought to try the other channels, sometimes the simplest things elude. Have original rocks for 3.000 and 3.155 and will see what happens. Just have to move the channel selector, Duh.


RF


-----Original Message-----
From: B. Smith <smithab11 at comcast.net> 
Sent: Monday, February 17, 2020 1:51 PM
To: Ray Fantini <RAFANTINI at salisbury.edu>; Military Radio Collectors Association <mrca at mailman.qth.net>
Subject: Re: [MRCA] MAK Transmitter osc

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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