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Preselector Racal MA-6197




 
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Author Topic: Preselector Racal MA-6197  (Read 211 times)
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Patrick J. / KD5OEI
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« on: October 11, 2021, 11:16:04 PM »

Not yet found info on these units, but they appear similar to the MA-197B preselector.
The MA-197B and the MA-6197 both cover 1-30MHz in 6 ranges:
1-2, 2-4, 4-8, 8-16, 16-24, and 24-30MHz.

The externally very similar MA-197B manual is online here;
https://www.pa3esy.nl/ontvangers/racal/racal-algemeen/pdf/MA_197B.pdf

These things are of the highest quality, cast chasssis, etc.. a relay-rack dream. Signals more than 5% off-tune are attenuated >85dB, while desired signals are boosted 6dB. Pretty nice.They can also be daisy chained so one antenna can serve more than one preselector. The weight is only 45 lbs.

I have not opened the MA-6197 yet, but I have a suspicion. The MA-197B has 6 ranges, but it is more or less pass-through on the 1-2Mhz and 24-30MHz ranges, meaning those have RF protection but no pre-selection.

So, why would there be this mysterious model, the MA-6197, which only shows up in a few old 'for sale' ads with zero additional information?
Could this be an unusual model made for secret squirrel?
Could it be that all 6 ranges have pre-selection?
Waiting for cooler weather to check.
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« Reply #1 on: October 12, 2021, 01:20:48 PM »

Curious, how is the 197 a "protection unit" unless it forms a large fusible link in and
of itself??

Did not see anything that does such a job in the sketch-a-matic?
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« Reply #2 on: October 13, 2021, 11:57:18 PM »

There are only the claims in the technical manual to officially support the protection statement. It's intended to be used with a separate antenna for transmitter and receiver. It is implied by its stated purpose of shipboard or communications central usage that a receiver used with this should be able to take a few volts RF at the antenna input.

The schematic as a whole highlights the inherent voltage limitation afforded by the input and output stages.

But to the manual first, about the meaning of protection there:

The manual for the MA-197B says its aerial coil windings are heavy enough to withstand radio-frequency e.m.f's of up to 40 volts on the (75-95 ohm) input. It prevents damage by "break-through" (what may that mean, but any sort of coupled signal?) to the receiver it is linked to when operating close to a transmitter. An unwanted signal off-tune by 5% is attenuated by >85dB. I believe this is the protection for RF of different frequencies than the receiver's.

For on-frequency protection or output voltage standpoint, It is supposed to put out 8-300mV of RF according to the testing section of the manual. There is no test included for what happens at its output when the rated 40V of RF on the same frequency as the tuned frequency hits the front end of it, but the claim is the tuner will survive, and upon reading the diagram it has a cascode connected class A 12AU7 voltage amplifier design (single ended) driving the 75 Ohm output load of the receiver through a transformer and a broad pi filter with 220 Ohms resistor in series with the coil. I believe all of this is the main protection for the receiver on the same frequency as the transmitter.

Most receivers should deal with this, if not all receivers of reputable make, but who knows? Once there is time to dig into it, more will be known about its maximum output voltage limits.

The schematic:
The lower section of the 12AU7 has 1.8V DC across its 150 Ohm emitter resistor, so the current is 12mA. This is with 135V on it.
The upper section will be the same current, so 11mA at 270V across the whole thing, approximately.
This would be around 3W input. If the circuit were for audio, perhaps the output would be up to 1 Watt if the gain of both parts of the cascode are counted. But it's RF, so bets are off, however the though experiment can continue with that value.
One thing is that the drive to the output stage is limited to 3.6V peak to peak by the 1.8V bias on the lower 12AU7.
The RC data shows about voltage gain of 12x, but there is no data with a small 150 Ohm cathode resistor and we do not know the impedance presented to the plate by the RF output circuit. The 100V RC amplifier data with a 47K plate load (like R10 and R11 in the schematic) shows a 1K cathode resistor and a 1.22mA current. so the proportions seem linearly related. Anyway I stick to the 1 watt, which is probably larger than actual.
The 220 Ohm pi filter series resistor and the 75 Ohm load come to about 300 Ohms.
1 Watt into 300 Ohms is 0.058A.
0.058A through 75 Ohms is 4.33V. (250mW).
However, the actual output will be reduced by inefficiencies, the actual match provided by the RF transformer is not known, and the pi filter loss isn't precise. So, 4.33V is unqualified speculation. It is not so harsh in the intended environment.

Those are my thoughts. Could be all wrong. Later some experiment will tell.
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