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SCR-274-N BC-696A, Modulation




 
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Author Topic: SCR-274-N BC-696A, Modulation  (Read 67 times)
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SM6OID
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« on: Yesterday at 07:03:31 AM »

Hi!

I'd like to share my AM results so far with you.

OK I have two BC-696A that originates from the Danish Air Force.
One of them is "highly rebuilt", new cables and components, including removing the roller inductor.
The other one is close to stock, only the antenna relay has been removed AND it has a modified frequency range, 2.8-3.6 MHz, it is said by a former Danish AF Tech that is was used for their communication with a station on Greenland on 2.8xx MHz.

For those of you not familiar with these nice set, use google!
But in short, it's a MOPA configuration comprising two 1625 parallel finals, about 40 W CW nominal output.
The ATA (early Navy versions) and SCR-274-N are screen grid modulated while the ARC-5 series are plate modulated.

So, here we go...
First CW, with approx. 200 V on the MO plate and 800 V on PA plate, screen grids via a resistor of 27 k from 800 V.
Result is approx. 100 W out, it sounds ok, its a MOPA so the tone is "nice and round".
The voltages are stabilized.
So, then how to modulate?

Looked around and with in my reach I found an singe ended output transformer.
Connected the PA screens to 200 V via the transformer "primary" and the "secondary" now became primary and hooked up to a AF generator.
Transmitter was tuned to max smoke CW and now I had around 40+ W carrier with 200 V on the PA screens.
Turned up the AF generator and I began to see a modulated envelope!
Set the modulation as high as "practically possible", about 85-ish%.
Readjusted the transmitter loading (external tuner) and screen grid voltage a little, the envelope looked OK, trapezoid pattern also indicated the it was entirely usable around 80-85%!

Then I hooked up an old radio from mid -50's as audio source, music sounded OK too.
Yesterday I made an audio test over the air with another ham 50 ish miles away, 40+ W carrier and probably 75-80% modulation to minimize the risk of distortion.
This time I had a car audio amp from a Saab 9000 as "mod amp"
Played Dream a little dream of me, with Mama Cass.
The "listeners report" I received was very positive!
Next step is to feed my own voice and make some QSO's.

In the future, a power supply needs to be constructed, the current set up is terribly bulky and occupies too much space.

Future also involves modulating my "brand spanking new" ATA (5.3-7.0 MHz) that will stretch well in to the lower part of 40 m phone. Had it on the air CW, making a stout 100+ W with 800 V on PA plates.

Attached a picture, the thing that looks like a toaster is the BC-696A, on top of the scope the modulation amplifier can be seen, the PS for MO and screen grid, modulation transformer can not be seen.

Needless (?) to say, I'd like to lay my hands on the original modulator, both the BC-456B and the MD-7/ARC-5, but it appears that they are about as rare as hens teeth...
  




* SM6OID BC696A screen mod.jpg (142.07 KB, 784x441 - viewed 30 times.)
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