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SCR-274-N BC-696A, Modulation

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Author Topic: SCR-274-N BC-696A, Modulation  (Read 3171 times)
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Posts: 217

« on: July 14, 2019, 07:03:31 AM »


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 759 times.)

RADIO: 51J-4, R-390A, SP-600 JX-21, BRT-400, Set No 19, T-47/ART-13, RF-590, SRT CR91, BC-312D, BC-348Q, HF-8020/8030/8010A/8090,  and much more...

ENGINE: Zvezda M50 F6L (V12), Rolls-Royce Meteor mk4B/2 (V12), Rolls-Royce B80 (inline 8 ) and much more

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Posts: 663

« Reply #1 on: July 28, 2019, 04:53:37 PM »

Forget the idea of using a MD-7 even if you can find one. The mod transformer is the size of a walnut. I have two ARC-5 (ATA) stations on the air- one mobile, the other as a base. Both are plate modulated and use a commonly available ART-13 mod transformer. The mobile uses 6146's in class B, the base uses 807's in class AB-2 and has hi fidelity audio. Both systems can be powered by plugging into a Heathkit HP-23 power supply. In the transmitters I have 12 volt relays with rectifiers on them so the rigs will work on 12 volts AC or DC. Same for the modulator's relays. I perform extensive updating of bypass caps in the transmitters: added cathode and screen bypasses. I also run the oscillator at 250 volts in the base and 300 in the mobile system, as you get more grid drive. The SCR/274/ARC-5/ATA all suffer from a lack of sufficient grid drive. Still I get a good 100 watts out with 100% plate modulation. By the way the magic number for the PA screen resistor is 22K. Good luck with your project!

73DE W2JBL     

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Posts: 217

« Reply #2 on: August 01, 2019, 03:51:01 AM »


Thanks' for the Words of wisdom, experience and ideas are always very welcome!
I have understood that the MD-7 is not a high quality audio modulator, my idea was to run the transmitter as it was intended.

The idea with a ART-13 mod iron is something that consider, they are not something that you find easily here in Sweden, will probably have to look elsewhere, should not be a huge problem.

I'd love to find a mounting rack for two or three transmitters, that would be nice.

One of my transmitters a BC-696A is "modified" in such a manner that it is a good candidate for HiFi AM conversion with a ART-13 modulator, the more I think about it the better it sounds...

Have a few other projects that needs to be taken care of, including my Dodge D400 1967 Fire Truck... 

RADIO: 51J-4, R-390A, SP-600 JX-21, BRT-400, Set No 19, T-47/ART-13, RF-590, SRT CR91, BC-312D, BC-348Q, HF-8020/8030/8010A/8090,  and much more...

ENGINE: Zvezda M50 F6L (V12), Rolls-Royce Meteor mk4B/2 (V12), Rolls-Royce B80 (inline 8 ) and much more
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