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MD-7/ARC-5 Modulator, on air




 
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Author Topic: MD-7/ARC-5 Modulator, on air  (Read 214 times)
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SM6OID
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« on: November 04, 2020, 04:30:27 PM »

Hi!

So, after some shelf time, it was time to make the MD-7/ARC-5 modulator to com a alive.
Known issues were the need for new brushes, all four of them, commutators how ever indicated minimal use.
Dynamotor also need to have the bearings cleaned up and re greased.

So, first issue encountered was B+ "550 V" was "leaking to ground".
When applying B+, 100 V or so, a quiescent current was noted, much higher than expected.
Raising the voltage higher just made the thing worse, a quick glance at the diagram revealed that the prime (only) suspect was a 1.2 F cap between HV line and GND.
Lifted the wire to the cap, and all was OK.

Then, second issue was terribly low modulation, much less than 25% when screaming into a good T-17 carbon mic.
The circuit is simple, carbon mic to transformer to feed the grids of 1625 in P-P.
Checked mic bias current, found to be 35 mA, so that was not the main issue.
With the scope checked grids of the 1625, very low signal swing.
Checked HI side of mic transformer, plenty of signal.
Checked LOW side of mic transformer, plenty of signal here too, well that was not expected!
The LOW side is feed with bias current via a 680 Ω resistor and there is a 20 F cap to GND.
Reached out and found a cap of nearly the same value and just hooked it up, and that is the end of issue number two.

I use the MD-7/ARC-5 together with SCR-274N series transmitters, as is, and it appears to work fine.
For those of you not familiar with these nice transmitters, the ARC-5 series are plate/screen modulated while the SCR-274N are screen grid modulated. (that modulator is the BC-456, bought one a while ago that eBay global shipping confiscated due to being very dangerous goods)

I have done one modification with the MD-7/ARC-5 modulator, and that is to increase the carbon microphone bias a bit, my wife, who is an engineer, explained the concept of using a microphone...
Apparently the reason for using a microphone is that the person that is supposed to hear the message should hear it, not the rest of the family in the house... And to make that work satisfactory, the mic and associated circuitry must have enough gain.
Otherwise, using a transmitter and modulator is not necessary, at least for local QSO's I have been told...

The HV now comes from a stabilized power supply, 550 V, but once I get the dynamotor running, it will be used with dynamotor.
If anyone out there has a DY-8/ARC-5 dynamotor or a DM-33-A, let me know!

So far I have had one QSO on AM, it is doing approx 30 W of carrier and peaking 85-90% modulation.
 
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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...

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KA3EKH
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« Reply #1 on: November 05, 2020, 11:28:01 AM »

I have a couple ARC-5 transmitters that I played around with that were hacked radios to begin with so had no problems changing them more. The one is a 80 meter transmitter that I changed the filament string in the transmitter for operation on 12 volts DC and used an old NARCO MX-12 power supply for running both the transmitter and a old RCA marine receiver with the Narco providing B+ for the receiver and the B+ and HV along with modulation for the transmitter. The Narco develops around five watts of audio at 400 volts DC and I am able to get about five watts of output from the radio.
The biggest issue I have is that the loading of the PA had a huge effect on the quality of the modulation from the transmitter, have to listen and set the loading for best sound quality.
Have you run into any issue with how heavily you load the radio and its audio quality?
And what are you doing to match 50 Ohms on the output? I just use the stock output tank and roller inductor and open wire to a short maybe ten foot vertical.

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