MD-7/ARC-5 Modulator

[W2JBL]

     i have had the good fortune of coming into some very nice ARC-5 stuff recently. among the treasures are an MD-7 modulator, containing two 1625's connected a small mod transformer. the former owner bulldosed most of the wiring but the hardware remains, and a few parts. looking at the schematics i have it shows ONE 1625 in the unit, which is used as a screen modualtor. i can find no MD-7 schematic showing two tubes and used as a plate modualtor. however this combination is referred to in my old CQ Magazine surplus schematics handbook. So- the question is what do i have here?-two screen modulators on one chassis or a plate modulator?

    meanwhile back in the JBL basement bunker i had a mint unconverted bone stock T18/ARC-5 on 160 last evening, modulated by a crude lashup of PA amp and back connected output transformer. works great with 75 watts out. this was a new rig never powered up, and worked perfect after a 64 year sleep. ya gotta love WW2 surplus!

[WU2D]

Chris,

The BC-456A is a single tube screen modulator. The MD-7 is a push pull modulator. Both of these units held a dynamotor. I had a screen modulator but never got it tamed. I have attached a couple of schematics.

73's Mike WU2D

[W2JBL]

 take a look at the two files you sent me- they are both the same schematic! these also are the same as the ones i downloaded earlier from the 'net. if you look at them they are both a single ended 1625, as in the screen modulator. if we both got the same info, something is wrong in Boonton NJ (at the  A.R.C. plant). my unit is clearly labled MD-7/ARC-5 and the old CQ magazine document shows that same schematic of the screen modualtor. what da F#*$ is going on here?

[WU2D]

Chris,

Its called an error. And apparently an old one! I found at least two people with the same BITC%. They did not fix it though or nobody told the list servers who keep the military documents.

I just sent a PDF of athe ARC-5 Maintenace manual to you. It is 29 Meg. If that fails to get to you I will scan the pages that have the MD-7 info and send them tonight.

73's Mike

[k4kyv]

I used one of those modulation transformers in my very first plate modulated rig some time in 1960 or 61. I couldn't get enough audio running a pair of 807's class AB1, so I rewired them to triode connected using the 20k resistors between screen and control grids, and feeding audio to screens.  I was able to fully modulate another pair of 807's in a final running 100w input.  The only problem was that the thing sounded tinny, with negligible lows.  I suspect that little mod transformer didn't pass much of anything below about 400~.  At the time I had never heard of shunt feed using a reactor to improve the lows.

I later upgraded to a "real" modulation transformer, a brand new Stancor multi-match rated at 125 watts of audio, with a single-ended 6L6 driver stage.  I thought I was in tall cotton then. 

[WU2D]

Half Day off today for a physical.

Chris the file is at http://bama.edebris.com/manuals/military-kg7bz/arc5/

73's Mike WU2D

[The Slab Bacon]

FWIW, that is the same mod transfoma that I am using to scream mod my 4-1000 rig with. A pair of 2E26s for the modders, the main output winding puts the shake on the screen grid, and the tertiary winding puts a little shake on the control grid biass. Works FB for me, been running for 6 or 7 years now.

                                               The Slab Bacon

[W2JBL]

well if i use the MD-7 it's going to be mobile, so i'm not much worried about no low end. i happen to have a correct and brand new 14 volt dynamotor for it too, inherited from my father. i'm fed up with using riceboxes for AM mobile, and when i do it this time i'm going back to a tube rig with some testosterone. no more 25 watts and ALC trouble. i started AM mobile about 10 years ago with an AM marine radio rebuilt to 40M and it worked slicker than snot on a rubber boot. should have stayed with that but went to an FT301 until i got my present car, and gave up. when 40 comes back i'm gonna do it again but with an ARC-5 transmitter and a receiver yet to be determined. Mike- i got the schematics Okfine from you, and thanks!