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Archaic Behemoth BC-191/BC375




 
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Author Topic: Archaic Behemoth BC-191/BC375  (Read 19946 times)
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WU2D
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« on: November 28, 2014, 10:47:17 PM »

The Surplus Radio Conversion Manual Vol 2 sates that "few amateurs have had the temerity to attempt to operate the transmitter as it was originally designed. In fact the conversion is to turn the VFO into an amplifier stage and drive it with a BC-696 Command Transmitter as a VFO. The CQ Surplus Schematics book goes further: It states things like: " The design engineers (at General Electric) responsible (for the design) almost had heart attacks when they received an ARC-5 for test comparison and found it to be a fraction of the size and weight, infinitely more stable and put out more power."

Well I have somehow avoided this thing over all these years of playing with surplus. I finally got one from Steve WA1HUD last Spring. This one was a BC191 in pretty tough shape missing tubes, the relay, connectors, many covers, tuning units and is apparently modified. Some surplus "conversion artist" had managed to replace the 211 (VT-4C) Master Oscillator with a crystal controlled 814 Pentode! This actually made sense and I think it could be a MARS or CAP conversion that was at least feasible for stabilization. Of course the tubes on all of these rigs went from worthless junk to valuable because of the audiophiles so they are always devoid of the 211's and Type 10 speech amplifier.

Faced with these facts, I decided to bite the bullet and purchase 2 Chinese 211's. These will be the modulators. But for the RF, I am going to attempt to use a pair of 814's which take the same filament voltage. The first very hard thing I did was to remove the meter panel. Wow. Over 20 screws with locknuts that spin, switches, meters and knobs all had to carefully be removed to get this panel off. But otherwise I could not see what the conversion was. I am glad I did because it was a mess. The mods were nasty and the solder was oxidized. This is going to be a process...         


* DSCF1006.JPG (2236.83 KB, 3648x2736 - viewed 526 times.)
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Jim/WA2MER
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« Reply #1 on: November 29, 2014, 06:41:14 AM »

That's going to be some project, but I bet it'll be fun and worthwhile once you eventually get it on the air. Good luck with it.

I bought all three of the Surplus Radio Conversion manuals back in the day, and even then I was disappointed to have learned that some of their conversions amounted to little more than destruction and/or parts salvage. An extreme example was the conversion of the MD-7.  If my memory serves me correctly the "conversion" began with stripping the chassis of all parts.
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W3RSW
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« Reply #2 on: November 29, 2014, 08:30:42 AM »

People P&M about all the nasty and otherwise conversions of mil. equipment.

However think of the thousands of hams, SWL's and budding geniuses (i i) that got their start and smarts by digging into quality rigs and parts hence obtaining the necessary education to advance the art past those GE engineers.

I cajoled, begged and waited years for a "real" electric train, a Lionel.
Parents finally relented, got a nice starter set when I was 11 and promptly ran it to death two days after Christmas.  Well I didn't dare tell dad that it lugged down and wouldn't come up to full speed.  Pretended that I was just running it slow if someone came in the room.  Took the engine apart and reassembled endless times as my knowledge of it and a lot of electric and radio stuff grew.  Finally as a young adult acquired the knowledge to remove the drivers (wheels) that hid the motor's rotor, then discovered a simple solder joint on one of the rotor winding's commutator had come loose allowing only two of the three sections to work which had caused the crippling of the engine all those years.
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« Reply #3 on: November 29, 2014, 09:32:34 AM »

People P&M about all the nasty and otherwise conversions of mil. equipment.
The line between stating an opinion on conversion techniques and P&M-ing is apparently a fine one that I seem to have stepped over.  Shocked
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W3RSW
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« Reply #4 on: November 29, 2014, 11:13:43 AM »

Oh no, you really haven't. I was Just referring to all the myriad conversion articles with passionate followers over the years now considered passť because of the scarcity of remaining, unmolested equipment.

Times change and the conservation impulse is certainly a noteworthy objective.
But many hams got solid, self taught educations based on otherwise unavailable or way to costly equipment that for the most part was constructed for a war planned to last eight years but ended in four.

Conversions at the least costly simply changed a rig from military assigned channels to amateur, perhaps by simple crystal substitution along with a homebrew power supply. More complicated changes, of course, ranged all the way to the bare chassis conversions mentioned earlier. ..and from the perspective of hams back then, with thousands of rigs available sometimes for the asking otherwise dumped, why not?

For another perspective of life back then, where were the conservationists when rigs were wholesale de-mil'd by Cat track crushing?  You had to live through the Cold War and nightly nuke dreams to understand the serious mindset of the military and most of the populace at large. Again, this is "you" general, hopefully not to be taken personally. I apologize for any misunderstanding. From you call, looks like you lived through that era too.(Sort of a smiley here but for the implications of what might have been.)

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« Reply #5 on: November 29, 2014, 01:12:19 PM »

Each to his own. It's all a continuum. To be hard nosed about it, these items were made for military use. Putting them on the ham bands, even with zero mods could be called misuse, improper, whatever. Some years ago, I had a guy (a self-styled purist, or so he thought) give me an earful about ART-13 mods and how his was going to be "proper" with no mods and a carbon mic. When I asked if he was only going to use it in a B-17 at 10000 feet, he went QRT.  Tongue

I just can't get worked up about this stuff, either way. Do what you want and have some fun.
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w4bfs
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« Reply #6 on: November 29, 2014, 02:10:30 PM »

hi Steve .... some folks just can't handle the truth .... for them their perceptions are reality ... do what you want with the '375 and have a good time
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Jim/WA2MER
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« Reply #7 on: November 29, 2014, 02:36:51 PM »

Rick:

No apology is necessary because I didn't take your comments personally (NJ types like myself are not easily offended, although offending others is one of our natural talents). I'm a bit disappointed that my message didn't come across as intended, but that's my own fault for not being more careful in my writing. I'm not a purist. My initial comment had more to do with what was considered a conversion as opposed to a salvaging of parts. I modified many a military radio in my day, including ARC-5s (who didn't have one of those?), ART-13s, ARC-3s and a BC-342. I agree with the others, do what you want to your own stuff because it's your own stuff. It doesn't belong to history or future generations, it belongs to the guy who bought it. I have no difficulty getting to sleep at night knowing that by the time I wake up there might be one less bone stock ARC-5 or KWM-2 in the world.
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« Reply #8 on: November 29, 2014, 04:35:54 PM »

Wow that modified oscillator was certainly more sophisticated than I imagined - I carefully studied it as peeled the onion and removed it. This is a crystal controlled electron coupled oscillator with a directly heated 814. This is a strange bird.

It was oxidized and pretty busted up but I did manage to capture the essence in a schematic. If this was a ham mod, it was a real engineer that did the deed. Take a look at this schematic and instead of discussing the ethics, let's look at the beauty. There certainly is something to be learned here. What are the two 100K resistors doing? Explain the keying method done with K1. What should be done with pin 4, the suppressor grid? What is all the tapping about on the tank? What is the Gimmick for?   


* BC191F_ECO.jpg (3104.2 KB, 3648x2736 - viewed 459 times.)

* BC191_OSC.jpg (402.75 KB, 2545x2036 - viewed 452 times.)
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W2PFY
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« Reply #9 on: November 30, 2014, 12:07:10 AM »

I think the gimmick is to keep the tube just on the edge of self oscillation to help kick start the crystal and the two 100 k resistors are just voltage dividers
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WU2D
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« Reply #10 on: November 30, 2014, 10:02:39 AM »

Here is may take and I still have some questions. Yup the two resistors are a voltage divider for the screen. A crystal oscillator with 1000V on the plate!!! This at least brings the screen down to 500V. I still think it would have been a crystal cracker.

The Gimmick is a typical method to add more feedback. This is not a Colpitts Oscillator, it is an old fashioned tuned Plate Tuned Grid (TPTG) type so it relies totally on inter-electrode capacitance for feedback. The Gimmick helps to give more encouragement to oscillation.

Notice how the 1000V return is connected to the 200K resistor, not ground. This trick sets up an artificial negative cutoff bias for the tube equal to 1/5 of 1000V or -200V. When you close the key (and relay K1), this is shorted out and the 68K provides normal operational bias. You do not require a separate bias supply. We call this grid block keying. Pretty sophisticated for the time (GE used this method on the original Harley oscillator as well).

I am not going to give everything away: What should be done with pin 4, the suppressor grid? What is all the tapping about on the tank?

Mike WU2D







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W3RSW
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« Reply #11 on: November 30, 2014, 10:09:48 AM »

Oops, was typing almost the same thing as you.

Well it's not an ECO. It's a simple crystal oscillator; note the 68 k good resistor is RF grounded by .01uf. If it were an ECO some sort of RF isolation choke would be in the cathode circuit with split feedback bridge in the grid circuit making the grid /cathode the oscillator and the grid/screen/plate the amplifier.

Interesting divider in the keying circuit.  Ground the suppressor grid. It's not grounded or left floating.  Can't tell about the tapped plate coil until I see the rest of the circuit, how it's coupled to following stage.  If the lower tap is very low impedance, perhaps it is intended as a direct sampling feed. If really low, say 50 to 300 ohms, could have been used as a low power transmitter.

One things for sure, a modern xtal, even a Ft243 would probably crack.

Why the one Meg. R from B+ to ground? Serves no purpose but week bleed of HV, but don't know the T/r switching so if the Osc. Stage can be used as a single stage xmitter and the B+ untied to later stages, then, yes, a local bleed might be helpful, but then again, for CW , major, regulation bleed must be required at the power supply anyway.  Got me ?
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« Reply #12 on: November 30, 2014, 10:17:53 AM »

Oh, you mean this is a quiz?
Ok, I'll bite.
Hook that low impedance coil tap to the suppressor grid.
That'll light things up !  Grin
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WU2D
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CW is just a narrower version of AM


« Reply #13 on: November 30, 2014, 11:33:43 AM »

Yes the 1M is 4 250K resistors in series as a bleeder. I have not studied the dynamotor yet so I do not know about a heavier bleed.

The suppressor grid should be grounded as you said. With DC on the filament this works great. But with AC, the engineer did something interesting. He connected the suppressor grid to the filament with a pair of 47 ohm 2W resistor from SupGrid to each side of the supply. This trick is normally done to create a virtual centertap for keying a directly heated tube at the filament. I would imagine that you would not want residual noise or hum on the suppressor grid of a tube specifically designed for suppressor grid modulation. This approach was interesting because I have not found it anywhere else in the literature. 

The BC191/375 is a true MOPA. It uses a Hartley power Oscillator much like the pre-war BC-AR-230/430 and later ARC-5's, but it directly feeds the PA grid rather than using a secondary link and tapped link neutralization. So the off center tap to the 814 sets a center RF ground zero phase. The lower tap is the feed to the 211 Final's grid. The top tap is to the neutralization cap to the plate circuit. So it is "direct feed" to a triode final. All of the tubes in the original set including the 211 modulators share a common 1000V supply. Thus the set is infamous for FM-ing and critical on neutralization.
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WU2D
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« Reply #14 on: November 30, 2014, 10:25:11 PM »

Well here is the final schematic showing the suppressor grid approach. I have ripped this XTAL circuit completely out of the set and have installed plates to cover the old 211 Johnson socket holes. These have been outfitted with 814 sockets. I am ready to start rebuilding. I began to trace the circuit starting with the tube filaments. The modulator checks out. But I already have an open circuit on the RF filament I have to investigate and I see old electrical tape splices - UUUGH! Yes some more mods seem to be hiding in there.

Oh and look a the the picture and you will see the whole relay assembly seems to be missing.

This is going to be fun.


* BC191_OSC.jpg (434.53 KB, 2624x2249 - viewed 348 times.)

* WhereMyRelay.jpg (2515.25 KB, 3648x2736 - viewed 467 times.)
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IN A TRIODE NO ONE CAN HEAR YOU SCREEN


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« Reply #15 on: December 06, 2014, 03:18:51 PM »

All the parts and goodies you could want...down South, too. Cheesy

73DG

http://antiqueradios.com/forums/viewtopic.php?f=15&t=261432&sid=a06bcec38247656d7434c50beecab721

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« Reply #16 on: December 27, 2014, 07:33:11 AM »

Well time for an update on the BC-191 project. I discovered that 4 of the banana plugs had been removed from the tuning drawer interface and I had to replace those. You might remember that they had modified the rig for XTAL operation with an 814 oscillator in place of the first 211. I made the banana replacements from standard banana plugs sawed to length, drilled and tapped and stuck 6-32 stock in and sweat soldered them. They look OK.

Getting the Filaments wired back to some semblance of normal was a chore.

Timmy's idea of using an 814 in the final in place of the 211 and sticking another triode connected 814 in for the oscillator will be interesting if it works. One benefit of using a pentode in the final is the reduced drive requirements and the promise of more stability. In the end it still is a triode power oscillator driving a PA in pure MOPA mode. My goal is to be able to use the tuning units as designed with no mods and get 814s to provide a low cost solution to replacing the costly 211's. And of course I want to cure the normal 191/375 maladies of drift and FM-ing and make it completely usable on 40 Meters.


There are gotchas. First of all as K4KYV will tell you that triode connecting a pentode with a low screen voltage rating is a bust. So you can no longer run 1000V on the oscillator if you do that. But as Timmy says, running 1000V on a free running oscillator is also nuts. They have three 2500 Ohm resistors in series feeding the oscillator off the 1000V.


So all I have to figure out how to get about 250 - 300V for the oscillator which should be a nice voltage range for stability in a conventional Hartley. Maybe I can put in a resistor to ground as a tapped bleeder. And by doing this simple trick, the voltage variation due to AM modulation should be 4X less for starters. If this voltage is too low for proper drive, I could chance raising the resistor to form a 500V tap as long as I limit the current with a resistor between the plate and screen. Comments? The ICAS rating on the screen of an 814 is 400 VDC.

The screen of the 814 PA will come off the modulation transformer through a screen dropping resistor for the conventional AM modulation hookup. Comments here?

Next I have to figure out what the tangle of wires chopped off where the relay once lived is all about...

Mike

  
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« Reply #17 on: December 27, 2014, 12:30:59 PM »


Mike,

   I follow this thread with interest. I once had a BC-223 in a similar state as yours. When the 801 tubes disappeared in a move to Texas, I let the rig go after cannibalizing the main 7 gang tuning capacitor.

   I once used an 814 in a Viking I after bridging the power supply to 1500 volts. It worked well until the tube shorted internally (g1 to g2 I think). After replacing it with another, the same thing happened. I ended up wiring an 813 into the 814 socket, and that worked without failure for several years. Could run 200 watts input, and with an external plate modulator, that V1 rocked.

  Thinking about your situation, if it were me with the equipment in my shack, I'd set the oscillator stage as a pentode amplifier, with variable screen voltage to set the drive to the output 211. Then drive that 814 with a Knight V44 self contained VFO. This has a high level 80M output, so perhaps you could get 80-40 out of the rig using the 814 as a doubler on 40m. This would make a MO-Buffer-Buffer/doubler-PA (4 tubes in RF path). I guess that would be cheating now wouldn't it?  Cheesy

Jim
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WU2D
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« Reply #18 on: December 28, 2014, 10:31:08 PM »

Well I have the two 814 sockets wired and it is time to tackle the relay circuit. Before I attempt to replace the front panel and meters, I really do want to try to see if I can get the oscillator running and verify the bias situation. Now I am going to slow it down a bit.

As far as handling the screen grid on the PA, I did try to figure a max of 20 mA on the screen and dropping 700V, because the max screen is 300V and 400V ICAS on the 814. This should limit the screen dissipation to 6 Watts or less. This was going to take a screen resistor of around 35K to be safe. The question was if I simply wanted to tie the screen resistor below the modulation transformer (screen self modulation) or tie it above the transformer (Conventional combined plate and screen modulation).

I ended up using the guidelines of Dean McGorill. http://www.amwindow.org/tech/htm/scrnmod.htm

So split it between two resistors of around 56K and 100K and added the two caps, 0.0015 screen to ground and .005 across the 56K to the screen. This is silly for a transmitter of this type of course but there was room and we shall see what happens.

As you can see I am adding parasitic suppression on the plates and even on the grids. The 814's will have a lot of gain compared to the 211's. I also added two 0.01 bypass caps right at the fils on the sockets. Is this overkill? Likely yes. 


* 814PA_.jpg (2421.46 KB, 2794x2505 - viewed 364 times.)
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WU2D
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« Reply #19 on: January 03, 2015, 02:36:16 PM »

The relays were tough to wire. I had no relay and 10 cut off wires poking out at me. Wow was it tough to trace these! I ended up using a 4 pole relay combined with a larger 2 pole relay in parallel to give me more than enough flexibility to replace the original, at least until I find the real thing. Truthfully the pair of 12V relays are likely a lot quieter than the real thing anyway!

I was encouraged to hear some clunking when I keyed from any of the 4 jacks (2 for a key, 2 for a T-17 microphone and one built in test key switch). While I am in there, I am really starting to clean up switch, socket and jack contacts and the pots with Deoxit. Previously I had done only some cleaning and lube on the controls to free things up.

I also plugged in a VT-25 (Type 10) and an 814 into the sockets and they both lit up but that is the limit on a bench supply!

I mounted a test plate with a pair of SO-239's on it. I am missing covers and truthfully I do not expect covers to go on for some time anyway and I need a way to test.

Next I will focus on getting the oscillator happy with the triode-connected 814 in place of the 211 MO. I have a tapped resistor voltage setup but to start I will try to run the oscillator on a simple bench supply so I can see what I have and what voltage it likes. Timmy seemed to think that 500V or half voltage would work , but I am thinking less since I have a real Beam Pentode final setup and should not need as much drive.



* BC191_Antenna_Cons.jpg (3207.78 KB, 2736x3648 - viewed 404 times.)
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Tom W2ILA
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« Reply #20 on: January 06, 2015, 05:52:11 PM »

Mike,
From the CQ Technical series 1960:
BC-375, BC-191
The BC-191, built by GE, building 89, 1 River Road, Schenectady, NY is an archaic behemoth whose design was finalized in 1935 and was produced in tremendous quantities for war destruction in planes and other expendable vehicles.  The design engineers responsible almost had heart attacks when they received an ARC-5 for test comparison and found it to be a fraction of the size and weight, infinitely more stable and put out more power.  The BC-191 for 14 volts the BC-375 for 28 volts otherwise they are almost identical.  There are a few moderately usable parts and the tuning units make nice cabinets.

I have been following your build and can't wait to hear your restomod on the (shameless net plug here) Old Military Radio CW Net on Sunday night 3570KC at 2100 Eastern.

Great work.

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WU2D
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« Reply #21 on: January 15, 2015, 10:29:05 PM »

UPDATE! Well I have both sockets in the RF path converted over to the 814's now and I popped in the Chinese 211's in the MOD sockets and a Type 801A in the speech and they all lit up nicely. I also have confirmed that the triode connected 814 oscillates in the Hartley oscillator. The grid blocked keying worked at the modest 200 VDC I was testing with.

The 40 Meter plug in looked great but don't let that fool you; I can assure you that these things need a complete clean up inside with lubrication on all shafts and verniers and deoxit on the contacts to make the them anywhere near stable.

Next I will do the fairly complex filament adjustment from the manual. The 12VDC from my homebrew power supply is just adequate to handle the fils. Then we should be close to a smoke test with 1000 VDC on both stages to see if we have some power out!

This weekend I will try to get down to Steve, WA1HUD to look for an 80M plug in and a couple of missing covers and that elusive RELAY! My hambone dual relay solution is just a quick way to test the thing.


* First Light.jpg (2616.43 KB, 4212x3111 - viewed 414 times.)

* DSCN1787.JPG (3549.41 KB, 4608x3456 - viewed 389 times.)
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« Reply #22 on: January 16, 2015, 11:01:45 AM »

Mike - do you have pic of the unit with the front cover on?  I want to do a show 'n tell

Al
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CW is just a narrower version of AM


« Reply #23 on: January 16, 2015, 08:29:39 PM »

Not sure what you are looking for Al but here are a few pics. The BC-375 was the 28V version for planes. The BC-191 was also put in planes with 12 but it really was used in ground stations and trucks and command cars and even in jeeps.


* BC-375 (1).JPG (56.19 KB, 534x434 - viewed 293 times.)

* IMAG1173.jpg (143.84 KB, 1600x957 - viewed 343 times.)

* AAFJEEP0008webcrop.JPG (32.58 KB, 636x415 - viewed 305 times.)
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Tom W2ILA
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« Reply #24 on: January 16, 2015, 09:03:09 PM »

Nice progress.  Do you have any add'l pictures of that jeep?  Very few of those WWII Willys or Fords were produced with 12V electricals in place of the standard 12V.
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