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Archaic Behemoth BC-191/BC375




 
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Author Topic: Archaic Behemoth BC-191/BC375  (Read 19272 times)
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WU2D
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CW is just a narrower version of AM


« Reply #25 on: January 18, 2015, 07:56:30 AM »

Wow Tom! I have to assume that you are correct in surmising that 99% of AAF unit jeeps would logically use the same equipment as the planes and it would be 28VDC?

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« Reply #26 on: January 18, 2015, 08:01:36 AM »

THE HORROR THE HORROR...

A visit to Steve's basement yesterday to attempt to find an 80M tuning drawer and some metal (The elusive Relay, a top and the right ANT side and a front to finish the project was exciting, dangerous and surreal all at once. Imagine being buried alive in radio parts, freezing to death and being found as a ham radio mummy some thousands of years later.


* TheHorror.jpg (1919.38 KB, 2448x3264 - viewed 432 times.)
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« Reply #27 on: January 18, 2015, 04:24:52 PM »

When I took this project on, I had some goals beyond just restoring the beast.
1. I wanted to see if the now expensive 211 (VT4-C) tubes could be replaced by 814's.

2. I wanted to see if the thing could do AM and CW on the modern ham bands, at least on 80M.

3. I wanted to see if all of the harsh talk about this radio was true.

4. I had a goal of achieving 40M CW and AM operation.

I can report that the 814's are able to do 60 and 80 Watts out respectively with 80M and and 40M tuning drawers with 1000V. That is commensurate with the 211s. No problem - same filament- good drive - it works.

But - I am stunned. After several days of exploring how this thing loads up, I am mostly shocked by the horrifically bad architecture and neutralization scheme. Yes, a triode connected 814 will not require as much capacitance as a 211, I thought, and it should not be so far off that I need to change the variable or tap point in the tuning drawers- or so I thought...

In frustration I tapped up the 80M oscillator coil - down the coil, put capacitors in series to lower the neutralization feedback. But every control influences every other. Ultimately you can not properly neutralize the transmitter. An un-neutralized MOPA is no proper transmitter.

Was this thing in the same war as the TCS, ARC-5, BC-610 or BC-653 or 654?

 
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« Reply #28 on: January 18, 2015, 07:41:28 PM »



Was this thing in the same war as the TCS, ARC-5, BC-610 or BC-653 or 654?

 

Yes, I think it was in the same war, and if you remember, we won that war.  Do you know why we won that war?  Because our radio sets were far better than the other guys.

Fred
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WU2D
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CW is just a narrower version of AM


« Reply #29 on: January 24, 2015, 01:46:53 PM »

I was curious if the BC-191 was flown or if it was simply for ground communications. Turns out it was. It was the main liaison transmitter on the C-47 in a setup known as the SCR-187-A

So there were a lot them were manufactured for planes. The C47 was equipped with:
SCR-211-N Frequency Meter
SCR-AJ-183 Radio Set (Command) Pre-ARC-5
SCR- 187-A Radio Set (Liaison) including the BC-191 and BC-223
SCR -265-G Automatic Radio Compass
(or SCR-280 Radio Compass)
SCR-578-A Sea Rescue Transmitter
RC-32 Filter Equipment
RC-39 Marker Beacon
RC􀅪􀀩5 Inte rphone Eq uip ment
T-17 Microphone


* C47 copy.jpg (253.55 KB, 1132x840 - viewed 287 times.)

* ACR-187 copy.jpg (204.8 KB, 960x738 - viewed 330 times.)

* SCR-AJ-183 copy.jpg (519.48 KB, 1041x803 - viewed 285 times.)
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WU2D
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« Reply #30 on: January 24, 2015, 03:07:31 PM »

The damage so Far: And I said that I was going to lay off any modifications. Well it turns out that the choice of 814's to replace 211's while sounding good, was really a bigger stretch than anticipated. You see, a 1924 something Western Electric low MU triode beast is not the same as a "modern" 1935 beam power pentode. Laws no. Even though this combo promised an easier to get and cheap solution to the tubes and the filament requirements were identical, these tubes could not be more different. The triode connection in the series fed Hartley worked out OK, but the voltage restriction based on the 814 screen would mean I should not run it any higher than 300-400 VDC rather than the 1000V on the 211. Actually that worked out because who in their right mind would operate a free running master oscillator at 1000V ?

But the PA final change out was really the issue. The 814 has a lot of gain and much lower inter-electrode capacitance. Technically it would not need neutralization to be stable according to the book, but not in this box! Taming it in such an open box with very long wires and meager bypassing, and with a plug in drawer approach, proved difficult indeed. There is no way that the 1925 vintage Rice direct grid neutralization on the MO tank in the drawer was going to work with an 814, heck it barely works with a 211. Grudgingly I removed the TU-6-B 80M drawer and prepared to blast. Somebody had already shot a couple of holes in it and it did not have much paint left so no foul so far. My idea was to emulate an ARC-5 version of the rice circuit which I know works with Pentodes (like the 1625). But when I saw that oscillator tank assembly I was not sure how I could get a tapped secondary over (or inside) the MO coil.

But then I spotted the PA tank. That looked more open. My idea was to convert to Standard Plate Hazeltine. With Hazeltine, you get the negative feedback by tapping the PA tank B+Feed which is bypassed, and connecting the neutralization cap on the opposite side of the tank as the tube plate side. This almost worked. I had way too much feedback for a pentode. This connection would be perfect for a 211 however! So if you do not like FM on your AM, think about this approach.  Turns out I needed to put the B+ bypassed "center tap" only 2 TURNS up from the cold side of the tank! Now I am neutralized - way good. Along the way I had to add parasitic suppression and more bypassing. Here is the before and after in simplified form... 

I still have a ways to go before I have a clean CW note, but it is a lot better.  Mike


* BC191_RF_Simplified copy.jpg (212.7 KB, 1258x802 - viewed 311 times.)

* BC191_RF_Simplified_WU2D_MODS.jpg (231.28 KB, 1258x802 - viewed 287 times.)
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Tom W2ILA
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« Reply #31 on: January 25, 2015, 08:51:30 AM »

Mike, on the jeeps bit.
The vast majority of jeeps went to Army contracts.  These were 6V. 
There were a number of other contracts for the Navy/USMC but the details escape me.  Again, most were 6V.  There were again some jeeps produced in 12V including the MZ radio Jeep with a TCS set.  A couple of these exist and have a separate PTO driven 12 volt system.
USAAF - I don't have any info.
While there may have been 28 volt vehicles produced during WWII I am not familiar with any.  The mil migration to 24 volt vehicular systems began with the M-series vehicles of the 50's.
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WU2D
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« Reply #32 on: January 25, 2015, 10:20:39 PM »

I put covers on it today and it was doing 60 Watts into a dummy load and sounded passable on CW. So I put the beast on the air tonight for the maiden voyage on the Old Military Radio Net CW Edition on 3570 kHz. Oh they heard me all right. I was LOUD but the note was bad. She jumped frequency a couple of times too, and it chirped. Not well behaved for having so much work put into her. Wow this is going to be a tough nut. I do not think I have ever put so much work into a radio with so little to show.


* BC191_MaidenVoyage.jpg (2262.55 KB, 3210x3356 - viewed 351 times.)
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« Reply #33 on: January 27, 2015, 06:01:19 PM »

I found the pedigree of the beast. It was a 1930-31 design by GE for the new Air Mail planes, the RT-76-A. The topology remained though military development in the 1930's.

Peeling the onion, I was wondering why the 211 modulators were idling very high at close to 50 mA each instead of the Class B value of around 10 mA per tube. To my horror I discovered that they derived bias for the modulators and the speech amp from the PA grid leak! Well the 814 pentode PA was not acting like a 211 in yet another important way. Geeze. I tried introducing a 10K resistor to the bottom of the series, but only got a little improvement. I measured -17 VDC on the modulators. I do not want to run A2.  I think I will throw in the towel and tie in some external fixed bias. Between the lack of regulation and decoupling on the B+, the direct stage coupling with questionable neutralization and this bias trick, you might start to figure out why these things FM. 
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« Reply #34 on: January 27, 2015, 10:59:46 PM »

I made a small bias supply.  With -90VDC connected to the top of the bias pots, I can now properly adjust the Class B 211 modulators to proper resting current. They want 10 mA each or a total of 20 mA for Class B. Bias for the modulator and speech amp is no longer dependent on the RF Grid leak bias on the final. I have a total of 15K of grid leak for the final amplifier. This may be too high, but it is a good place to start. I still want the HV cutoff grid leak bias active so all tubes draw zero key up so I kept the returns the same at the bleeder tap point.

The speech bias pot seems scratchy and may be bad. I am modulating! The gain is very low and I can only load to 35 Watts out and get decent modulation.

So I will work on the speech amp tomorrow.

I am going to try an FM test later this week with my ICOM receiver. I will put a tone into the thing and tune for minimum FM as a critical neutralization test.


* SimplifiedSch_BIAS.jpg (270.94 KB, 1374x866 - viewed 312 times.)
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« Reply #35 on: January 28, 2015, 02:28:06 AM »

Seems R-614 is also part of the grid leak resistance.  The sa bias must be bias for the speech amp.  15K ohms seems a good starting point for the grid leak.  You can figure the resistance by knowing what negative grid voltage is required for class C and knowing what the grid current should be for the tube you're using.

Fred

Read some of the earlier posts,  FMing very often is caused by an oscillator that is running on the same frequency as the output stage.  An oscillator running on 1/2 the output freq and doubling up to the output freq will eliminate FMing.

Not sure if this is completely true with a crystal oscillator.  I think I saw a schematic for a 814 crystal oscillator which I think you're using.
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« Reply #36 on: January 28, 2015, 11:18:01 AM »

Perhaps the authors of the CQ Surplus Conversion Manual deserve a little more credit than they are given today?  Within the time period and with the demands of producing something the BC-191 may have been useful at the beginning of the war but more modern sets like the ART-13 and the use of VHF pushed the  BC-191/375 aside into the trash heap of history. WW2 began with things like the BC-191/375, GO-9, TBW and TRF receivers and ended with useful transmitters like the ART-13 that stayed in service until the sixties and seventies. I would doubt that no one in their right mind would have been using a BC-191 in an aircraft after seeing an ART-13.  Not much that was used at the beginning of the war was considered useful by its end except things like the BC-348 receivers and the BC-610 transmitters, but this is just my speculation.
It is interesting to see that with your level of skill and expertise the amount of problems youíre having with this challenge, I donít think I would try the BC-191 challenge.

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« Reply #37 on: January 28, 2015, 10:31:16 PM »

Yes R614 is part of the grid leak but it is a low value parasitic suppression resistor.

They used 10K for the Grid leak for Class C bias on the old 211 PA and my gut feel of 15K for the 814 turns out to be right on purely by chance. Actually I kept the 4K 10W that was already there and added a 10K, 10W so 14K. Take a peek at the 814 conditions at 1000V with 10 mA on the grid and a goal of -150V bias.

Calculating you simply get 15K/10 mA = 150V. Not much to that math! 

I can not get 100% modulation over about 35 Watts out. That is disappointing. I do want to try another Final. It does not look too bad on the scope with pretty good symmetry. You may remember that I selected a 39K screen resistor and this also agrees with the spec., so I think I have the Final set up correctly. Drive may be another story, but the 814 should be a lot easier to drive than the original 211! I am tapping lower on the Harley coil for less chirp on CW.

 I adjusted the bias on the speech amp dynamically with a sine wave and went for least distortion at high level modulation. The 801A that I am using in the Type 10 socket is a beautiful triode with impressive power out specs.

Next I have to get back on my power supply and make sure I have enough C in my filament circuit. I am getting a little shy on voltage now that I am lighting 5 tubes.

I am getting close to an on air test. The Covers are staying on. Next I will try the dynamic neutralization test with an ICOM RX in the FM mode.



* 814_ClassC.jpg (206 KB, 642x885 - viewed 277 times.)
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« Reply #38 on: January 28, 2015, 11:12:26 PM »

I have one of these BEASTS, it is on my to do list when I retire. We have a number of guys here in California/Nevada who have these on the air, all sound bad to you don't want to know. The main reason I have one, with a few spare 211's, is that this is what my father would have used as the Bombardier/Navigator/Radio Operator in WWII on a B-24.

    A few years ago we had a BC-375 Shoot Out on the Sunday Morning Vintage Military Radio Net, we had 4 BC-191's and 3 BC-375's check in. Here is an article I wrote that ran in Electric Radio and a nice New England Fall BC-375 picture.

* Vintage Military Radio Net BC 375.doc.docx (14.3 KB - downloaded 497 times.)
* Vintage Military Radio Net BC 375.doc.docx (14.3 KB - downloaded 111 times.)

* Fall BC-375 Low Res.jpg (261.79 KB, 768x966 - viewed 333 times.)
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WU2D
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« Reply #39 on: January 31, 2015, 09:01:27 AM »

Maiden voyage this morning on the Old Military Radio Net. Lucky that there were 3 other BC-191/375 transmitters on for an unbiased  comparison. All FM and none are anywhere as good as a stock Apache mind you. You basically must have at least 8 kHz of BW to "hear" them properly.

I started with a carbon microphone, switched to a carbon mic replacement which is a T-17 with an electret and Darlington and finally a low Z amplifier with a dynamic microphone.

No Soap. They heard me but my 191 was by far the worst performer in terms of audio quality and FMing. This nut is not cracked.  5 weeks of hard work and using modern tubes have produced a worse situation than the stock units running 211's!

After eliminating neutralization, coupling and bias PULLING as the major FM causes, it is time to investigate the oscillator regulation  which is related to frequency PUSHING being the FM source. My approach will be drastic. I will attempt to separately regulate the oscillator to investigate.




 


* DSCN1813.JPG (3361.61 KB, 4608x3456 - viewed 318 times.)
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« Reply #40 on: January 31, 2015, 09:11:15 AM »

Mike,

Just out of curiosity, is the FM'ing at the audio frequencies or is it "modulation envelope warbling"?

By the way, since my father was a ham,  as kids in the late 50's and early sixties we had some of the tuning units to play with.  In the winter our picnic table was stored in the basement and we would throw a large tarp over the table and set up the tuning units and other junk underneath.  This was our space capsule.
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« Reply #41 on: January 31, 2015, 10:49:53 AM »


No Soap. They heard me but my 191 was by far the worst performer in terms of audio quality and FMing. This nut is not cracked.  5 weeks of hard work and using modern tubes have produced a worse situation than the stock units running 211's!

After eliminating neutralization, coupling and bias PULLING as the major FM causes, it is time to investigate the oscillator regulation  which is related to frequency PUSHING being the FM source. My approach will be drastic. I will attempt to separately regulate the oscillator to investigate.




 

wow!   this one is being tough  it would be interesting to change the mo to a buffer and excite separately to see iffin it would settle down ... I like the lockout / tagout idea .... we used them in the nuclear biz .... massive woe to anyone who disregarded the tag !
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« Reply #42 on: February 02, 2015, 09:48:56 PM »

Your suggestion of external drive is exactly what was suggested by the CQ conversion handbook VOL II. They basically turn the VFO into a neutralized buffer. But that is cheating.

It was quite disheartening to have to remove the hundred or so screws all covers and strip it down to where I started. Removing the metering panel was again - much fun.
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IN A TRIODE NO ONE CAN HEAR YOU SCREEN


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« Reply #43 on: February 02, 2015, 10:32:52 PM »

One (other) problem on these is the "family" of TU's.

Many have stated the TX and the set of TU's were married at the factory, and swapping them was an invitation to problems.  I suppose this was due to the N+1 rubric of handbuilding them.

Complete matched sets were much easier to get going according to the blurbs in the '46 to '50 ham rags.  I am sure how they were indicated, maybe by serial #s.

73DG
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« Reply #44 on: February 14, 2015, 09:05:31 AM »

Two weeks later and another update on the beast. Let's just say that many long held myths are being dispelled as I go through the transmitter to try to find the source of the characteristic FM-ing.

1. It is the B+ 1000V power source. This has always been a great theory. After all they tie all of the B+ on the BC-375/191 to the same point.
2. It is the Filament Source, especially on AC.
3. It is the tapped HV Bleeder below ground bias system for grid block keying.
4. It is the direct MO tank feed with no buffering.
5. It is the High unregulated Voltage on the MO stage.
6. It is the high Grid Drive requirements of the 211A PA.
7. It is the weak Neutralization scheme.
8. It is the funky PA Grid leak that is used to bias the Modulators.
9. In general lack of proper bypassing.


All of the above, while showing the limits of late 1920's design practice and all having minor effects, have been systematically mitigated or replaced with proper design practice. None of the above is the root cause. But let me tell you - I am getting close to understanding what is going on, and you are not going to like it.
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« Reply #45 on: February 21, 2015, 07:19:28 AM »

Oh it gets worse - much worse, but I will get to that.

On the positive side I finally have a working RF section now with a steady CW signal and a passable note on 80M CW.

And the FM situation is solved.

This was accomplished by converting the Hartley power oscillator to an electron coupled oscillator in the form of an 837 (12 V Fil) driving the 814 final. The screen of the 837 HAD to be regulated and I added a 150 VR tube. The neutralization is tapped plate on the 814. The keying is standard grid blocked on both stages. The tuning unit had to be modified extensively for this of course but very few new components are involved. It is just really a rearrangement of connections in the drawer. So the transmitter is now solid on 80M at 70W out and with 3 mA on the grid of the 814 and a very conventional late 1930's MOPA design. Is this still a BC-191? Hell no. 

So my attention moved to the 211 modulators. I am running fixed bias now and made the mistake of not keying the modulator protective bias with the grid block on the RF stages (keeping the modulators at operating Class B bias at all times). The problem here as with any high power audio amplifier, is that you now have the potential to operate the amplifier without a load (the transmitter when transmitting presents a safe impedance load). Well as you can guess I achieved a situation where there was too much swing with no load and it arced over likely taking out the modulation transformer. Replacing a modulation transformer in a BC375/191 is way up on the scale of crazy. I have changed on modulation transformers on ART-13s and they are cake compared to what I am about to try. 



* VR_Tube_BC191.jpg (2821.72 KB, 4608x3456 - viewed 322 times.)

* DSCN1818.JPG (3384.93 KB, 4608x3456 - viewed 278 times.)
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« Reply #46 on: February 21, 2015, 09:31:28 AM »

Very nice rig and progress Walt.
Be careful setting modulator bias and -tone pots next to the now very open HV plate line on that 814.  A long fibre screwdriver might be in order.
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« Reply #47 on: February 21, 2015, 04:30:59 PM »

Rick, you can not see it but the plate lead is sheathed with clear tubing.  Mike
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« Reply #48 on: February 22, 2015, 08:02:29 AM »

Oops, and your Mike too.
Thanks. I looked closer and saw that even with the 811's , etc. some of the adjustments look to be directly behind the tubes, particularly on the left of the partition. Guess you need a double right angle driver to adjust them.  Grin
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« Reply #49 on: February 22, 2015, 10:04:15 AM »

Congratulations Mike.  I can't wait to hear you on the air.
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