Just to see if my BC-375 would even "light up", I hooked up a 24VDC 10A supply to the filaments via SO-41 (A+ to 45, 46, 54, and A- to 53, 47 and 56). Switch on, start bring it up on the Variac. The Variac fuse blows at 45% setting and the insulation overheats on the wire going to pin 47 - on the ground side if I'm interpreting the schematic correctly.

Why is this happening? All 3 (53, 47 and 56) read continuity between each other and the frame. The schematic appears ambiguous to me on the polarity issue. Am I applying power correctly?

This thing blew a 24VDC/10A switching PSU last month and I assumed it was because of the cheapie PSU.

Where next?

45, 46, and 54 should go to the positive side of the battery. 53, 47, and 56 should to to the negative side of the battery or to ground. You should show some continuity across the paralled pins from the plus side to the minus side, as you are measuring across the filaments.

But it should not be a short and blowing fuses. Check around the tube sockets and the 24/28 volt switch in the tube compartment and also check that the connections on the big filament compensating resistor are correct. Something is probably shorting something out.
Curt

Wafer sockets come to mind with their problems?
I'm not familiar with the set.

Jim,
There are no wafer sockets in this set! It has five tubes; one 10Y and four 211's. All bayonet style, ceramic and aluminum.

The BC-375 is a 28v version of the BC-191 that was designed around 1932.

A real manly transmitter!

John

John....

You know what I'm talking about though. I realized that after I posted.

Tracing the filament wiring in the BC-375E can be very intimidating to some. The circuit with the series dropping resistors and switches is very complicated, especially around the compensating circuitry. The internal 24/28 volt toggle switch adds complexity to the CW/Voice/ MCW switching that cuts in or out filaments of some of the tubes. Probably one of the most complex filament circuits of any radio ever built.

And I suspect that it took them all of the three years from when the BC-191 was designed in 1932 to 1935 when the BC-375E was finalized.

What really messes these rigs up is when some ham thinks he can rewire the filaments and run them off an AC supply. That is NOT the way to convert them!

Even in about 1948 when they were sold complete with all the tuning units and dynamotor for $69.95 from an outfit in Boston, MA called Radio Shack they had a four page booklet packed with them that Radio Shack printed up showing how to convert them for ham use. The power supply they showed used a hefty plate transformer good for 350mA at 1000 volts DC and used a pair of 866A mercury vapor rectifier tubes and the filament supply was a hefty transformer powering a big selenium rectifier to obtain the 28 volts of DC.

That is where my first BC-375E came from. I bought it at a hamfest back around 1986 from an elderly ham who worked for Boeing in Renton for many years. He told me he thought it would be a cheap ham transmitter and ordered it from Radio Shack and when it arrived, he never did anything with it. Other priorities in life took over, like babies and such and he was inactive for quite a while, but he did maintain his license.

He stored it in a closet and forgot all about it. Then he found it and no longer wanted it, so he took it to the W7DP hamfest in Milton-Freewater, Oregon and that is where I bought it. I had been looking for one for many years, but never found any. This rig still had the cardboard box and the tuning units were also in their cardboard boxes. No price was marked on it, so I offered him what I had in my wallet, which was fifty dollars and he said I will help you load it up.

It did not have any tubes in it, and he said he had the tubes for it, but could not remember where they were stored at, so we exchanged addresses and about a year later, I got a letter from him saying he found the tubes that came with it and wanted to know if I wanted any other junk from the old Command Sets. I said sure, so we made arrangements for me to travel to meet him at his home in Kent, Washington the following Saturday afternoon.

It was a long trip for me, but I loaded up my Datsun pickup with Command Set goodies and he handed me a small cardboard box about the size of a large priority mail flat rate box with the tubes in it. Inside were four VT-4C/211 tubes and one VT-25A tube and the pamphlet from Radio Shack on how to convert it to ham use.

So I got a lot of memories of the BC-375E. Funny how things go, but within two years of that, I picked up two more of them. So now I have three of them. That brand new one that I got first is still stored in its original cardboard box, along with the tuning units. One have done nothing to, and the third one, I did manage to make up some cables for it and the dynamotor and I did manage to light up a 60 watt light bulb pretty bright with it while the dynamotor was hooked to my bank of four six volt golf cart batteries. The relay that is keyed and controls everything from keying to antenna switching is very noisy and not the thing for a shack that is in a house occupied by others. But in a B-17 flying in Harms Way at 25,000 feet, I don't think anybody cared about the noise the relay made!
Curt

I pulled the top and rear covers looking for something obviously out of place to no avail. Shake, shake - nothing ratling or falling out.

Thing that confuses me on the schematic is that the cord pin-out seems wrong - a mirror image of the sockets it's supposed to plug into. My question: is the pin-out for SO-41, the one next to the B+ input labeled like I was looking at from the side of the rig?

If, so - the wire to pin 47 still overheats. I put it on a beefier Variac and this smoke occurs at around 25% of full power.

Mebbe I need to run #8 to all of them..........

In the mean time, I'll start checking circuit continuity. there's a dead short in there somewhere..............

Pins 53, 47, and 56 are all jumpered together and is the negative or ground. I can't understand why one of them would cause smoke when the others that are connected the same way don't.
Curt

I got the manual out and started looking at the wiring diagrams but quickly got a headache. That is indeed a complex arrangement when you see it for the first time...the primary wiring goes to a connection panel, the dynamotor, compensation gizmos and all over the place. I would suggest making photocopies of the pages and tracing the filament lines out with a colored marker to stave off insanity.

Unless it's a new transmitter right out of the box, you have to expect that someone was hacking on it.

I think I would remove all the tubes before tracing things out so that the filaments don't mask a short somewhere.

The history of the BC-375 is an interesting one. I got into it a little bit when I was researching the new mid-1930s aircraft radio position I've been trying to assemble at http://aafradio.org/flightdeck/1935/1935.html The first instantiation of it was the RT-76-A, a commercial offering that was developed by GE for mail and transport aircraft.

I posted a .pdf from the IRE article of August 1932 which suggests that they actually began working on it in 1931 - http://aafradio.org/docs/GE_RT-76-A.pdf It used four UV-211 tubes, and a UX-841 as the speech amplifier, but the circuit should bring a smile to anyone who has worked on a BC-375 - it is almost identical. There are some resemblances even then to the progeny that ballooned during WWII.

The first truly military version was the BC-AA-191, which has a contract date of 16 July 1935, although one of the tuning units I found for the set has a date of 9 March 1934. ( http://aafradio.org/garajmahal//BC-AA-191.htm ) Like the RT-76-A, it used a unique shock mount that didn't resemble the later ones, and a separate antenna tuner was required that was eventually built into the BC-375.

The first transmitter that truly reflected the later configurations was the BC-191-A, which had an initial contract date of 18 May 1936. It used plug-in tuning drawers that were nomenclatured TU-*-A, and had antenna terminals on the upper right corner so that they could be used with the BC-AA-191 as well. Below is a snapshot that shows the minor external differences between it and all the later 191s (and 28v -375's.)

The BC-191-B and all the BC-375s used the later black face meters (though I have seen at least one -B model with the white face version.) I haven't yet found an initial contract date yet for the first of the BC-375s, though I suspect it was in 1940, when designers began to realize the 12 volts wasn't going to cut it in new aircraft with burgeoning avionics employment. The changes needed were minor, with a new dynamotor and filament circuit modifications.

73,

Mike- my reference to the 1935 date for acceptance of the BC-375E was Walt Hutchins, KJ4KV.

On July 14, 1935, the B-17 Flying Fortress made its debut. One of its features were a 24/28 volt electrical system, due to the size of the aircraft and all the electrical components aboard it. The BC-191 was saved for ground operations for the most part after that. Some of them were even mounted in Jeeps, and I have an old book that shows a picture of a BC-191 being Jeep mounted.

But General Electric saw the coming of 24/28 volt aircraft electrical systems and made considerable changes to the BC-191 and updated it with the built-in antenna tuner and a few other minor details and called it the BC-375E. Why it was tagged with the "E" suffix is unknown and nobody seems to know why. They were all "E" versions.

But when you look at the overall prospective, they were obsolete by the time we went to war. If you study the development and use of the AN/ART-13, they were not that much behind in the deployment of the BC-375E, but they were vastly more modern and used more updated tubes and circuitry.
Curt

Gee, I dunno, Curt. I'm sitting here looking at a set of installation drawings for the B-17C that have a BC-191 installed. Before we transferred it to the Air Force Museum, I had the opportunity to crawl through the Swoose, the only B-17D in existence, to determine what I would have to do to restore the avionics. Going over the history of the aircraft, I came across an entry where it was later converted to 28vdc, so up through at least that model they appear to have used 14v systems.

On the BC-375, I have a BC-375-D, and the tag says the contract was 30 June 1941. The operator's manual covers the BC-375-C, D, and E model, so there were at least those variations. The preliminary instruction manual for the E is dated May 1942. They were all 28v, as you said.

I know Walt fairly well...I'll ask him about the perception next time I send him a note.

73,

Mike- I am not doubting what you say in the least. I am finding it very fascinating. Now you got me going. Do you know what any differences were between the C and D models as compared to the E model?

I find it very interesting about the early B-17's having a 12/14 volt electrical system, also. I once saw some pre-war promotional stuff that Boeing put out, probably in competition with Consolidated Aircraft. In it it mentioned a newer higher voltage electrical system that allowed easier engine starting in remote airfields where an APU (Auxillary Power Unit) was not available. So I had assumed that they were all 24/28 volt systems by time we entered the war.

I guess the more we think we know about some things, actually the less we know about them.

My manual on the BC-375E is dated January 5, 1943, by the way.
Curt

You, me, and maybe one other person reading this thread...
Poor radio-tuber is still trying to solve a problem and here we're talking about contract dates...



I can't honestly see any differences based on a quick scan, Curt, other than the nomenclature tag info on the E model changed from listing the exact date of the contract to a code in the contract number suffix that simply indicated the year. 1940 and 1941 saw a huge upswing in military acquisition contracts, and sequential contracts were let as the numbers needed grew higher. For that reason, I think the contracts through the first E contract in 1942 were simply separate contracts where they incremented the suffix and increased the total number of sets per contract. Same was true of the BC-348. Then they got smart, or the war pressures by then limited the competition. GE won them all anyway, so they simply issued contract mods from then on, eliminating the need to change the suffix from -E.



I don't have the numbers handy at the moment, but I seem to recall the B-17 had a decent increase in numbers during the -D model production. I'm not certain, but I suspect that's when they began making the switch to 28 volts, probably early during the contract run. The original YB-17 had a BC-191-A installed, according to the drawings I have here - aircraft numbers listed on the drawing are 36-149 to 36-161. (There's a cool serial number database at http://www.joebaugher.com/usaf_serials/usafserials.html that goes back to 1930 and has ultimate disposition of each aircraft.) The liaison receiver was the venerable BC-224-A. The command set was the SCR-AE-183, which is one reason I'm pursuing the balance of an -AE set for the 1930's position in the shack. As long as we're talking about receivers, the ash receiver was pure 1936 Ford, which as I mentioned on my website ended up requiring Ford to continue manufacturing them for Boeing through at least 1945 for the B-29. Some engineers are just lazy...



Except for the preliminary manual I mentioned, that's the earliest -E manual with a T.O. number on it that I have as well. The date on the preliminary -E model manual is 1 May 1942, the one for the -D model is 1 Sept 1941, and the one for the -C model is 3 August 1939. The -A and -B models may have been only for pilot run quantities, since I've never seen any reference to them.

That's about all the arcane minutia I can dig up this early in the morning...

73,

Radio-tuber, I haven't forgotten you. Geoff Fors gave you excellent advice, at least from my experience. The only thing I'll add is to go through the nine combinations of feeding DC voltage to only two of the possible pins at a time until you hit the pair with a short. Then you can go back through the wiring diagrams with a diminished set of alternatives. The variac is a good method of controlling the current, but I'd also put an ammeter on one of the power supply leads so you don't have to rely on your sense of smell to notice when you have the short. At some point you will likely need to unstrap a ground lug or unsolder a wire to pin it down, but sometimes that's the only choice.

"You, me, and maybe one other person reading this thread... "

Count me in...

Maybe I'm a glutton for punishment, but this stuff reads like a detective story. After 20 yrs in the Mil. Spec. production end, I got out of most of it when Grumman said bye bye to Long Island, dates, facts and figures from a bygone era shows me that it was the "Same 'ol-same'ol" back then also.

Don't feel bad about this background info, fellas. Interesting reading!

So far, all the wiring looks OK visually using a big flashlight. No blatant hacking, but that side of this baby is VERY densely wired. Am beginning the point to point resistance measurements as presented on pg. 32 of the 1943 BAMA manual. The idea of the 9 combination test is good. I'll do that also.

I'm using #18 wire for the individual socket connections (and female end of bullet connectors - a perfect fit!).

The first time I "fired" the thing up, both wire clusters jumped as if I'd bumped the workbench. That takes beaucoup amps to do, for sure. why it's all coming back on a single negative lead is odd. Schematic shows all of them connected together like the positive also.

If your lights flicker today, hopefully it won't be me, hihi!

PS: the chart on pg. 32 says "Point 8 of tuning unit plug-board". Any idea how these are numbered. I assume left-right when viewed from the front.

That's correct. Looking from the front, it's the 8th terminal from the left end. Those are just convenient terminals to measure the continuity and resistance of 1118 and the primary of 1164 in that particular step.

I am always amazed at the dearth of unambiguous designations in the 1930's manuals (e.g., GE just kept reproducing the same format clear to the end on the BC-191/375 production.) It's almost as if you have to be a member of a secret club, having both the intelligence and mental agility to be able to flip back and forth between three or four drawings and correlate all the blasted information. I have a strong feeling that the training classes included what the Navy still calls "gouge", which in this case would be a mimeographed sheet of component values, terminal numbers, and the like. Either that of each tech would have to create/share his own. Or maybe WWII techs were just naturally smarter than I am...

Don't answer that...I probably already know the answer.....

73,

Basic continuity checks are intermittent between iterations. I must have thick oxide non-conductive coating on the pins on these sockets. Tough getting the same answer twice between adjacent pins and between them and ground...........ARRRRGH!

Tried using the bullet connector females and can't even get consistent readings between them and base of the associated pin (wiggle, wiggle, test).........I checked this w/3 DVM's on the chance I had a bad set of test probes.

So.....is there a "Miracle Oxide Remover" that I can apply to these contacts? Pleast don't tell me I have to remove the panel above TU along w/ the plug assembly, unsolder everything and disassemble the sockets for cleaning...............I'm low on Cold 807's.

Maybe I can find some sort of Dremel-like buffer to get the crud off w/o snagging and destroying any of the pins. At the worst, it makes a nice display when the Collings Foundation planes visit.