One of my all-time favorite war movies--except for the faky scene where they removed the fifty(?) calibre MG's from the Fortress and fired them from the hip?

I have read that the first Flying Fortresses were equipped with .30 caliber machine guns. After seeing what the RAF was doing during the Battle of Britian, that finally convinced the powers to be to convert to .50 Caliber Brownings in the U.S. Bomber fleet.
Curt

Perhaps that explains it. And all those years I thought that part was just kinda fake...
I do remember them cutting off the tail, to install a tail gun, that all Fotresses eventually came with from the factory. That definitely looked like a .30 cal...
Also heard on the Military Channel last night that the P-40 originally had two .30 mg's and two .50's, then later switched to six .50's.
Can anybody identify the radio equipment in that Fortress?

Traced an intermittent modulation problem to a cruddy mic jack (as usual), and got the '375 finally set up last night. Lights up a 200W bulb very nicely!

I have it out at FD and running into a 40M Butternut vertical. It LOVES a real antenna! 7290 AM and all tuned up yields 320mA w/a slight flicker on the plate current meter. I'm bringing my scope out and will take a gander at the modulation envelope. I'll be on 7290, 3880 and 3983 tonight. I'll be calling as W6ZZK, the Club callsign.

Gimme a listen if you can!

Jim,
Very good! Will give a a listen o the TCS.

John

Well, I was over-enthusiastic (as usual, hihi). New member of the club is a TV broadcast engineer. He thought my signal sounded funky (has hum), scoped the 375's output and "320mA w/a slight flicker on the plate current meter" is ~30% modulation. Time for an outboard audio amp.

Also, a "minor" detail: I neglected to neutralize it after re-loading the tubes, so the PA is generating spikes every ~10kHz either side of the carrier - AKA: "Broadband w/o powerlines". That's where the carrier hum is probably coming from. Good power output, just all over the spectrum. When they said to re-neutralize it after installing a new PA tube, I guess it was for a good reason.

Back to the manual to read up on neutralizing............ I'll get that baby there yet.

It sounds like you're getting there, slowly but surely.

The 10 KHz modulation does indeed sound like a neutralization problem... but it may also be necessary to also go with measures to kill parasitic oscillations.

You may have to throw away the book and do your own experimentation on that. In the 1930s and 40s they DID do neutralization, but spurious emissions standards back then weren't as tight as they are today. The manual measures may not be enough.


Mr. T. W9LBB

This is ambarassing, but neutralization on 40 was messed up by yours truly several months ago while attempting to "adjust the TU calibration".

I may be preaching to the choir, but you know those disk-shaped "neutralization cap"-looking things behind the front panel? They are for MO alignment, NOT neutralization.

The neutralization caps are in each TU behind the frequency chart and they are are NOT for transmitter calibration as I thought (insert Gomer Pyle here: "Surprise, surprise, surprise!").

The adjustment thumbwheel is locked in place by a pair of metal pincers that you unlock by turning a fat setscrew next to the thumbwheel port 90 degrees.

I'm going to do the neutralization "by the book" and see how close I can get. My pal w/the $$pectrum analyzer will look at the output and see what it looks like, then we'll take it the rest of the way.

Comments about harmonics are reasonable and I'll watch for those. The PA output runs thru a parasitic supressor, IIRC. If necessary, I'll run it all thru an outboard Pi-network or somesuch.

Those dime size discs that you mentioned are the temperature stabilizers. They form one plate of a capacitor of a few pF and the stator of the variable capacitor the other plate. The arm or that thin metal piece that the discs are attached to is made of bimetal. As the temperature changes, these arms bend and cause the discs to move closer or further away from the stator of the main capacitor, hence varying the shunt capacity.

They may sound crude, but they did the job and did it quite well. I don't recall any other designs that used something like that. But remember that the BC-375E was designed in 1935. Since it was a MOPA set and not crystal controlled, temperature compensation of the master oscillator was a paramount requirement for aircraft use.

The aircraft could be parked on the tarmac waiting for its mission to come up and with the summer sun shining on it, the internal temperatures could be over a hundred degrees. Then the aircraft takes off and in less than an hour be at 25,000 feet or more and the temperature in the radio compartment well below -40 degrees.

Still with such a wide temperature excursion, the transmitters were expected to hold frequency so they could communicate with the base, which could be hundreds of miles away. Even though the design of the entire transmitter was obsolete by the time WW2 broke out, they soldiered on thru the war and got the job done.

The VT-4C/211 is NOT an ideal tube to use in a self excited oscillator. Too big with its massive internal structure that expanded and contracted a lot with heating, so more need for temperature compensation.

Try building a self excited oscillator using one of those tubes and try running it at 12mc and see just how unstable they can be. It is interesting to bring one of the tuning units inside the house after sitting overnight outside in the cold winter time and watch the bi-metal strips on those discs move the discs as the tuning unit warms up to room temperature.
Curt

This thread is OLD, but I thought I'd bring the tale to a close. In short, I finally got the BC-375 running correctly about 2 months ago. I thought it ran "OK", but continued comments about FM-ing from several Saturday night net regulars got me off my duff.

Step 1 - Rebuild the solid state PSU: It now delivers 1100 VDC at 400mA w/8% regulation. I have a NOS power transformer installed that delivers 1800VAC/ 400mA, a 25-4 H swinging hoke, 25K/100W bleeder to get the B+ stable (all thanks to Dennis W7QHO), and rebuilt the filter cap string to handle the higher voltage.

Step 2 - Revisit the bias values for the Speech Amp and Modulators - way off. Dropped the Mic Input to 4 out of 10, then reduced the output coupling to 50W as recommended.

Step 3 - I borrowed a modern AM/FM transceiver from a pal up here. That was the real solution: disconecting the B+ from the PA tube and neutralizing while watching pass-thru on a scope only takes you so far. To nail it, I put on the head set and spoke loudly into the mic while listening to myself 3983 FM = you really can hear yourself quite clearly! When I couldn't hear myself, I started blowing loudly into the mic and tweaked the neutralization that extra hairs-breadth it takes to get the last of the FM. Now it puts out a clean AM signal, and CW has minimal chrip.

I also built a beefy PSU to feed a PE-73 I located cheap and it has similar (8 to 10%) regulation as the solid state PSU. My BC-348-R is slaved to the '375 as it was in the air and the pair work well together. I'll have them out for FD this year and will be running 7290 daytime, 3870-3880 at night. I'll try and be on 3885 for the West Coast MRCG net at 2030, but bands may be too crowded w/SSB to do any good.

What an adventure, and what an education this has been!

73 all!

Well done, Jim. If folks back in the late 1940s had been as careful, many of these sets might have served honorably in ham's shacks.

Of course, then many of them would never have survived to today, I suppose. Double edged sword....