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THE UNITED AMERICAN BOSCH 48C

    And to think, I was going to give this one away! I picked up this radio 11 years ago along with an Atwater Kent (still in the trunk of a stolen car, so its gone. It was a model 40, I believe) and another breadbox model (a homebrew, also gone). It was so heavy and looked like a WWII receiver. I only thought this because of the variometer at the front end. I have never seen one of these except in magazines.

    It is a 7 tube TRF radio using 3 24a's, a 27 as detector, 2 45's for audio output and an 80. The first stage is a variometer. The 24a's are tetrodes, for low capacitance and higher sensitivity. The 27 is an indirectly heated triode biased for detection. The 45's are AC directly heated power triodes. And the ubiquitous 80 full wave rectifier. There are three controls, a volume, a tuning and a front end trimmer. This is much like the Majestic Model 90 from about the same year. It looks as though it was part of a console model, because it has a plug in the back for the field coil of a dynamic speaker as well as a plug for the speaker voice coil (it has the audio output transformer already). It also has sort of an outline on the front face plate which looks like the opening of a cabinet.

    As usual, I began by applying power to the unit. But before I did I removed all the tubes and disconnected the tuner section from the power/audio section. Well, I got AC at the typical points where I would find AC, and... no smoke. I then put in the 80 tube. I measured a decent DC at areas I expected it, and... no smoke. I then put in the 45's. I got less than ideal voltage measurements. It turns out that the voltage divider resistor was blown, except for part of it. It has four connections on it. The second from the left (from the point of view that I was looking at it that is) is connected to ground. The far left is connected to the center tap of the hum balance resistors (I think). It was blown. Then from the ground tap to the right was the tap for the screen grid power. It was intact. It read 1500 ohms. It is supposed to have 70 volts on it. It had none. The other end to the far right is connected to B+. It had 250 volts on it. There was no resistance reading on it.

    So, I had a couple of things in my favor. The transformer worked, and I had at least two parameters that will help me determine the values of the divider. The one for the amplifier I will have to wing, because there was no reading on it. I put a 1500 ohm 10 watter just for posterity. I think that it is a good value. I then determined that if I needed to drop 70 volts across 1500 ohms, I had about 47 milliamps through it. So to drop the remaining 180 volts I would need about 4000 ohms. I made a makeshift 4500 ohmer by stringing three 1500 ohms 10 watters in series. This is because I had plenty of them in my junk box. Well, it worked. (I first used four 1/2 watt 22K ohm resistors because I thought that there would not be too much current, but... there was smoke!) I had all the voltages out that I needed to power both the amp and the tuner section. So I connected the tuner section. Well, I then got nothing. Not even smoke, so that was good. I found that the power going to the tuning section was zero. So I promptly disconnected the tuner and removed the audio choke, because it looked bad, even though it showed a resistance across it. I figured it may be shorted to the can, since the choke was inside a metal can. But I still got a zero volt reading.

    I then did the thing I should have at first. I measured the resistance at the point where I was measuring the voltage in the tuner section. Zero ohms. I checked the place where I removed the choke. There is a capcitor there, an RF choke and another capacitor. It turned out to be the capacitor connected at the RF choke and the plate of the 27. It was shorted. Imagine that, a defective mica cap! They are usually quite durable. I then replaced it with a ceramic version (I know, but I never had any real trouble with ceramics) and reconnected everything and ureka! it worked.

    I put only an antenna and was recieving many stations at the low end of the band. I usually get more sensitivity at the high end of the band. I think this has to do with the variometer at the front end. I then hooked in a ground. Of course, I got even better reception. The sound is very clean and loud. I also get the high frequency whistles with the weaker stations because of signals mixing with distant stations. I then realized that I should really put the audio choke back in. Lo and behold no more whistling, and even the interference from the distant stations was gone. Can you imagine that, though? The radio was so sensitive that I got interference from distant stations on the more local ones!

    I then realigned it, with all the sheilding in place, because the thing was oscillating like crazy without the shielding, and the thing is a monster performer. The only thing that is lacking is the tuning dial. It is present, but most of the lines and numbers are rubbed off. I do know where I can get one, and will update this page and pictures when I do.

July 3, 2001

A few months ago I got the dial, actually two of them, and installed one of them. It fit almost perfectly. It looks terrific. I wil have a picture very soon.

PICS



    These are images of the Bosch front view, the tuner section without the shield (which is held in place by friction clips on top and two thumbscrews at the bottom), and the amplifier/power supply  section. Notice I use modern knobs. I do not know what the original knobs were.

    I will leave the old look. Though I think in the near future I may clean and repaint it. The faceplate will be difficult to do because it is a steel plate with a painted wood grain. I may use a sheet of wood veneer over it. It might look better. Who knows. I may even build a cabinet for this one to live in and mount a speaker. The speaker I am currently using is a cheap hi-fi (really lo-fi) 8 ohm speaker. So I can use a better quality 10 or 12 inch. It should be powerful enough to drive a 20 or 30 watt speaker sufficiently. 45's in push-pull can likely put out 5-8 watts.

    I now know that this is the best radio of my collection for its quality construction, superior reception and clean sound. And... no smoke!
 
 
 

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