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Reviews Categories | Transceivers: VHF/UHF+ Amateur Base/Mobile (non hand-held) | BC-1335 (SCR619) Vintage Military radio Help


Reviews Summary for BC-1335 (SCR619) Vintage Military radio
BC-1335  (SCR619) Vintage Military radio Reviews: 1 Average rating: 5.0/5 MSRP: $Unknown
Description: Vintage FM transceiver used in World-War II and Korea
Product is not in production.
More info: http://www.fernblatt.net/sch/bc-1335.gif
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You can write your own review of the BC-1335 (SCR619) Vintage Military radio.

AC5XP Rating: 5/5 Jun 23, 2004 16:12 Send this review to a friend
Ahead of its time  Time owned: more than 12 months
BC-1335 (SCR-619) , a 27 to 39 MHz WWII military FM transceiver
By Loek d’Hont AC5XP


The BC-1335 lightweight FM transceiver is part of the SCR-619 radio installation which was developed during the later years of WWII, primarily for vehicle use. It came into service around 1944, as an improved version of the BC-659 FM transceiver. The BC-1335 saw service all the way into the Korean War and beyond.

The BC-1335 is in the same family of radios as the BC-620, BC-659, WS-88 (UK) and ER-40 (French) and possibly a few others that I'm not aware of. This family of radios uses frequency modulation, works in the 20 to 70 MHz band, uses crystal control for the receiver (one for each channel) and uses an early method of phase-lock-loop to lock the transmitter frequency to the receiver crystal. The latter concept results in only one crystal being needed for each individual transmit-receive channel.

The BC-1335 can be adjusted anywhere between 27 MHz and 39 MHz, assuming the proper crystal is inserted. The original crystal pack for this radio allowed frequency setting in 100 kHz intervals. If you have this crystal pack, the obvious choices for operation on the ham bands would be 29.3 MHz, 29.4 MHz and 29.5 MHz (crystal channels 293, 294 and 295). The radio is equipped with individual LC tanks for each of the two switchable channels (thus a set of two tanks for each point in the circuit that requires retuning when changing the channel). As a result, the two programmable channels can be widely separated from each other in frequency; they don’t have to be close together as some other radios of the era required.

If you have ever compared the schematic diagrams of the BC-1335 and the BC-659, you will recognize the similarities (both radios share many of the same concepts) but there are distinct differences, which will become apparent when one looks at these radios, both the outside as well as "under the hood". When doing such comparing, it becomes apparent that the BC-1335 was developed to solve some of the operational issues with the BC-659 radio. To name a few differences:

• Size. The BC-1335 is less than half the size of the 659, with the power supply integrated in the 1335 radio itself. The BC-659 uses a separate unit as a power supply on which the radio unit is mounted. In a way, the BC-1335 was an early attempt toward "miniaturization".

• Used tubes. The BC-1335 uses sub-miniature tubes that still can be found in abundance today, where the BC-659 uses larger, very difficult to get tubes (although both transceiver tube types use low voltage, direct heating for the filaments)

• In-field re-channelization method. The BC-1335 has been made very user-friendly (a relative term of course) for this job, with pre-set charts for each tunable filter, improved coil adjustment dials, built-in "magic eye" as adjustment meter and light bulbs as tuning indicators. The BC-659 is much harder to field-adjust when re-channeling, and requires external equipment when doing so.

• The notorious anode battery that is needed to operate the BC-659 (even when using the vehicular power supply!) is eliminated for the BC-1335 design. The BC-1335 uses a "bias oscillator" instead to generate the negative bias voltages, and the built-in vibrator power supply for the remaining anode HTs. Its bias oscillator is in a way a predecessor to the modern switching power supply.

• Internal room for spare tubes, fuses, crystals and spare vibrator (latter only for the BC-1335, not the BC-1335A)

• The BC-1335 has a vastly improved mechanical design for the discriminator tank over the BC-659, as this discriminator tank determines the ultimate transmitter stability in this radio concept (explained later)

• The BC-1335 has a 50-ohm coaxial output as well as a base for a whip. The BC-659 supports only the whip antenna.

Don't take me wrong; the above does not mean you should decline any good deals for a BC-659 radio that might come along. With all it quirks, the BC-659 is of older vintage and accomplishes the same as the BC-1335 which in itself probably makes the 659 a more desirable collectors item. After all, if we only would collect improved radios, we might as well collect the latest ham rigs; can't get more up-to-date than that! However, those of you who have monitored BC-659 offerings from the several Internet sources lately must have noticed that this radio has reached comparable madness regarding pricing as some Collins tube equipment. The BC-1335 is (still) very acceptable priced though; I bought all my units below $200 which I think is a fair price for this radio when in good condition.

In general, I like WWII tube radios. They were developed during turbulent, historically significant times by people we now call the Greatest Generation and as such the radios of those days deserve our respect and preservation efforts. To quote Jay Leno: One does not really own vintage equipment; one simply preserves it to be passed on to the next generation.
WWII radios were developed in an era where wireless technology underwent tremendous, rapid advances. To name a few: The introduction of FM, the wide use of quartz crystals instead of free running oscillators, the introduction of usable portable equipment, the invention of the magnetron tube, it is just a small example of the advances that were introduced in radio technology during World-War II.

When you think about it, it actually is quite remarkable that today’s radios are conceptually not at all that different compared to what was invented then. They are still of the same design in terms of the general radio concepts. Where real improvements have been made in later years is component miniaturization and frequency synthesis. Regarding the latter, it might be worth mentioning that most of the concepts behind modern frequency synthesis were known during WWII, but could not be realized due to limitations in available components. (Ever seen a programmable divider realized in tube technology? I rest my case!) The actual radio concepts behind it all, have not changed that much. The only truly new radio concept of today has been digital signal processing, where filters and (de)modulators are realized in the digital domain instead, reaching performance that just cannot be realized in the traditional analog way.

Back to the BC-1335 review! The radio is quite complex in terms of tube count, it has as many as 18 electron tubes, one voltage stabilizer tube and two ballast tubes. I think this rivals tube rigs like the much more recent Collins KWM-2!

The receiver front-end has two RF stages followed by the mixer stage. The mixer is followed by 3 IF stages of which the last stage is the limiter. IF frequency is 4.3 MHz which I think is identical to the BC-659. The limiter stage is followed by a Foster Seely phase/frequency discriminator, with the two diodes in this circuit being hollow-state. One diode tube has an indirect-heated cathode to isolate the cathode from the filament wiring, a necessity for the Foster-Seely discriminator function (all other tubes in this radio are of the direct-heated filament type to save filament current consumption)
The radio has no squelch, so the FM hiss is constantly present. This is actually strange if one considers that other radios of the time like the BC-1000 (SCR300) actually have a squelch.

The BC-1335 transmitter uses a symmetrical, high-power dual-triode oscillator (almost like a latter-day multivibrator circuit) which is a departure from the BC-659 oscillator concept. For the rest, the circuit is pretty much identical to the BC-659: A balanced doubler circuit that follows the oscillator (through a sort of full-wave rectifier concept) followed by a push-pull RF amplifier that is capable of giving off around 3 watts. The transmitter feeds either a whip antenna base (on the side of the radio) or a 50-ohms SO239 coaxial connector. The BC-659 as far as I know has no 50-ohms output, although I have once owned a late version BC-620 (the BC-659 sister radio) which actually did have a 50-ohm connector on the back.
The FM modulator is formed by means of a reactance tube, the latter being loosely capacitive-coupled to the oscillator tank. The reactance tube creates both the FM modulation as well as the transmitter frequency correction (explained below), basically turning the oscillator into an early-style VCO circuit.

The transmitter oscillator is not crystal controlled, but locked to the receiver crystal as follows. When the transmitter is keyed-up, the receiver (which is always on, also during transmit) will receive the transmitter signal which eventually will reach the IF and the FM discriminator. Assuming the transmitter is operating at the same frequency where the receiver receives, the FM discriminator will be perfectly balanced and generate no error voltage. If the transmitter oscillator tries to drift, the discriminator will generate an error voltage which is then fed back to the reactance modulator in a negative feedback which will force the transmitter back on frequency. The frequency correction loop that is created this way, will correct for slow drifts but not for the fast frequency changes caused by the actual FM modulation (the control loop "correcting" the FM modulation away would be undesired!). The latter is accomplished by the introduction of a loop filter with a cutoff frequency below the lowest modulation frequency.
Sounds familiar? It should be! It is basically the same as what a modern PLL does.
Needless to say that this concept will only work within the discriminator's hold-range; if the transmitter drifts beyond this range no corrections will be applied. The radio is designed such that the worst-case transmitter drift will always be within this hold range, assuming the discriminator was properly set in the center to begin with. Now you will understand why the discriminator/transmitter oscillator adjustment is very critical when re-channeling the radio.

The BC-1335 originally used a reference battery voltage to compare against the discriminator voltage when making this critical adjustment. A built-in magic eye tube is used to assist in this process. If the reference cells are still present in your BC-1335, it is very likely they have passed on to a better life a long time ago(my battery cells had for sure!). A better way to make the discriminator adjustment without these reference cells being present is to first measure the voltage extremes ("peaks" in the S curve) that the discriminator generates when sweeping an external frequency source across the particular BC-1335 radio channel (use your ham radio at low power into a dummy load as an external signal source for this measurement) and to record the voltage that corresponds with the center of that discriminator "S" curve. This is then the same voltage that you will need to look for when making the transmitter oscillator adjustment with the radio's AFC loop active. You also might want to monitor the BC-1335 transmitter frequency on your ham rig when you make this adjustment, to make sure you did not lock the transmitter to a "false dip" which sometimes can happen.

All other BC-1335 adjustments are pretty straight forward and are explained quite well on the alignment guide present on the inside of the lid (together with a diagram of the radio, printed on the inside of the lid, quite convenient)

Some problems you might run into when getting the radio on the air is the synchronous vibrator used in its power supply. This device is a potential source of problems. If you turn the radio on and hear no hum noises from the vibrator and the radio hardly draws any current, you got a defect vibrator. I had two New-Old-Stock (never used) vibrators that did not work at all. I think that over time, the rubber seal on the bottom gasses out to the inside of the can and corrodes the internal vibrator contacts. This vibrator is extremely difficult to find (it is the one with the six pins of which two are thicker than the rest) so if this is your problem it is serious. I am in the process of trying to modify a defect vibrator into a transistor version whilst keeping the outside look of the device identical. I actually hate having to do this, as modifications to antique radios are in fact a crime. Since this vibrator is a replaceable part I could tolerate it, if I ever find the real thing again I simply can plug it back in.

Other important issue is the operating voltage. The radio can be switched for either 6 volts or 12 volts DC operation. 12 volts might seem to be the obvious choice but this is not necessarily so. When operating the radio in 12 volt mode, ballast filament tubes are used to burn off the excessive 6-volts not needed for the tube filaments. I do not feel comfortable with this concept; a slight malfunction in it will burn-out the sensitive radio tubes (1.5V DC direct-filament types). So I prefer to run the radio in 6 volt DC mode using an external step-down regulator when using it mobile from the cigarette lighter plug. This also filters out vehicle voltage spikes before they can reach the radio. Current draw for this radio is around 4 to 5A when transmitting.

How does the radio behave on the air? Not bad at all! The modulation sounds very good and is not too wide for it to be received properly by a Japanese ham rig. The used transmitter AFC concept works well enough to the extent that a narrow 15 KHz FM radio on the other side has no problems with the signal. The receiver is very sensitive but also pretty wide; IF passband is as much as 75 KHz as I once measured.
The 3 watts RF power is a fairly comfortable power level for this kind of radio, it is more power than most of its contemporaries (the BC-1000 is only 300 mW) and also more than some other portable tube military radios that were developed much later for this band (PRC-6, PRC-9, PRC10, PRC25 to name a few)

And it's quite a thrill to use a radio that is actually 60 years old and still works well. Think about it; do you believe that that new IC-7400 you are now drooling over will still work in 2064? Or that you actually can get replacement parts for it by then (like for instance a new LCD screen)? If you're still around, you probably will have an easier time to find replacement tubes for your BC-1335!

 


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