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T-368 AM/CW/RTTY Transmitter

 

T-368 Transmitter T-368 Line art   T-368 Front Photo T-368 Photo

History

Produced in large numbers during the 50's and 60's, the T-368 transmitter was used by the military as a multi-purpose unit. Affectionately known as the  "T-3" a great many were placed in stationary RTTY service, while others were used for AM and CW operation. As a medium power transmitter the T-368 was overbuilt for continuous 7/24 service. Originally designated as the T-368/URT, the T-368 has since undergone several updates, culminating in the T-368F model. Major updates were the addition of a vacuum T/R relay, and modified protection circuits. Most of the older non-lettered models have undergone MWOs (Modification Work Orders) for service updates. The MWO tags are typically affixed to the front of the decks.

Physical

The unit weighs about 700 lbs., and is roughly cube shaped. Height and length are close to 41". It's modular construction allows easy access for service. The lower deck, and the most heavy, is the power supply section. The middle deck, and next lightest, is the modulator, and the top deck  is the RF section. All decks are equipped with Cannon connectors and can easily be removed for service and troubleshooting. It fact it is possible to "hot swap" defective decks while the power is still energized with no adverse effects.

Conversion to Amateur Use

The T-368 requires very little in the way of conversion for use on the amateur bands. With its 450 watts of carrier, and heavy duty construction, the T-3 makes an outstanding amateur transmitter. A few simple safety and convenience changes are all that is required. See the Conversion button for more information.

Tube Complement

The T-368 uses a total of 29 vacuum tubes. Aside from the "6000" tube mentioned later, all are quite readily available from commercial vendors and hamfests. A pair of  4-125A tubes are used for modulators, and a single 4-400 for the PA. Power output is 400-450 watts carrier in the AM mode. The unit is quite frequency agile, capable of a tuning range from 1.5 Mc to 20 Mc with the built-in exciter unit. The exciter tube, know as a "6000" is a bit unusual, and becoming more difficult to find. I have replaced the rectifier tubes in my T-3 #1 unit with solid state found that the unit runs considerably cooler. Although this does raise the supply voltage somewhat, I believe that the design offers significant engineering headroom as not to cause a problem. The tube lineup can be viewed in detail by clicking on the Tube Use button.

Cooling

Two blowers are utilized to keep the units interior temperature within a safe operating range. One blower located on the RF deck force cools the 4-400 final tube. The second blower, and the larger of the two, is located on the rear cover of the unit. This blower forces air into the power supply deck. Despite having two blowers, the T-3 relies to a large extent upon convection cooling. Cool air is drawn through a filter located on the rear of the back cover, forced into the unit by the larger blower, and exhausted via a vent in the top right rear of the unit.

Power

Power requirements are simple, 110-120VAC at 20 amps. A stiff service is required to allow the T-368 to run at it's full potential. Some of the earlier units, the non-lettered models, were supplied with a power supply that could be used on 230VAC, but the later units lack this feature.

Audio

In it's stock form, the T-368 suffers from classical military limited frequency response in the audio. When used with the ubiquitous D-104 mic the audio is acceptable. However, with a few simple and almost painless mods the unit can easily produce audio very close to broadcast quality.

Mods

One of the more popular audio mods, and one which I use in my T-3 #1, is the N9FOY speech amplifier mod. It is based loosely upon the AF amplifier used in older RCA Broadcast Transmitters. Click on the N9FOY Button on the left side of the screen for more information. Steve has also designed a simple circuit that allows individual bias adjustment of the modulator tubes.

A simpler audio mod, and one which I use on T-3 #2, is described under the KG2IC Mod button on the left side of the screen. This mod does not offer the audio quality of the N9FOY, but does make a noticeable difference over the stock unit. As a plus it retains the stock speech amp, and is easily reversible.

Tim WA1HLR has designed an extensive series of mods for the T-368 transmitters. These mods encompass both the audio and the RF sections of the unit, and the results are quite impressive. The RF deck mods permit a considerable improvement in the transmit audio. Click on the WA1HLR Mod button on the left side of the screen for details and schematics.

Service Points

Although the unit is exceptionally rugged, over time some weak points have appeared. They are quite minor, especially in the light the vintage of the T-3, now approaching 40+ years in age! One of the more common problems has to do with the safety interlocks on each deck. Over time they pit and do not allow a good connection. The fix is simple, just bypass the interlock units by jumpering out the interlock wires on each deck. My second T-3 suffered from this problem, and since the interlocks were jumped out it has worked just fine. A possible fix for this problem is listed under the Interlocks button. A word of warning here. If the deck interlocks are bypassed, the high voltage (2,400 volts) will not be disabled when the decks are removed!

The exciter tube, a type 6000, is also becoming a bit rare, although alternatives are possible. Fair Radio was stocking new replacements for this tube. It would be an excellent idea to obtain a few spares.

As these units were often used in continuous service, it is not unusual to find the blower bearings are worn and noisy. Both of my units suffered this fate. The blower motors are serviceable, and replacement bearing can be readily found at a good bearing supply warehouse. The cost of replacement bearings was quite reasonable at $1.83 each.

The plate current meter is susceptible to failure as it carries HV on it. The later meters were mounted with an insulated standoff ring. The exact replacement meters are becoming quite scare, although another meter maybe substituted in it's place. A various number of mods are available to correct this problem. The majority deal with removing the HV from the meter circuit. The  WA1HLR Mod in particular works quite well, and offers improved transmit audio as well.

Additional Equipment

The BC-939-B antenna tuner of BC-610 fame was also used with the T-368. The top of the T-368 cabinet contains mounting studs to secure the BC-939-B. Tuning range of this unit is from 2MC to 20MC.

The MA-29A/U carbon mic was designated to be used with the T-3.

The ME-165/G combination dummy load/SWR/Power meter is an especially useful accessory to have. The ME-165/G is a 600 watt dummy load and Power meter which is invaluable for tuning up the T-3. 

Operation

As it was designed for military service, the T-368 is exceptionally simple to operate. It is also equipped with numerous safeguards both for the protection of the transmitter, as well as the operator. The T-3 employs a overload protection relay which engages when excessive plate current is detected. A thermal protection switch is also placed over the 4-400 final tube to protect it in the event of blower failure. Any frequency between 1.5KC and 20KC can be readily selected with the built-in exciter unit. The two large vacuum variable capacitors used in the loading and tuning circuit allow rapid and painless frequency changes. The design of the Pi-L tuning network allows the T-3 to match a wide range of antenna loads.

Summary

The T-3 is really overbuilt for amateur service, and it is a pleasure to operate on the air. When used in intermittent amateur service, the unit will easily outlast its owner. Service parts are readily available from a variety of vendors, see the Resources button for more info. Failures are infrequent, and when they do occur, it is usually a simple matter to diagnose and correct.

There is very little, short a commercial broadcast transmitter, that can offer the "brick on the key", 7/24 service of a T-368. Given some simple mods, and a little time, the T-368 is capable of outstanding transmit audio. When matched with the R-390A receiver, the T-368 indeed offers a bulletproof and formidable vintage AM station.

See you on the air!

              

Copyright 2006
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Last modified: 10/17/06