Tube Radio Australia

SCR-522 restoration

The Band Changing mechanism Video: https://www.facebook.com/tuberadioaustralia/videos/1840611612843446/

Tube Radio Facebook Page: https://www.facebook.com/tuberadioaustralia

The SCR522 aalso known as T.R 5043 was used in many WWII aircraft and also was in service after the war in both Military and later in civilian aircraft. These were used by the RAF and RAAF and US air force.
They were a very widely used Very High Frequency Transmitter / Receiver operating in the 100 to 150 MHz range. This equipment was recovered from three locations, One in Bundeena, and the units were heading for scrap, the Dynamotor was found in a cleanout sale in Guilford NSW, and the other parts from the Wyong swap and donations from the guys in the Ausmilsig group as well as other radio friends. Special thanks to Dave Prince, Ray Robinson, Morris Odell, Brian Clarke, for supplying parts needed to complete this job. The unit is very well built and also heavy for a low power output of, (12 to 18 watts) weighing in at 22 kg and another 16kg for the associated Dynamotor power supply. They are a mechanical masterpiece given that it selects 4 channels that are preselected by setting and locking the cams on the receiver and transmitter. Two cams for the RX, and 4 cams for the TX. The solenoid then ratchets the cams to the position selected from the control box. The link above shows this in operation and it really scares you the first time it does it…

General Description

The SCR 522 is the American version of the British TR 1143 transceiver. Although external connections for both units are the same and major units are interchangeable, the internals are quite different. (This may answer the question of why there are such weird sockets on this equipment.)

The American SCR522 has its own British type number as well, just to confuse the issue, which is TR5043.

The radio sets were designed for use in U.S. Army aircraft to provide two way radio telephone communication between aircraft in flight and between aircraft and ground stations.

Technical Data

They operate on any one of 4 crystal controlled channels lying within the frequency range 100-150 mc. Remote control only was available.

Transmitter channel D was frequently used as a special use channel, which was automatically selected at regular intervals by the action of Contactor Unit BC 608A, which caused a signal to be transmitted for 14 seconds in every minute for DF purposes.

The set operated from 24 volts D.C. and obtained auxiliary voltages from a Dynamotor Unit, type PE-94. The transmitter required 11.5a and the receiver, 11.1a at 28v. The dynamotor produced 300v HT, minus 150v grid bias (Tx) and 13v for heaters, relays, channel change etc.

The transmitter contained the following tubes: 2 x VT-118 (832), 3 x VT134 (12A6), 1 x VT-I98A (6G6) and 2 x VT-I99 (6SS7).

The transmitter tuning was adjusted by plugging in a 0-l ma meter from the Test Set.
The positions and operations are as follows:

   1. 1st Har Amp. Plate    50ma FSD
   2. 2nd Har Amp. Plate    l00ma
   3. P.A.  Plate.                  l00ma
   4. RF Indicator Diode    1ma.
   5. P.A. Grid                     2ma

The transmitter output was 12 to 18 watts.

The sets were very popular on 2 metres, with transmitters in use up to the 1970's, when they were replaced with even smaller black boxes of the SSB type.

Unfortunately the 522 Tx was prone to TVI problems, and had to be heavily shielded to obtain peace from the neighbours. Many an Australian Amateur started in the hobby with an SCR522 transmitter and a crystal locked VHF converter, feeding an HF receiver from the services.

The 522 was even converted to operate on 6 metres -

Photo of the Main chassis Reciever and Transmitter with the TEST control box plugged into the front.

Photo of CHASSIS DURING STRIP DOWN

Photo of the two transceivers and the Dyno unit awaiting service.

Photo of transceiver one repainted after initial work

Photo of reciever section type 1 with no noise limiter

Photo of the Trasnmitter chassis and the modulation transformer

Photo of the reciever type 2. More on this later with detials of the differences ion the under side of the chassis

Photo of the type 1 reciever Note the the right there is no additional tubes on the under side.

Photo of the type 2 reciever section Note the tube added to the under side, this was a noise limiter modification.

Photo of the type 2 top side very similar from the top.

Photo of the junction box and other chassis waiting to be worked on.

Photo of the Dynamotor 300v -150V and 14.5 volt. This is the 28V input type there is a 12V typoe as well.

Photo of the transmitted signal on the spectrum analyser looking at the parasitic frequencies either side of the fundamental .

Photo of the test dummy load, this is the correct load for this from the test set.

Photo of the display frame being made and painted

Photo of the unit mounted, with the test meter and junction box assembled.

Photo of the Choke in the transmitter that had failed, this caused the Modulation pre amp tube to be missing

the HV to the plate, although I had RF I had no modulation, Note the ACME stamp on the replacement from another chassis.

The transmitter always had RF output and the mike circuit was working as I could hear it in the headphones, just no Modulation.

So l traced the Mike into the TX where it goes from a Balanced Mike input through a transformer T158 and relay R131, there was signal…..

Looking at the 6SS7 yes a SS7 never seen one, there should have been output, so I looked at the input to the two 12A6’s Nothing on either grid…?

Hmmm, well a quick check of voltages and there was no HV to the 6SS7, so looking at the Choke 126 I found it Open.

I tacked a little choke I had across the dead choke and it made no difference. Hmmmm must need that specific value…

Looking at the spec it was a 2000 Ohm, 1mv choke, so I when looking at other dead chassis and found one… Funny thing the transformer brand was ACME… yes the Road Runner ACME.. I always thought

that it was a joke by Warner Bros.. Does this mean it’s going to fail spectacularly in a cloud of smoke, like it did in the movies… who knows..