The 601 Series of transmitters [601-605] were British and were developed during WW2.  When they were rolled-out,  the intention was to replace all the American equipment which the RN had gratefully received from the USN,  and which went on to help win the war.

By the end of the 1950's they had been added to and modified so much that unless you kept your eye on the modifications state, you could so easily 'lose the bubble'. Here are two small files which show the state of the changes, and, more relevant to our case, the changes made to emissions.  I can understand an onlooker viewing these lists and thinking that the navy were trying to get a 'quart out of a pint pot'!

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When RATT 2 was introduced bringing on stage HF FSK RATT Transmission, the 5AB transmitter {see the thumbnail below for further details},

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part of the 601 series transmitter, was modified in exactly the same way as was the 89Q, to take the GK185A FSK Keyer.  There was one difference in its application with a type 601 transmitter, an important one, for gone were the tricky bits of working out the crystal value of the GK185A output crystal because of the doubling and trebling effect of the transmitter - it was now automatically done for you.  Shortly afterwards, now with more 601 series transmitters in the Fleet than 89Q's, the death knell for the GK185A was spelt.  The 5AB was completely re-built and returned to the Fleet as a 5AB/A transmitter, this time with its own built in FSK Keyer as a plug in unit.

The 5AB/A did the same job as the GK185A of course, but in a different way, although it was still a carrier shifter and not a tone or sub-carrier shifter {the 601 series were not SSB transmitters}. This rather poor picture of a Type 603[5] transmitter shows the 5AB/A transmitter as the bottom but one unit - No 3.

The unit has a hinged door to the left giving access to the plug in Oscillator Unit, with a switch and a small calibrator tuning control beneath it. The switch is to select one of 9 crystals in the range 2.75 to 4.75 MHz with 250 kHz spacing. The crystal frequency is fed to a calibrator and also into the calibrator comes the output of the VFO. Since the crystal is accurate, the VFO is set to its standard by varying the vernier control looking for maximum signal on the bottom of two meters. Once set, the output frequency is a MIXTURE of VFO and Crystal which the navy called PCC [Partial Crystal Control] : CLICK HERE for more information on PCC. The selected crystal also fed one input of a Mixer. The now accurate VFO was connected to the incoming teleprinter signal, the Marks and Spaces.  It had a small oscillating range of from 1000 to 1250 kHz only, and even this was to accommodate the wide input required when a FAX [Facsimile] transmission [A4] was being made: for FSK it could have been smaller in range. The 5AB/B was fixed for ARRANGEMENT 2 @ 850Hz Shift [Mark highest frequency in the ether] and the frequency outputs tailored made with assigned frequencies ending in decimal point 425 for example, so the choice wasn't great. In the following drawing you will see the basic block diagram of the 5ABA.

Consider transmitting on a 5Mhz frequency of 5750.425 kHz. The range switch [see table] would be set to 3-6 and the crystal chosen is No 9 - 4750 kHz. You can observe in the table that throughout,  3 to 6 MHz is treated as a fundamental and is not doubled or tripled in frequency by the 601 series associated transmitter. The teleprinter Mark condition results in the Reactance Stage creating a frequency of 1000.850 kHz in the VFO and the Space a frequency of 1000 kHz. These frequencies are fed to the mixer which also receives the output of the crystal, 4750 kHz. When the Mark ADDS to the crystal, the output becomes 5750.85 kHz and the Space becomes 5750 kHz, the frequencies being either side of the assigned frequency and in Arrangement 2.  Finally the output of the transmitter 5AB/A was fed to the 601 series transmitter proper as is shown in this block diagram below.  For "RF Input from 5AB" read  from 5AB/A.