Greetings to all.I have for some time been enquiring about the availability of a manual for a Racal RA121D ISB adaptor unfortunately,to no avail.This adaptor has a 118kHz crystal oscillator installed as well as the normal 118kHz variable oscillator.I have the details of the RA98 118k crystal oscillator but need the connection details within the 121.Can any kind soul assist?

Brian g

Brian, I have a RA121D I restored, but house move cannot at present find manual, I think I got mine from Mullion Cove. Hopefully someone else can find information but I will keep looking for my manual.
Andy G3UEQ

开云体育

Greetings all,

I'm currently working on what claims to be an RA121A (Ser. No 493). However it bears more resemblance to an RA121D since it has the Crystal-Controlled 118KHz oscillator in the middle of the chassis. It also has the 6AU6 Buffer Stage between C30 and SB1F, and the RA17/RA117 switch on the chassis which was not present on the last RA121D that I worked on.

Does anyone on this group have the 'official' circuit diagram for the buffer stage? I have downloaded the diagram for the crystal oscillator from the files folder and see that it is designated Fig. 9. Since the main schematic is Fig.7, I'm assuming therefore that if there is a schematic for the buffer stages, it will be Fig 8. It's easy to work out the diagram from the components, but at least one of the resistors is a different value to the previous one I worked on ... so I'm curious to know what the official diagram says.

Finally, I am a tad puzzled by the two zener diodes, MR2 and MR3. These are listed as type Z2A110F which is a low power 11V type. The ones fitted in this particular RA121 look like they might be replacements judging by the way they are badly attached, plus they are type CV7144, small glass bead construction and 11V. As I photograph everything that I work on, I looked back at the two 121s that I have previously worked on. One had the original 'chunky' Z2A110Fs fitted, whilst the other had modern glass bead types. It looks like the Anodes (red sleeve) of the Z2A110Fs are connected to ground, as one would expect ... however, the Zeners in the other 121s were mounted the other way round, with their cathodes connected to ground, i.e. the end with the stripe.

I checked the CV7144s and confirmed them as 11V, and re-mounted them with their Anodes grounded. I suspect that confusion may have been due to the way that they are represented in the main schematic, where a plus sign is adjacent to the Anode and a minus sign adjacent to the Cathode. Thoughts anyone?

Cheers,
Ian, GM0UHC
www.chavfreezone.me.uk