[Radio Wrangler]

So, you've found the fault in your set (wrong RF valve) and now you're looking at design inadequacies. The RA17 AGC system is not very good even when working properly. The overshoot with the first word or so of CW is known. They come from an era when most CW operators would switch AGC off and drive the IF gain manually, along with the input attenuator.

Here in Europe, we had megawatt broadcasters in 7.1MHz upwards and the 40m band was 7.0 to 7.1MHz. Receiver overload while trying to hear distant stuff was a bad problem with almost all receivers. The RA17 front end with three mixers before you even get to any narrow selectivity was well known as an underperformer in the overload stakes.... Actually, the plain RA17 flavours had a narrow capacitor tuned filter in the 2-3MHz IF to get rejection of the image from mixing next to 100kHz. That helped a little. I had the RA117E. This variant has no tracking 3-4MHz filter, it has an extra IF and bandpass filters, so it misses out on even that narrowing (This was so the 'kc/s' LO could be switched to external for use with an exciter or synthesiser.) The RA117E first stage was different too, a double triode cascode. This stage went into some of the later RA17 models.

The whole structure of many stages without much selectivity is a recipe for intermodulation trouble. Using one of these receivers in Europe meant you had to always use the RF preselector (RA117 had a wideband position on the preselector range switch - you never used it!) and to run with RF attenuation in most of the time.

I think the AGC design must have been designed to try to not make the overload issues any worse, in terms of how gain was distributed at various levels of signal. The detector of course comes right at the end of the receiver selectivity and sees essentially just the wanted signal, while many stages, up to the 100kHz IF get tenderised by any strong signals within up to a whole MHz block.

Racal followed up the RA17 family by going transistorised. The RA1217. This is a very cute receiver, but with the RA117 block diagram. With the weedy overload behaviour of early transistor circuitry, it was a very limited receiver. AGC was better done, it had gone to crystal block filters with proper dedicated USB, LSB and CW bandwidths (plus AM and RTTTY) all depending on optional fit. There was a proper product detector. The British Army bought loads and many went in Land Rover mounted radio vehicles, using whip antennae of modest size. The small rack height of the RA1217 was the attractive feature. It was convenient not having to rack up an SSB adaptor with it, but the poor RF/IF dynamic range was a major limitation and a setback from even the RA117.

Eventually, someone at RACAL cottoned on to the issues and their next generation, the RA1772 paid a lot of attention to RF linearity and overload. They licensed the use of the Rafuse D-MOS mixer (from MIT?) and went straight to a crystal filter at ... was it 34MHz? This set fixed a lot of problems. But it still needed manual operation of separate control knobs for its preselector, agc was better in some ways, but it was a hang circuit which would hold for half a second before decaying to higher gain. This meant short impulses could deafen it. Around the same time, much more expensive receivers from Marconi (H2900) and GEC (RC410) had servo-motor controlled preselectors tracking the tuning of their synthesisers.

After the RA1772, RACAL went backwards again. The RA1792 retained the Rafuse mixer, but replaced the preselector with a bank of suboctave filters. It is very much a cost cutting exercise. For a while their second hand prices were rather high.... a later generation receiver from RACAL had to be better, didn't it? But eventually reality sank in... even the bearing for the tuning knob was cheapened to a nylon bushing!

So you may be stuck with the AGC flaws of your RA17. Doing much about it risks worsening overload problems at various signal levels and offsets.

I've never had an R390, they'r fairly uncommon over here. I have had Marconi H2900 and GEC RC410 (GEC and Marconi competed against each other even though GEC owned Marconi) Racal receivers have been RA117E and I still have an RA1217 and RA1792. I've always had my eye open for an RA1772 at a good price, I think this was their best.
GM4ZNX was on the air for many years with an NOS RA117, and a Redifon GK203 multimode exciter plus a home built PA. I had an HP spectrum analyser looking at the RA117 IF to see what was about.

David