The RA1792
Page 12
Final thoughts … and links to useful pages

My own RA1792 was left over from an propagation experiment that Racal started (I think) in the mid 1980s. To cut a long story short, it and the accompanying 1723 drive unit were ‘stored’ in an environment which exposed them to extremes of temperature … cold in the winter and hot in the summer. This ultimately destroyed the LCD displays in both cases. So when I ‘inherited’ the kit, both were in a sorry state. The 1792 performed well as a receiver but I could never get the 1723 to do anything, so I parted company with it some time ago.

The issue with the ‘bleeding’ displays was resolved by replacing the entire front panel assembly and the PSU was modified to support the back-lit displays. Ultimately I replaced all the tantalum bead capacitors with new ones. It had a tendency to make loud crackles when switched on, accompanied with an ominous flickering of the back-lights! Rather worryingly, changing the tants didn’t resolve this issue, and everyone who I mentioned this to said the same thing … “it’ll be a tant about to pop” … and I tended to agree too. However, I had many other radios to attend to, having started my RA17 restoration service, so the flickering back-light problem was put on the ‘back burner’.

Last week I finished re-building five RA1792s so I decided to tackle what was obviously a fault on my own 1792. I hooked an oscilloscope up to the +5V rail at the PSU and switched on. Huge spikes were visible for several seconds before the trace settled down to a smooth flat line. I switched off the RX and unplugged each of the ribbon cables to the individual boards in turn. Ultimately all I had was the PSU, yet the spikes were still there. So the fault was in the PSU! That was a relief. The PSU in the 1792 is about as simple as they come. Looking at the schematic there were really only two possible suspects … two axial-lead 1uF tantalum capacitors … one either side of the 5V regulator. These were replaced with aluminium types, yet the problem persisted.

Then I thought “what if the fault is on the other side of the mains transformer?”. I had joked a few times that it might be a dirty mains switch! I dug out a scrap 1792 front panel complete with switch and mains cable. It was filthy! After cleaning it up, I substituted the switch and cable assembly for my own one and switched on. BINGO! No flickering back-lights, no loud crackles and no voltage spikes on the +5V rail. I didn’t leave it at that. This had been an intermittent problem, so I was taking no chances. But after several hours of operation and switching it off and on many times, the problem has gone … a dirty mains switch!!!

Although I have other receivers including an RA1772 and an RA3701 to name two, I do like the RA1792. Its 16KHz roofing filter offers the widest IF bandwidth in my collection, giving superb AM reception quality when used with an external speaker. The 1Hz tuning resolution is super smooth and switching modes and bandwidths is a breeze.

One thing that does let the type down though is the paintwork. Yes, there are lots of RA17s out there with tattered looking front panels, but some of these are pushing 60 years old! The 1792 has only been around half that time.The problem appears to be a lack of primer. I have a bunch of front panels where the grey paint is coming off in large flakes; notably where labels or stickers had been placed. Where the paint has come off there is bare metal exposed … no primer!!

Useful RA1792-related links

Keith’s vintage Racal Racal Enthusiast site has a mass of information on the RA1792.
Dave Schofield’s alternative to the infamous display problem.
Manuals and schematics at the Premium Receiver group.
Replacement Processor boards and more from Phill Simmons
Finally … Replacement LCD modules for your RA1792.