OBSERVATIONS GM1SXX 29 Dec 2009
I've owned an example of the superb RACAL RA 1772 HF receiver
for a number of years, having purchased it as a refurbished radio from Telford
Electronics in Shropshire. This radio receiver has given me excellent service
and I consider it to be the best HF radio receiver I've owned or used.
It's sensitive, selective, ergonomically superb, and is a joy to use. It is
however 'getting on' a bit in years, like myself, and of course with age,
comes aches and pains! The first problem I had was a faulty 723 regulator
in the -7 volt line. This was a few years ago, and because of the scarcity and
cost of obtaining a replacement round canned Ua723, I chose to build an
off-board regulator using a modern negative voltage adjustable regulator IC.
This proved to be easy to do and restored the radio to a working condition.
I've had other problems too. The PSU is a linear type and supplies
+20, +12, +5, -1V and -7V and is fitted on a single PCB with pass transistors
mounted on the rear apron of the set. If the RA1772 has a design flaw, I'd have
to say that it's in the PSU. It's cramped and runs hot, so has a tendency to
'bake' cabling, leading to brittle insulation that can crack and fall off, which
of course can lead to short circuits:-)
This 'Observation' has some large images that may take time to load. Just be patient.
My most recent receiver fault also turned out to be PSU related. The bridge rectifier listed as D2 on the schematic disintegrated... at least the positive output tag somehow separated from the rectifier leading to a total failure of the receiver. D2 generates most of the heat that emanates from the PSU. My first thought was to replace it with a potted bridge rectifier purchased from Maplin, so I proceeded to go about replacing the faulty bridge rectifier with a new one.
In the photo above, you can see the rear panel of the receiver folded down and the PSU board is just visible in the jumble of wires on the left of the photo.
Here you can see the offending rectifier as removed from the radio. The positive connection tag just lifted free as I was about to unsolder the leads from it!
This photo shows the PSU board with the new metal cased rectifier bolted into place and ready to connect up. The PCB pads marked ML1 are the site of the original Ua723 that was sued in the -7V supply, and which I removed years ago.
The wiring connected to the pins at the bottom of this photo suffers badly from being 'cooked' over many years of use. The insulation tends to become very brittle, and the wires inside oxidise, none of which is a good thing. In the photo sandwiched between the new rectified and the small IR bridge rectifier to the right of it are some pins that have thin 'fuse wire' soldered across them in normal use, to provide on-board fuses for the various supply lines. There are a number of these links used in the PSU board and ALL of them must be present and correct for the radio to function. In this photo, I had removed the links. I fitted new ones later.
I'm not sure what sort of 'fuse wire' RACAL used for these links, but I use a single strand of wire culled from normal stranded hookup wire as replacements. Works fine for me! The links protect the transformer secondaries in the event of a fault condition.
This photo shows the eventual setup I used! Since the bridge rectifier D2 produces most of the heat generated by the PSU, I decided to take it 'off-board', so I unbolted it and placed some 'copper island' pads where the rectifier had sat, so I had convenient connection points for the new wires I installed to connect to the rectifier. The advantage of this arrangement is that I take the heat generated to another part of the radio chassis where it can't do so much harm. The RA1772 is very 'tight' for space so I had to do some thinking about where I could place the rectifier to dump the excess heat while being a safe position to avoid short circuits etc.
My eventual solution was to bolt the rectifier to the rear of the card-cage that holds all the modular boards. This has decent ventilation while providing a large surface into which the heat can be dissipated. The wires from the rectifier are routed back to the PCB where the old one used to live. Unfortunately, I'm a bit like Henry Ford when it comes to wire colour... black, black, or err, umm... red! Ideally I'd have preserved the RACAL wiring colour-scheme.... but where to buy grey wire with blue stripes evades me! I used a load of small ty-wraps to tidy up the wiring and although not as nice as the original lacing-cord, it does a job of work.
The bridge was fitted below the level of the edge of the card-cage in order to clear the cage lid when it was re-fitted.
In this photo, you can see the modified PSU board and my earlier repair to the -7V supply, where I used a regulator fitted in the space vacated by the -7V supply's pass transistor I ripped out along with the 723 regulator associated with the -7V line, identified on the PCB as ML1. I used an LM337T regulator and a pair of fixed resistors and decoupling capacitors for the new -7V supply. Because this is a non-standard mod- I also labelled it as I have now also done with the D2 rectifier.
You can see the -7V supply more clearly in the photo above. The heavy wire goes to an earth tag on the chassis. I used 'copper island' PCB pads cyano-glued to the heatsink to make the various connections. This repair has seen daily use for several years now without any problems.
This photo shows the relocated rectifier D2 and associated wiring. The box to the right is the RF stage, preselector etc.
The old and new rectifiers for comparison. The new one is rated at 10 amps.
Almost back together again! You can see how little space there is around the PSU board on the left side of the photo and also how much metal is available for the rectifier to dump heat into. The rather 'chunky' mains transformer can be seen to the left of the photo in it's steel box. RACAL didn't do things by halves. It's a very nice varnish impregnated job, and very quiet indeed. The grey box situated between the transformer and the front panel is the ovened 5MHz frequency reference for the synthesizer. To the left of the loudspeaker (on the rear of the front panel) you can see the 30 position band-switch. I imagine this was a 'special' for RACAL and it's beautifully made. One thing you can't see is the tuning encoder which is an optical quadrature type constructed as a bespoke item for RACAL. It's beautifully made and a complete delight to use. Silky smooth.
Here you can see the receiver with the card-cage screening cover in place. The edge of the rectifier just clears the lip of the cover... whew!
Copy of the PSU PCB overlay... from the RACAL service manual. D2 causes most of the heat (and trouble).
Back together and working. 331? That's the Glasgow Airport NDB navigation beacon... just for a quick test!
I'm a happy bunny tonight :-)
The unit below the 1772 receiver is the RACAL MA1720 transmitter exciter. It's next on the list for my attention. It works 100% but I still need to sort out a PA and filter box for it.
73 Al.
GM1SXX