The Repair and restoration of another early Racal RA17 . . .
As expected, RA17 N659 now performs beautifully. Interestingly, the similarities between it and my own one (N664) are not limited to serial and inventory numbers however. Both had broken Meter switches. Both had open circuit meters. Both had loudspeakers that although appeared to work, delivered extremely distorted audio. It is also interesting to note that in both receivers it was the 3rd IF (100KHz) unit which produced the worst degraded capacitors, although this might simply be because the 3rd IF is the unit with the most tubular capacitors. Statistically, 33% of the carbon composite resistors were high by a factor of at least 33%, reinforcing the need to replace the resistors as well as the capacitors. As was the case with my RA17, N659 was retested after each module was refurbished, and at each stage in the process the general receiver noise figure was seen to drop; a receiver full of leaky capacitors and out of spec resistors produces a lot of unwanted pops and crackles.
As I said, the front panel meter was unserviceable. I don’t know why, but it now appears a common failure in RA17s. The good news is that it is not difficult to fit a new movement into the old casing. In this case I used a 100uA movement suitably shunted to bring it up to 200uA. The new movement had to be held in place with epoxy adhesive since the holes did not match. I also had to file a couple of notches in the scale-plate to accommodate the slightly wider movement (see right).
The fact that I have now come across two seemingly functional loudspeakers that produce extremely distorted audio is very curious indeed. My initial theory was that perhaps corrosion had formed between the pole-pieces and this in turn was interfering with the travel of the voice-coil. Having dismantled one of the loudspeakers, I am no longer convinced that corrosion is the culprit. Whatever the cause, the loudspeaker is not difficult to replace with RS Components part number 364-3357 being an excellent substitute albeit a higher impedance.
See the photograph on the left . . .
Finally, and proof that everything was done by hand back in the 1950s. I spotted this when giving the chassis a final clean. Clearly someone in the machine shop had drilled a hole in the wrong place. No Problem! Just fill it in and try again!
