COMMUNICATIONS RECEIVER TYPE RA. 17
The Repair and restoration of an early Racal RA17
The Racal RA17 is undoubtedly the product which firmly established Racal as a world leader in its field. Had it not been for Collins’ unwillingness to allow Racal to manufacture their product under licence using non-US sourced parts, the RA17 might never have been designed.
As an former Racal Defence man myself, I felt it appropriate (and a challenge) to restore a very sad looking early RA17 (Ser. No. N664)
As can be seen from the photograph on the left, it was in a very sorry state indeed. Lying for over a year in a damp unoccupied property had probably not done it any favours. The front panel was splattered with some unknown substance. Interestingly though, when I switched it on, it did work, sort of. There was a lot of noise, crackles etc. Sensitivity was low, and the resolved audio was exceedingly distorted. Never having heard an RA17 in operation I had no bench mark to compare it with. However, I had recently read that many fans of the RA17 swear that they can still out-perform many of today’s modern receivers. With this in mind, and with a newly repaired RA1772 in the shack, I decided to put it to the test. But first I was going to have to do some serious work!
Note that the switch knobs in the photograph above are not the usual skirted type. This and the low serial number (N664) lead me to think that this is possibly a MK1. It took me a long time but I eventually stumbled upon Steve’s Boatanchors RA17 page, where the first three pictures are of a MK1 RA17 (Ser No. N721) . . . Complete with non-skirted knobs. So I reckon mine is definitely a MK1.
Obviously before embarking on this project I needed to get myself a manual and circuit diagram. The first one that I found was for an RA17L which I downloaded mainly out of curiosity. I eventually located and downloaded an RA17 manual with a very nice circuit diagram which I printed out on eight A4 sheets and carefully taped together. At his point, having cleaned up the front panel and with the lid removed, I switched it on again. Something caught my eye. A wisp of smoke. Not a good sign! I switched it off. Turning it upside down and removing the main bottom panel, it was clear where the smoke was coming from. But not only that, the tag-board in question was bent. Further examination revealed that one of the screws securing the side gusset plate to the chassis was too long and was pushing the board away from the chassis. At some point, the side plate adjacent to the 100KHz module had obviously been removed and when re-fitted, one of the long screws was substituted for one of the shorter ones. This however was not the cause of the smoke. R80 (2K2) in the 100KHz IF strip was completely cooked and was now only measuring 200R. A quick investigation revealed that the resistance to chassis from the ‘cold’ end of R80 was only about 400R. This implied about 200V across R80! 18W dissipated in a half-watt resistor is bad news! The actual cause of this low resistance fault was most unusual. I ‘chased’ it back through the 100KHz and Xtal filers. The input to the latter comes from the 3rd mixer via PL6 on the 2nd VFO module. Disconnecting SK6 from PL6 removed the fault. PL6 is connected to the Anode of V11 via R79 (2K2). However there was a low resistance evident across PL6 which was cleared simply by cleaning the rear of PL6. Inside the 2nd VFO unit there is a small vertically mounted tag board. In this case it was clear that several of the resistors had been severely stressed and showing signs of ‘cooking’. Huge amounts of wax had pooled towards the bottom edge of the board. It is my belief that waxy smoke had contaminated the rear of PL6 and formed a low resistance path to chassis.
