The Repair and restoration of another early Racal RA17 . . .
Something which I have to conclude is peculiar to the MK1 is the orientation of the Silvered Mica capacitors in the AGC circuit on the front tag board. Note that they along with several resistors and a 10nF tubular capacitor are mounted horizontally as opposed to vertically. ALL diagrams that I have encountered show these mounted vertically. Also note the vacant tags on the other large tag board where in later models V24/(later MR8) would be fitted . . . Evidence that the AGC circuitry did indeed undergo minor evolutions. Not shown here, but it might be worth a mention also . . . The MK1 does not appear to have been fitted with the simple 30MHz LP filter between the antenna connector and the input attenuator. . . Now, back to the 100KHz IF . . .
As with my RA17, the 100KHz IF was home to some of the worst cases of degrading components. Most of the large tubular capacitors turned out to be little more than high-value resistors and curiously, R90 and R91 (both 4K7) measured less than 300R!!, even though they didn’t show any outward signs of stressing. Also, it was noticed that the screening cans for V18 & V21 were loose. This was because someone had fitted a pair of the shorter CV4007s (6AL5W) instead of CV4025s. I therefore fitted a pair of new CV4025s.
3rd IF (100KHz) before refurbishment
3rd IF (100KHz) after refurbishment
I will be honest here, unlike the 3rd IF, the BFO is a horrible unit to work on . . . But worth the time and effort in the end.
